@article {6303, title = {Plant responses to changing rainfall frequency and magnitude}, journal = {Nature Ecology \& Evolution}, year = {In Press}, keywords = {LTER-KNZ}, author = {Feldman, A.F. and Feng, X. and Felton, A.J. and Konings, A.G. and Knapp, A.K. and Biederman, A.J. and Poulter, B.} } @article {6068, title = {Repeated extreme droughts decrease root production, but not the potential for post-drought recovery of root production, in a mesic grassland}, journal = {Oikos}, volume = {1}, year = {2023}, pages = {e08899}, abstract = {

Global climate change is expected to cause more frequent extreme droughts in many parts of the world. Despite the crucial role of roots in water acquisition and plant survival, our understanding of ecosystem vulnerability to drought is primarily based on aboveground impacts. As return intervals between droughts decrease, root responses to one drought might alter responses to subsequent droughts, but this remains unresolved. We conducted a seven-year experiment that imposed extreme drought (growing season precipitation reduced 66\%) in a mesic grassland. Plots were droughted during years 1\–2 (\‘Drought 1\&$\#$39;), or years 5\–6 (\‘Drought 2\&$\#$39;) or both. We quantified root production during year 6 (final year of Drought 2) and year 7 (first year after Drought 2), when all plots received ambient precipitation. We found that repeated drought decreased root mass production more than twice as much as a single drought (\−63\% versus \−27\%, respectively, relative to ambient precipitation). Root mass production of the dominant C4 grass Andropogon gerardii did not decrease significantly with either one or two droughts. A. gerardii root traits differed from subdominant species on average across all treatments, but drought did not alter root traits of either A. gerardii or the subdominant species (collectively). In year 6, root production in plots droughted 4 years ago had not recovered (\−21\% versus control), but root production recovered in all formerly droughted plots in year 7, when precipitation was above average. Our results highlight the complexity of root responses to drought. Drought-induced reductions in root production can persist for years after drought and repeated drought can reduce production even further, but this does not preclude rapid recovery of root production in a wet year.

}, keywords = {LTER-KNZ}, doi = {10.1111/oik.08899}, url = {https://onlinelibrary.wiley.com/doi/10.1111/oik.08899}, author = {Slette, I.J. and D.L. Hoover and M. D. Smith and Knapp, A.K.} } @article {KNZ002034, title = {Is a drought a drought in grasslands? Productivity responses to different types of drought}, journal = {Oecologia}, year = {2021}, keywords = {LTER-KNZ}, author = {Carroll, C.J.W. and Slette, I.J. and Griffin-Nolan, R.J. and Baur, L.E. and Hoffman, A.M. and Denton, E.M. and Gray, J.E. and Post, A.K. and Johnston, M.K. and Yu, Q. and Collins, S.L. and Luo, Y. and Smith, M.D. and Knapp, A.K.} } @article {KNZ002035, title = {Asymmetric responses of ecosystem productivity to rainfall anomalies vary inversely with mean annual rainfall over the conterminous U.S}, journal = {Global Change Biology}, volume = {26}, year = {2020}, pages = {6959-6973}, keywords = {LTER-KNZ}, author = {Al-Yaari, A. and Wigneron, J.P. and Ciais, P. and Reichstein, M. and Ballantyne, A. and Og{\'e}e, J. and Ducharne, A. and Swenson, J.J. and Frappart, F. and Fan, L. and Wingate, L. and Li, X. and Hufkens, K. and Knapp, A.K.} } @article {KNZ002001, title = {Mass ratio effects underlie ecosystem responses to environmental change}, journal = {Journal of Ecology}, volume = {108}, year = {2020}, pages = {855-864}, abstract = {

1. Random species loss has been shown experimentally to reduce ecosystem function, sometimes more than other anthropogenic environmental changes. Yet, controversy surrounds the importance of this finding for natural systems where species loss is non-random.
2. We compiled data from 16 multi-year experiments located at a single native tallgrass prairie site. These experiments included responses to 11 anthropogenic environmental changes, as well as non-random biodiversity loss either the removal of uncommon/rare plant species or the most common (dominant) species.
3. As predicted by the mass ratio hypothesis, loss of a dominant species had large impacts on productivity that were comparable to other anthropogenic drivers. In contrast, the loss of uncommon/rare species had small effects on productivity despite having the largest effects on species richness.
4. The anthropogenic drivers that had the largest effects on productivity nitrogen, irrigation, and fire experienced not only loss of species but also significant changes in the abundance and identity of dominant species.
5. Synthesis. These results suggest that mass ratio effects, rather than species loss per se, are an important determinant of ecosystem function with environmental change.

}, keywords = {LTER-KNZ}, doi = {10.1111/1365-2745.13330}, url = {https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2745.13330}, author = {M.D. Smith and Koerner, S.E. and Alan K. Knapp and M.L. Avolio and Chaves, F.A. and Denton, E.M. and Dietrich, J. and Gibson, D.J. and Gray, J. and Hoffman, A.M. and Hoover, D.L. and Kimberly J. Komatsu and Silletti, A. and K.R. Wilcox and Yu, Q. and John M. Blair} } @article {KNZ002008, title = {Precipitation amount and event size interact to reduce ecosystem functioning during dry years in a mesic grassland}, journal = {Global Change Biology}, volume = {26}, year = {2020}, pages = {658-668}, abstract = {

Ongoing intensification of the hydrological cycle is altering rainfall regimes by increasing the frequency of extreme wet and dry years and the size of individual rainfall events. Despite long-standing recognition of the importance of precipitation amount and variability for most terrestrial ecosystem processes, we lack understanding of their interactive effects on ecosystem functioning. We quantified this interaction in native grassland by experimentally eliminating temporal variability in growing season rainfall over a wide range of precipitation amounts, from extreme wet to dry conditions. We contrasted the rain use efficiency (RUE) of above-ground net primary productivity (ANPP) under conditions of experimentally reduced versus naturally high rainfall variability using a 32-year precipitation\–ANPP dataset from the same site as our experiment. We found that increased growing season rainfall variability can reduce RUE and thus ecosystem functioning by as much as 42\% during dry years, but that such impacts weaken as years become wetter. During low precipitation years, RUE is lowest when rainfall event sizes are relatively large, and when a larger proportion of total rainfall is derived from large events. Thus, a shift towards precipitation regimes dominated by fewer but larger rainfall events, already documented over much of the globe, can be expected to reduce the functioning of mesic ecosystems primarily during drought, when ecosystem processes are already compromised by low water availability.

}, keywords = {LTER-KNZ}, issn = {1354-1013}, doi = {10.1111/gcb.14789}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14789}, author = {Felton, Andrew J. and Slette, Ingrid J. and M.D. Smith and Alan K. Knapp} } @article {KNZ002014, title = {Rainfall-manipulation experiments as simulated by terrestrial biosphere models: where do we stand?}, journal = {Global Change Biology}, volume = {26}, year = {2020}, pages = {3336{\textendash}3355}, abstract = {

Changes in rainfall amounts and patterns have been observed and are expected to continue in the near future with potentially significant ecological and societal consequences. Modelling vegetation responses to changes in rainfall is thus crucial to project water and carbon cycles in the future. In this study, we present the results of a new model-data intercomparison project, where we tested the ability of ten terrestrial biosphere models to reproduce observed sensitivity of ecosystem productivity to rainfall changes at ten sites across the globe, in nine of which, rainfall exclusion and/or irrigation experiments had been performed.

The key results are:
(a) Inter-model variation is generally large and model agreement varies with time scales. In severely water limited sites, models only agree on the interannual variability of evapotranspiration and to a smaller extent gross primary productivity. In more mesic sites model agreement for both water and carbon fluxes is typically higher on fine (daily-monthly) time scales and reduces on longer (seasonal-annual) scales.
(b) Models on average overestimate the relationship between ecosystem productivity and mean rainfall amounts across sites (in space) and have a low capacity in reproducing the temporal (interannual) sensitivity of vegetation productivity to annual rainfall at a given site, even though observation uncertainty is comparable to inter-model variability.
(c) Most models reproduced the sign of the observed patterns in productivity changes in rainfall manipulation experiments but had a low capacity in reproducing the observed magnitude of productivity changes. Models better reproduced the observed productivity responses due to rainfall exclusion than addition.
(d) All models attribute ecosystem productivity changes to the intensity of vegetation stress and peak leaf area, whereas the impact of the change in growing season length is negligible. The relative contribution of the peak leaf area and vegetation stress intensity was highly variable among models.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.15024}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.15024}, author = {Paschalis, Athanasios and Fatichi, Simone and Zscheischler, Jakob and Ciais, Philippe and Michael Bahn and Lena R. Boysen and Chang, Jinfeng and De Kauwe, Martin and Estiarte, Marc and Goll, Daniel and Hanson, Paul J. and Harper, Anna B. and Hou, Enqing and Kigel, Jaime and Alan K. Knapp and Larsen, Klaus Steenberg and Li, Wei and Lienert, Sebastian and Luo, Yiqi and Meir, Patrick and Nabel, Julia E.M.S. and Ogaya, Rom{\`a} and Parolari, Anthony J and Peng, Changhui and Pe{\~n}uelas, Josep and Pongratz, Julia and Rambal, Serge and Schmidt, Inger Kappel and Shi, Hao and Sternberg, Marcelo and Tian, Hanqin and Tschumi, Elisabeth and Ukkola, Anna and Vicca, Sara and Viovy, Nicolas and Wang, -Ping and Wang, Zhuonan and Williams, Karina and Wu, Donghai and Zhu, Qiuan} } @article {KNZ002036, title = {Temporal variability in production is not consistently affected by global change drivers across herbaceous-dominated ecosystems}, journal = {Oecologia}, volume = {194}, year = {2020}, pages = {735{\textendash}744}, keywords = {LTER-KNZ}, author = {Avolio, M.L. and Wilcox, K.R. and La Pierre, K.J. and Lemoine, N. and Bowman, W.D. and Collins, S.L. and Knapp, A.K. and Koerner, S.E. and Smith, M.D. and Baer, S.G. and Gross, K.L. and Isbell, F. and McLaren, J. and Reich, P.B. and Suding, K.N. and Suttle, K.B. and Tilman, D. and Xu, Z. and Yu, Q.} } @article {KNZ001944, title = {Assessing precipitation, evapotranspiration, and NDVI as controls of U.S. Great Plains plant production}, journal = {Ecosphere}, volume = {10}, year = {2019}, pages = {e02889}, abstract = {

Productivity throughout the North American Great Plains grasslands is generally considered to be water limited, with the strength of this limitation increasing as precipitation decreases. We hypothesize that cumulative actual evapotranspiration water loss (AET) from April to July is the precipitation-related variable most correlated to aboveground net primary production (ANPP) in the U.S. Great Plains (GP). We tested this by evaluating the relationship of ANPP to AET, precipitation, and plant transpiration (Tr). We used multi-year ANPP data from five sites ranging from semiarid grasslands in Colorado and Wyoming to mesic grasslands in Nebraska and Kansas, mean annual NRCS ANPP, and satellite-derived normalized difference vegetation index (NDVI) data. Results from the five sites showed that cumulative April-to-July AET, precipitation, and Tr were well correlated (R2: 0.54\–0.70) to annual changes in ANPP for all but the wettest site. AET and Tr were better correlated to annual changes in ANPP compared to precipitation for the drier sites, and precipitation in August and September had little impact on productivity in drier sites. April-to-July cumulative precipitation was best correlated (R2 = 0.63) with interannual variability in ANPP in the most mesic site, while AET and Tr were poorly correlated with ANPP at this site. Cumulative growing season (May-to-September) NDVI (iNDVI) was strongly correlated with annual ANPP at the five sites (R2 = 0.90). Using iNDVI as a surrogate for ANPP, we found that county-level cumulative April\–July AET was more strongly correlated to ANPP than precipitation for more than 80\% of the GP counties, with precipitation tending to perform better in the eastern more mesic portion of the GP. Including the ratio of AET to potential evapotranspiration (PET) improved the correlation of AET to both iNDVI and mean county-level NRCS ANPP. Accounting for how different precipitation-related variables control ANPP (AET in drier portion, precipitation in wetter portion) provides opportunity to develop spatially explicit forecasting of ANPP across the GP for enhancing decision-making by land managers and use of grassland ANPP for biofuels.

}, keywords = {LTER-KNZ}, doi = {10.1002/ecs2.2889}, url = {https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.2889}, author = {Chen, Maosi and Parton, William J. and Hartman, Melannie D. and Del Grosso, Stephen J. and Smith, William K. and Alan K. Knapp and Lutz, Susan and Derner, Justin D. and Tucker, Compton J. and Ojima, Dennis S. and Volesky, Jerry D. and Stephenson, Mitchell B. and Schacht, Walter H. and Gao, Wei} } @article {KNZ001954, title = {Decadal-scale shifts in soil hydraulic properties induced by altered precipitation}, journal = {Science Advances}, volume = {5}, year = {2019}, pages = {eaau6635}, abstract = {

Soil hydraulic properties influence the partitioning of rainfall into infiltration versus runoff, determine plant-available water, and constrain evapotranspiration. Although rapid changes in soil hydraulic properties from direct human disturbance are well documented, climate change may also induce such shifts on decadal time scales. Using soils from a 25-year precipitation manipulation experiment, we found that a 35\% increase in water inputs substantially reduced infiltration rates and modestly increased water retention. We posit that these shifts were catalyzed by greater pore blockage by plant roots and reduced shrink-swell cycles. Given that precipitation regimes are expected to change at accelerating rates globally, shifts in soil structure could occur over broad regions more rapidly than expected and thus alter water storage and movement in numerous terrestrial ecosystems.

}, keywords = {LTER-KNZ}, doi = {10.1126/sciadv.aau6635}, url = {https://advances.sciencemag.org/content/5/9/eaau6635}, author = {Caplan, J.S. and Gimenez, D. and Hirmas, D.R. and N. Brunsell and John M. Blair and Alan K. Knapp} } @article {KNZ001965, title = {Global change effects on plant communities are magnified by time and the number of global change factors imposed}, journal = {Proceedings of the National Academy of Sciences}, volume = {116}, year = {2019}, pages = {17867-17873}, abstract = {

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (\<10 y). In contrast, long-term (\≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity\–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.

}, keywords = {LTER-KNZ}, doi = {10.1073/pnas.1819027116}, url = {https://www.pnas.org/content/early/2019/08/14/1819027116}, author = {Kimberly J. Komatsu and M.L. Avolio and Lemoine, Nathan P. and Isbell, Forest and Grman, Emily and Houseman, Gregory R. and Koerner, Sally E. and Johnson, D.S. and K.R. Wilcox and Juha M. Alatalo and Anderson, J.P. and Aerts, R. and S.G. Baer and Baldwin, Andrew H. and Bates, J. and Beierkuhnlein, C. and Belote, R.T. and John M. Blair and Bloor, J.M.G. and Bohlen, P.J. and Edward W. Bork and Elizabeth H. Boughton and W.D. Bowman and Britton, Andrea J. and Cahill, James F. and Chaneton, Enrique J. and Chiariello, N.R. and Cheng, Jimin. and Scott. L. Collins and Cornelissen, J.H.C. and G. Du and Eskelinen, Anu and Firn, Jennifer and Foster, B. and Gough, L. and Gross, K. and Hallett, L.M. and Han, X. and Harmens, H. and Hovenden, M.J. and Jagerbrand, A. and Jentsch, A. and Kern, Christel and Klanderud, Kari and Alan K. Knapp and Kreyling, Juergen and Li, W. and Luo, Yiqi and McCulley, R.L. and McLaren, Jennie R. and Megonigal, Patrick and J.W. Morgan and Onipchenko, Vladimir and Pennings, S.C. and Prev{\'e}y, J.S. and Price, Jodi N. and P.B. Reich and Robinson, Clare H. and Russell, L.F. and Sala, O.E. and Seabloom, E.W. and M.D. Smith and Soudzilovskaia, Nadejda A. and Souza, Lara and K.N. Suding and Suttle, B.K. and Svejcar, T. and Tilman, David and Tognetti, P. and Turkington, R. and White, S. and Xu, Zhuwen and Yahdjian, L. and Yu, Q. and Zhang, Pengfei and Zhang, Yunhai} } @phdthesis {KNZ001966, title = {Grassland sensitivity to extreme drought: Assessing the role of dominant species physiology and community functional composition}, volume = {PhD Dissertation}, year = {2019}, school = {Colorado State University}, type = {Ph.D. Thesis}, address = {Fort Collins, CO}, keywords = {LTER-KNZ}, url = {https://mountainscholar.org/bitstream/handle/10217/197274/GriffinNolan_colostate_0053A_15495.pdf?sequence=1}, author = {Griffin-Nolan, Robert} } @article {KNZ001970, title = {How ecologists define drought, and why we should do better}, journal = {Global Change Biology}, volume = {25}, year = {2019}, pages = {3193 - 3200}, abstract = {

Drought, widely studied as an important driver of ecosystem dynamics, is predicted to increase in frequency and severity globally. To study drought, ecologists must define or at least operationalize what constitutes a drought. How this is accomplished in practice is unclear, particularly given that climatologists have long struggled to agree on definitions of drought, beyond general variants of \“an abnormal deficiency of water.\” We conducted a literature review of ecological drought studies (564 papers) to assess how ecologists describe and study drought. We found that ecologists characterize drought in a wide variety of ways (reduced precipitation, low soil moisture, reduced streamflow, etc.), but relatively few publications (~32\%) explicitly define what are, and are not, drought conditions. More troubling, a surprising number of papers (~30\%) simply equated \“dry conditions\” with \“drought\” and provided little characterization of the drought conditions studied. For a subset of these, we calculated Standardized Precipitation Evapotranspiration Index values for the reported drought periods. We found that while almost 90\% of the studies were conducted under conditions quantifiable as slightly to extremely drier than average, ~50\% were within the range of normal climatic variability. We conclude that the current state of the ecological drought literature hinders synthesis and our ability to draw broad ecological inferences because drought is often declared but is not explicitly defined or well characterized. We suggest that future drought publications provide at least one of the following: (a) the climatic context of the drought period based on long-term records; (b) standardized climatic index values; (c) published metrics from drought-monitoring organizations; (d) a quantitative definition of what the authors consider to be drought conditions for their system. With more detailed and consistent quantification of drought conditions, comparisons among studies can be more rigorous, increasing our understanding of the ecological effects of drought.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.14747}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14747}, author = {Slette, Ingrid J. and Post, Alison K. and Awad, Mai and Even, Trevor and Punzalan, Arianna and Williams, Sere and M.D. Smith and Alan K. Knapp} } @article {KNZ001983, title = {Shifts in plant functional composition following long-term drought in grasslands}, journal = {Journal of Ecology}, volume = {107}, year = {2019}, pages = {2133 - 2148}, abstract = {

\ 1. Plant traits can provide unique insights into plant performance at the community scale. Functional composition, defined by both functional diversity and community-weighted trait means (CWMs), can affect the stability of above-ground net primary production (ANPP) in response to climate extremes. Further complexity arises, however, when functional composition itself responds to environmental change. The duration of climate extremes, such as drought, is expected to increase with rising global temperatures; thus, understanding the impacts of long-term drought on functional composition and the corresponding effect that has on ecosystem function could improve predictions of ecosystem sensitivity to climate change.
\ 2. We experimentally reduced growing season precipitation by 66\% across six temperate grasslands for 4 years and measured changes in three indices of functional diversity (functional dispersion, richness and evenness), community-weighted trait means and phylogenetic diversity (PD). Specific leaf area (SLA), leaf nitrogen content (LNC) and (at most sites) leaf turgor loss point (πTLP) were measured for species cumulatively representing ~90\% plant cover at each site.
\ 3. Long-term drought led to increased community functional dispersion in three sites, with negligible effects on the remaining sites. Species re-ordering following the mortality/senescence of dominant species was the main driver of increased functional dispersion. The response of functional diversity was not consistently matched by changes in phylogenetic diversity. Community-level drought strategies (assessed as CWMs) largely shifted from drought tolerance to drought avoidance and/or escape strategies, as evidenced by higher community-weighted πTLP, SLA and LNC. Lastly, ecosystem drought sensitivity (i.e. relative reduction in ANPP in drought plots) was positively correlated with community-weighted SLA and negatively correlated with functional diversity.
\ 4. Synthesis. Increased functional diversity following long-term drought may stabilize ecosystem functioning in response to future drought. However, shifts in community-scale drought strategies may increase ecosystem drought sensitivity, depending on the nature and timing of drought. Thus, our results highlight the importance of considering both functional diversity and abundance-weighted traits means of plant communities as their collective effect may either stabilize or enhance ecosystem sensitivity to drought.

}, keywords = {LTER-KNZ, ANPP, Climate change, community weighted traits, Drought, Functional diversity, plant functional traits}, doi = {10.1111/1365-2745.13252}, url = {https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2745.13252}, author = {Griffin-Nolan, Robert J. and D.M. Blumenthal and Scott. L. Collins and Farkas, Timothy E. and Hoffman, Ava M. and Mueller, Kevin E. and Ocheltree, Troy W. and M.D. Smith and Whitney, Kenneth D. and Alan K. Knapp}, editor = {Jones, Holly} } @article {KNZ001898, title = {Ambient changes exceed treatment effects on plant species abundance in long-term global change experiments}, journal = {Glob Chang Biol}, volume = {24}, year = {2018}, pages = {5668 - 5679}, abstract = {

The responses of species to environmental changes will determine future community composition and ecosystem function. Many syntheses of global change experiments examine the magnitude of treatment effect sizes, but we lack an understanding of how plant responses to treatments compare to ongoing changes in the unmanipulated (ambient or background) system. We used a database of long-term global change studies manipulating CO2 , nutrients, water, and temperature to answer three questions: (a) How do changes in plant species abundance in ambient plots relate to those in treated plots? (b) How does the magnitude of ambient change in species-level abundance over time relate to responsiveness to global change treatments? (c) Does the direction of species-level responses to global change treatments differ from the direction of ambient change? We estimated temporal trends in plant abundance for 791 plant species in ambient and treated plots across 16 long-term global change experiments yielding 2,116 experiment-species-treatment combinations. Surprisingly, for most species (57\%) the magnitude of ambient change was greater than the magnitude of treatment effects. However, the direction of ambient change, whether a species was increasing or decreasing in abundance under ambient conditions, had no bearing on the direction of treatment effects. Although ambient communities are inherently dynamic, there is now widespread evidence that anthropogenic drivers are directionally altering plant communities in many ecosystems. Thus, global change treatment effects must be interpreted in the context of plant species trajectories that are likely driven by ongoing environmental changes.

}, keywords = {LTER-KNZ, elevated CO2, nitrogen, Phosphorus, plant community, Warming, water}, doi = {10.1111/gcb.14442}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14442}, author = {Langley, A. and Chapman, S.K. and Kimberly J. La Pierre and M.L. Avolio and W.D. Bowman and Johnson, D. and Isbell, F. and K.R. Wilcox and Foster, B. and Hovenden, M. and Alan K. Knapp and Koerner, S.E. and Lortie, C. and Megonigal, J. and Newton, P. and Reich, B. and M.D. Smith and Suttle, B.K. and Tilman, D.} } @article {KNZ001863, title = {Asymmetric responses of primary productivity to altered precipitation simulated by ecosystem models across three long-term grassland sites}, journal = {Biogeosciences}, volume = {15}, year = {2018}, pages = {3421 - 3437}, abstract = {

Field measurements of aboveground net primary productivity (ANPP) in temperate grasslands suggest that both positive and negative asymmetric responses to changes in precipitation (P) may occur. Under normal range of precipitation variability, wet years typically result in ANPP gains being larger than ANPP declines in dry years (positive asymmetry), whereas increases in ANPP are lower in magnitude in extreme wet years compared to reductions during extreme drought (negative asymmetry). Whether the current generation of ecosystem models with a coupled carbon\–water system in grasslands are capable of simulating these asymmetric ANPP responses is an unresolved question. In this study, we evaluated the simulated responses of temperate grassland primary productivity to scenarios of altered precipitation with 14 ecosystem models at three sites: Shortgrass steppe (SGS), Konza Prairie (KNZ) and Stubai Valley meadow (STU), spanning a rainfall gradient from dry to moist. We found that (1)\ the spatial slopes derived from modeled primary productivity and precipitation across sites were steeper than the temporal slopes obtained from inter-annual variations, which was consistent with empirical data; (2)\ the asymmetry of the responses of modeled primary productivity under normal inter-annual precipitation variability differed among models, and the mean of the model ensemble suggested a negative asymmetry across the three sites, which was contrary to empirical evidence based on filed observations; (3)\ the mean sensitivity of modeled productivity to rainfall suggested greater negative response with reduced precipitation than positive response to an increased precipitation under extreme conditions at the three sites; and (4)\ gross primary productivity (GPP), net primary productivity (NPP), aboveground NPP (ANPP) and belowground NPP (BNPP) all showed concave-down nonlinear responses to altered precipitation in all the models, but with different curvatures and mean values. Our results indicated that most models overestimate the negative drought effects and/or underestimate the positive effects of increased precipitation on primary productivity under normal climate conditions, highlighting the need for improving eco-hydrological processes in those models in the future.

}, keywords = {LTER-KNZ}, doi = {10.5194/bg-15-3421-2018}, url = {https://www.biogeosciences.net/15/3421/2018/}, author = {Wu, Donghai and Ciais, Philippe and Viovy, Nicolas and Alan K. Knapp and K.R. Wilcox and Michael Bahn and M.D. Smith and Vicca, Sara and Fatichi, Simone and Zscheischler, Jakob and He, Yue and Li, Xiangyi and Ito, Akihiko and Arneth, Almut and Harper, Anna and Ukkola, Anna and Paschalis, Athanasios and Poulter, Benjamin and Peng, Changhui and Ricciuto, Daniel and Reinthaler, David and Chen, Guangsheng and Tian, Hanqin and Genet, {\'e}l{\`e}ne and Mao, Jiafu and Ingrisch, Johannes and Nabel, Julia E. S. M. and Pongratz, Julia and Lena R. Boysen and Kautz, Markus and Schmitt, Michael and Meir, Patrick and Zhu, Qiuan and R. Hasibeder and Sippel, Sebastian and Dangal, Shree R. S. and Sitch, Stephen and Shi, Xiaoying and Wang, Yingping and Luo, Yiqi and Liu, Yongwen and Piao, Shilong} } @article {KNZ001928, title = {Carbon exchange responses of a mesic grassland to an extreme gradient of precipitation}, journal = {Oecologia}, year = {2018}, pages = {1 -12}, abstract = {

Growing evidence indicates that ecosystem processes may be differentially sensitive to dry versus wet years, and that current understanding of how precipitation affects ecosystem processes may not be predictive of responses to extremes. In an experiment within a mesic grassland, we addressed this uncertainty by assessing responses of two key carbon exchange processes\—aboveground net primary production (ANPP) and soil respiration (Rs) - to an extensive gradient of growing season precipitation. This gradient comprised 11 levels that specifically included extreme values in precipitation; defined as the 1st, 5th, 95th, and 99th percentiles of the 112-year climate record. Across treatments, our experimental precipitation gradient linearly increased soil moisture availability in the rooting zone (upper 20\ cm). Relative to ANPP under nominal precipitation amounts (defined as between the 15th and 85th percentiles), the magnitude of ANPP responses were greatest to extreme increases in precipitation, with an underlying linear response to both precipitation and soil moisture gradients. By contrast, Rs exhibited marginally greater responses to dry versus wet extremes, with a saturating relationship best explaining responses of Rs to both precipitation and soil moisture. Our findings indicate a linear relationship between ANPP and precipitation after incorporating responses to precipitation extremes in the ANPP\–precipitation relationship, yet in contrast saturating responses of Rs. As a result, current linear ANPP\–precipitation relationships (up to ~\ 1000\ mm) within mesic grasslands appear to hold as appropriate benchmarks for ecosystems models, yet such models should incorporate nonlinearities in responses of Rs amid increased frequencies and magnitudes of precipitation extremes.

}, keywords = {LTER-KNZ, Climate extremes, Drought, Primary production, Rainfall, soil respiration}, doi = {10.1007/s00442-018-4284-2}, url = {https://link.springer.com/article/10.1007/s00442-018-4284-2}, author = {Felton, A.J. and Alan K. Knapp and M.D. Smith} } @article {KNZ001888, title = {Change in dominance determines herbivore effects on plant biodiversity}, journal = {Nature Ecology and Evolution}, volume = {2}, year = {2018}, pages = {1925-1932}, abstract = {

Herbivores alter plant biodiversity (species richness) in many of the world\’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis\—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.

}, keywords = {LTER-KNZ}, doi = {https://doi.org/10.1038/s41559-018-0696-y}, url = {https://www.nature.com/articles/s41559-018-0696-y$\#$article-info}, author = {Koerner, S.E. and M.D. Smith and Burkepile, D.E. and N.P. Hanan and M.L. Avolio and Scott. L. Collins and Alan K. Knapp and N.P. Lemoine and E.J. Forrestel and S. Eby and D.I. Thompson and G. Aguado-Santacruz and J.P. Anderson and Anderson, M. and A. Angassa and S. Bagchi and E.S. Bakker and Bastin, Gary and L.E. Baur and K.H. Beard and E.A. Beever and P.J. Bohlen and Elizabeth H. Boughton and Canestro, Don and Cesa, Ariela and Chaneton, Enrique and Cheng, Jimin and C.M. D{\textquoteright}Antonio and C. Deleglise and Fadiala. Demb{\'e}l{\'e} and Josh. Dorrough and David. J. Eldridge and Barbara. Fernandez-Going and Silvia. Fern{\'a}ndez-Lugo and Lauchlan. H. Fraser and Bill. Freedman and Gonzalo. Garc{\'\i}a-Salgado and Jacob. R. Goheen and Liang. Guo and Sean. Husheer and Moussa. Karemb{\'e} and Johannes. M. H. Knops and Tineke. Kraaij and Andrew. Kulmatiski and Minna-Maarit. Kyt{\"o}viita and Felipe. Lezama and Gregory. Loucougaray and Alejandro. Loydi, Dan G. Milchunas, and Dan.G. Milchunas, and Suzanne. J. Milton and J.W. Morgan and Claire. Moxham and Kyle. C. Nehring and Han. Olff and Todd. M. Palmer and Salvador. Rebollo and Corinna. Riginos and Anita. C. Risch and Marta Rueda and Mahesh. Sankaran and Takehiro. Sasaki and Kathryn. A. Schoenecker and Nick. L. Schultz and Martin. Sch{\"u}tz and Angelika. Schwabe and Frances. Siebert and Christian. Smit and Karen. A. Stahlheber and Christian. Storm and Dustin. J. Strong and Jishuai. Su and Yadugiri. V. Tiruvaimozhi and Claudia. Tyler and James. Val and Martijn. L. Vandegehuchte and Kari. E. Veblen and Lance. T. Vermeire and David. Ward and Jianshuang. Wu and Truman. P. Young and Qiang. Yu and Tamara. Jane. Zelikova} } @article {KNZ001865, title = {Codominant grasses differ in gene expression under experimental climate extremes in native tallgrass prairie}, journal = {PeerJ}, year = {2018}, pages = {e4394}, abstract = {

Extremes in climate, such as heat waves and drought, are expected to become more frequent and intense with forecasted climate change. Plant species will almost certainly differ in their responses to these stressors. We experimentally imposed a heat wave and drought in the tallgrass prairie ecosystem near Manhattan, Kansas, USA to assess transcriptional responses of two ecologically important C4 grass species, Andropogon gerardii and Sorghastrum nutans. Based on previous research, we expected that S. nutans would regulate more genes, particularly those related to stress response, under high heat and drought. Across all treatments, S. nutans showed greater expression of negative regulatory and catabolism genes while A. gerardii upregulated cellular and protein metabolism. As predicted, S. nutans showed greater sensitivity to water stress, particularly with downregulation of non-coding RNAs and upregulation of water stress and catabolism genes. A. gerardii was less sensitive to drought, although A. gerardii tended to respond with upregulation in response to drought versus S. nutans which downregulated more genes under drier conditions. Surprisingly, A. gerardii only showed minimal gene expression response to increased temperature, while S. nutans showed no response. Gene functional annotation suggested that these two species may respond to stress via different mechanisms. Specifically, A. gerardii tends to maintain molecular function while S. nutans prioritizes avoidance. Sorghastrum nutans may strategize abscisic acid response and catabolism to respond rapidly to stress. These results have important implications for success of these two important grass species under a more variable and extreme climate forecast for the future.

}, keywords = {LTER-KNZ}, doi = {https://doi.org/10.7717/peerj.4394}, url = {https://peerj.com/articles/4394/}, author = {Hoffman, Ava M. and M.L. Avolio and Alan K. Knapp and M.D. Smith} } @article {KNZ001886, title = {Drought consistently alters the composition of soil fungal and bacterial communities in grasslands from two continents}, journal = {Global Change Biology}, volume = {24}, year = {2018}, pages = {2818 - 2827}, abstract = {

The effects of short-term drought on soil microbial communities remain largely unexplored, particularly at large scales and under field conditions. We used seven experimental sites from two continents (North America and Australia) to evaluate the impacts of imposed extreme drought on the abundance, community composition, richness, and function of soil bacterial and fungal communities. The sites encompassed different grassland ecosystems spanning a wide range of climatic and soil properties. Drought significantly altered the community composition of soil bacteria and, to a lesser extent, fungi in grasslands from two continents. The magnitude of the fungal community change was directly proportional to the precipitation gradient. This greater fungal sensitivity to drought at more mesic sites contrasts with the generally observed pattern of greater drought sensitivity of plant communities in more arid grasslands, suggesting that plant and microbial communities may respond differently along precipitation gradients. Actinobateria, and Chloroflexi, bacterial phyla typically dominant in dry environments, increased their relative abundance in response to drought, whereas Glomeromycetes, a fungal class regarded as widely symbiotic, decreased in relative abundance. The response of Chlamydiae and Tenericutes, two phyla of mostly pathogenic species, decreased and increased along the precipitation gradient, respectively. Soil enzyme activity consistently increased under drought, a response that was attributed to drought-induced changes in microbial community structure rather than to changes in abundance and diversity. Our results provide evidence that drought has a widespread effect on the assembly of microbial communities, one of the major drivers of soil function in terrestrial ecosystems. Such responses may have important implications for the provision of key ecosystem services, including nutrient cycling, and may result in the weakening of plant\–microbial interactions and a greater incidence of certain soil-borne diseases.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.2018.24.issue-710.1111/gcb.14113}, url = {http://doi.wiley.com/10.1111/gcb.2018.24.issue-7}, author = {Ochoa-Hueso, Ra{\'u}l and Scott. L. Collins and Delgado-Baquerizo, Manuel and Hamonts, Kelly and Pockman, William T. and Sinsabaugh, Robert L. and M.D. Smith and Alan K. Knapp and Power, Sally A.} } @article {KNZ001885, title = {Legacy effects of a regional drought on aboveground net primary production in six central US grasslands}, journal = {Plant Ecology}, volume = {219}, year = {2018}, pages = {505 - 515}, abstract = {

Global climate models predict increases in the frequency and severity of drought worldwide, directly affecting most ecosystem types. Consequently, drought legacy effects (drought-induced alterations in ecosystem function postdrought) are expected to become more common in ecosystems varying from deserts to grasslands to forests. Drought legacies in grasslands are usually negative and reduce ecosystem function, particularly after extended drought. Moreover, ecosystems that respond strongly to drought (high sensitivity) might be expected to exhibit the largest legacy effects the next year, but this relationship has not been established. We quantified legacy effects of a severe regional drought in 2012 on postdrought (2013) aboveground net primary productivity (ANPP) in six central US grasslands. We predicted that (1) the magnitude of drought legacy effects measured in 2013 would be positively related to the sensitivity of ANPP to the 2012 drought, and (2) drought legacy effects would be negative (reducing 2013 ANPP relative to that expected given normal precipitation amounts). The magnitude of legacy effects measured in 2013 was strongly related (r2\ =\ 0.88) to the sensitivity of ANPP to the 2012 drought across these six grasslands. However, contrary to expectations, positive legacy effects (greater than expected ANPP) were more commonly observed than negative legacy effects. Thus, while the sensitivity of ANPP to drought may be a useful predictor of the magnitude of legacy effects, short-term (1-year) severe droughts may cause legacy effects that are more variable than those observed after multiyear droughts.

}, keywords = {LTER-KNZ}, issn = {1385-0237}, doi = {10.1007/s11258-018-0813-7}, url = {http://link.springer.com/10.1007/s11258-018-0813-7.pdf}, author = {Griffin-Nolan, Robert J. and Carroll, Charles J. W. and Denton, Elsie M. and Johnston, Melissa K. and Scott. L. Collins and M.D. Smith and Alan K. Knapp} } @article {KNZ001887, title = {A reality check for climate change experiments: Do they reflect the real world?}, journal = {Ecology}, volume = {99}, year = {2018}, pages = {2145-2151}, abstract = {

Experiments are widely used in ecology, particularly for assessing global change impacts on ecosystem function. However, results from experiments often are inconsistent with observations made under natural conditions, suggesting the need for rigorous comparisons of experimental and observational studies. We conducted such a \“reality check\” for a grassland ecosystem by compiling results from nine independently conducted climate change experiments. Each experiment manipulated growing season precipitation (GSP) and measured responses in aboveground net primary production (ANPP). We compared results from experiments with long-term (33-yr) annual precipitation and ANPP records to ask if collectively (n\ =\ 44 experiment-years) experiments yielded estimates of ANPP, rain-use efficiency (RUE, grams per square meter ANPP per mm precipitation), and the relationship between GSP and ANPP comparable to observations. We found that mean ANPP and RUE from experiments did not deviate from observations. Experiments and observational data also yielded similar functional relationships between ANPP and GSP, but only within the range of historically observed GSP. Fewer experiments imposed extreme levels of GSP (outside the observed 33-yr record), but when these were included, they altered the GSP\–ANPP relationship. This result underscores the need for more experiments imposing extreme precipitation levels to resolve how forecast changes in climate regimes will affect ecosystem function in the future.

}, keywords = {LTER-KNZ}, doi = {10.1002/ecy.2474}, url = {https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecy.2474}, author = {Alan K. Knapp and Carroll, C.J.W. and Griffin-Nolan, R.J. and Slette, I.J. and Chavez, F.A. and Baur, L. and Felton, A.J. and Gray, J. and Hoffman, A.M. and Lemoine, N.P. and Mao, W. and Post, A. and M.D. Smith} } @article {KNZ001883, title = {Trait selection and community weighting are key to understanding ecosystem responses to changing precipitation regimes}, journal = {Functional Ecology}, volume = {32}, year = {2018}, pages = {1746 - 1756}, abstract = {

1. Plant traits can be used to predict ecosystem responses to environmental change using a response\–effect trait framework. To do this, appropriate traits must be identified that explain a species\&$\#$39; influence on ecosystem function (\“effect traits\”) and the response of those species to environmental change (\“response traits\”). Response traits are often identified and measured along gradients in plant resources, such as water availability; however, precipitation explains very little variation in most plant traits globally. Given the strong relationship between plant traits and ecosystem functions, such as net primary productivity (NPP), and between NPP and precipitation, the lack of correlation between precipitation and plant traits is surprising.

2. We address this issue through a systematic review of \>500 published studies that describe plant trait responses to altered water availability. The overarching goal of this review was to identify potential causes for the weak relationship between commonly measured plant traits and water availability so that we may identify more appropriate \“response traits.\”

3. We attribute weak trait\–precipitation relationships to an improper selection of traits (e.g., nonhydraulic traits) and a lack of trait-based approaches that adjust for trait variation within communities (only 4\% of studies measure community-weighted traits). We then highlight the mechanistic value of hydraulic traits as more appropriate \“response traits\” with regard to precipitation, which should be included in future community-scale trait surveys.

4. Trait-based ecology has the potential to improve predictions of ecosystem responses to predicted changes in precipitation; however, this predictive power depends heavily on the identification of reliable response and effect traits. To this end, trait surveys could be improved by a selection of traits that reflect physiological functions directly related to water availability with traits weighted by species relative abundance.

}, keywords = {LTER-KNZ}, doi = {10.1111/1365-2435.13135}, url = {https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2435.13135}, author = {Griffin-Nolan, Robert J. and Bushey, Julie A. and Carroll, Charles J. W. and Challis, Anthea and Chieppa, Jeff and Garbowski, Magda and Hoffman, Ava M. and Post, Alison K. and Slette, Ingrid J. and Spitzer, Daniel and Zambonini, Dario and Ocheltree, Troy W. and Tissue, David T. and Alan K. Knapp}, editor = {Fox, Charles} } @article {KNZ001852, title = {Assessing community and ecosystem sensitivity to climate change - toward a more comparative approach}, journal = {Journal of Vegetation Science}, volume = {28}, year = {2017}, pages = {235 - 237}, abstract = {

Plant communities can vary widely in their sensitivity to changing precipitation regimes, as reported by Byrne et al., Mulhouse et al. and Sternberg et al. in this issue of Journal of Vegetation Science. But to understand why communities differ in their sensitivity, we argue that clearly defined metrics of sensitivity and coordinated research approaches are needed to elucidate mechanisms.

}, keywords = {LTER-KNZ}, doi = {10.1111/jvs.12524}, url = {https://onlinelibrary.wiley.com/doi/full/10.1111/jvs.12524}, author = {M.D. Smith and K.R. Wilcox and Power, Sally A. and Tissue, David T. and Alan K. Knapp} } @article {KNZ001811, title = {Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments}, journal = {Global Change Biology}, volume = {23}, year = {2017}, month = {4376-4385}, abstract = {

Climatic changes are altering Earth\&$\#$39;s hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.13706}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.13706}, author = {K.R. Wilcox and Shi, Zheng and Gherardi, Laureano A. and Lemoine, Nathan P. and Koerner, Sally E. and Hoover, David L. and Bork, Edward and Byrne, Kerry M. and Cahill, James and Scott. L. Collins and Evans, Sarah and Gilgen, Anna K. and Holub, Petr and Jiang, Lifen and Alan K. Knapp and LeCain, Daniel and Liang, Junyi and Garcia-Palacios, Pablo and Pe{\~n}uelas, Josep and Pockman, William T. and M.D. Smith and Sun, Shanghua and White, Shannon R. and Yahdjian, Laura and Zhu, Kai and Luo, Yiqi} } @article {KNZ001842, title = {Asynchrony among local communities stabilises ecosystem function of metacommunities}, journal = {Ecology Letters}, year = {2017}, abstract = {

Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species-level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1\–315\%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species\’ populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

}, keywords = {LTER-KNZ, Alpha diversity, alpha variability, beta diversity, Biodiversity, CoRRE data base, patchiness, Plant communities, Primary productivity, species synchrony}, doi = {10.1111/ele.12861}, url = {http://onlinelibrary.wiley.com/doi/10.1111/ele.12861/epdf}, author = {K.R. Wilcox and Tredennick, Andrew T. and Koerner, Sally E. and Grman, Emily and Hallett, Lauren M. and M.L. Avolio and Kimberly J. La Pierre and Houseman, Gregory R. and Isbell, Forest and Johnson, David Samuel and Juha M. Alatalo and Baldwin, Andrew H. and Edward W. Bork and Elizabeth H. Boughton and W.D. Bowman and Britton, Andrea J. and Cahill, James F. and Scott. L. Collins and G. Du and Eskelinen, Anu and Gough, Laura and Jentsch, Anke and Kern, Christel and Klanderud, Kari and Alan K. Knapp and Kreyling, Juergen and Luo, Yiqi and McLaren, Jennie R. and Megonigal, Patrick and Onipchenko, Vladimir and Prev{\'e}y, Janet and Price, Jodi N. and Robinson, Clare H. and Sala, Osvaldo E. and M.D. Smith and Soudzilovskaia, Nadejda A. and Souza, Lara and Tilman, David and White, Shannon R. and Xu, Zhuwen and Yahdjian, Laura and Yu, Qiang and Zhang, Pengfei and Zhang, Yunhai}, editor = {Gurevitch, Jessica} } @article {KNZ001802, title = {Drought timing differentially affects above- and belowground productivity in a mesic grassland}, journal = {Plant Ecology}, volume = {218}, year = {2017}, pages = {317 - 328}, abstract = {

Climate models forecast an intensification of the global hydrological cycle with droughts becoming more frequent and severe, and shifting to times when they have been historically uncommon. Droughts, or prolonged periods of precipitation deficiency, are characteristic of most temperate grasslands, yet few experiments have explored how variation in the seasonal timing of drought may impact ecosystem function. We investigated the response of above- and belowground net primary production (ANPP \& BNPP) to altered drought timing in a mesic grassland in NE Kansas. Moderate drought treatments (25\% reduction from the mean growing season precipitation [GSP]) were imposed by erecting rainout shelters in late spring (LSP), early summer (ESM), and mid-summer (MSM, n = 10 plots/treatment). These treatments were compared to two controls (long-term average GSP [LTA] and ambient GSP [AMB]) and a wet treatment (+30\% of the long-term average GSP [WET]). We found that LSP drought did not significantly reduce ANPP relative to control plots while the ESM and MSM drought did despite equivalent reductions in soil moisture. In contrast, the WET treatment did not affect ANPP. Neither the WET nor the drought treatments altered BNPP as compared to the controls. Our results suggest that forecasts of ecosystem responses to climate change will be improved if both the seasonal timing of alterations in precipitation as well as differential responses of above- and belowground productivity to drought are incorporated into models.

}, keywords = {LTER-KNZ}, doi = {10.1007/s11258-016-0690-x}, url = {https://link.springer.com/article/10.1007\%2Fs11258-016-0690-x}, author = {Denton, Elsie M. and Dietrich, John D. and M.D. Smith and Alan K. Knapp} } @article {KNZ001782, title = {Photosynthetic responses of a dominant C4 grass to an experimental heat wave are mediated by soil moisture}, journal = {Oecologia}, volume = {183}, year = {2017}, pages = {303-313}, abstract = {

Extreme heat waves and drought are predicted to increase in frequency and magnitude with climate change. These extreme events often co-occur, making it difficult to separate their direct and indirect effects on important ecophysiological and carbon cycling processes such as photosynthesis. Here, we assessed the independent and interactive effects of experimental heat waves and drought on photosynthesis in Andropogon gerardii, a dominant C4 grass in a native mesic grassland. We experimentally imposed a two-week heat wave at four intensity levels under two contrasting soil moisture regimes: a well-watered control and an extreme drought. There were three main findings from this study. First, the soil moisture regimes had large effects on canopy temperature, leading to extremely high temperatures under drought and low temperatures under well-watered conditions. Second, soil moisture mediated the photosynthetic response to heat; heat reduced photosynthesis under the well-watered control, but not under the extreme drought treatment. Third, the effects of heat on photosynthesis appeared to be driven by a direct thermal effect, not indirectly through other environmental or ecophysiological variables. These results suggest that while photosynthesis in this dominant C4 grass is sensitive to heat stress, this sensitivity can be overwhelmed by extreme drought stress.

}, keywords = {LTER-KNZ, Canopy temperature, Climate extremes, Drought, Ecophysiology, tallgrass prairie}, doi = {10.1007/s00442-016-3755-6}, url = {https://link.springer.com/article/10.1007\%2Fs00442-016-3755-6}, author = {Hoover, D.L. and Alan K. Knapp and M.D. Smith} } @article {KNZ001783, title = {Pushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years}, journal = {Global Change Biology}, volume = {23}, year = {2017}, pages = {1774-1782}, abstract = {

Intensification of the global hydrological cycle, ranging from larger individual precipitation events to more extreme multiyear droughts, has the potential to cause widespread alterations in ecosystem structure and function. With evidence that the incidence of extreme precipitation years (defined statistically from historical precipitation records) is increasing, there is a clear need to identify ecosystems that are most vulnerable to these changes and understand why some ecosystems are more sensitive to extremes than others. To date, opportunistic studies of naturally occurring extreme precipitation years, combined with results from a relatively small number of experiments, have provided limited mechanistic understanding of differences in ecosystem sensitivity, suggesting that new approaches are needed. Coordinated distributed experiments (CDEs) arrayed across multiple ecosystem types and focused on water can enhance our understanding of differential ecosystem sensitivity to precipitation extremes, but there are many design challenges to overcome (e.g., cost, comparability, standardization). Here, we evaluate contemporary experimental approaches for manipulating precipitation under field conditions to inform the design of \‘Drought-Net\’, a relatively low-cost CDE that simulates extreme precipitation years. A common method for imposing both dry and wet years is to alter each ambient precipitation event. We endorse this approach for imposing extreme precipitation years because it simultaneously alters other precipitation characteristics (i.e., event size) consistent with natural precipitation patterns. However, we do not advocate applying identical treatment levels at all sites \– a common approach to standardization in CDEs. This is because precipitation variability varies \>fivefold globally resulting in a wide range of ecosystem-specific thresholds for defining extreme precipitation years. For CDEs focused on precipitation extremes, treatments should be based on each site\&$\#$39;s past climatic characteristics. This approach, though not often used by ecologists, allows ecological responses to be directly compared across disparate ecosystems and climates, facilitating process-level understanding of ecosystem sensitivity to precipitation extremes.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.13504}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.13504}, author = {Alan K. Knapp and M.L. Avolio and Beier, C. and Carroll, C.J.W. and Scott. L. Collins and Dukes, J.S. and Fraser, L.H. and Griffin-Nolan, R.J. and Hoover, D.L. and Jentsch, A. and Loik, M.E. and Phillips, R.P. and Post, A.K. and Sala, O.E. and Slette, I.J. and Yahdjian, L. and M.D. Smith} } @article {KNZ001801, title = {Reconciling inconsistencies in precipitation{\textendash} productivity relationships: implications for climate change}, journal = {New Phytologist}, volume = {214}, year = {2017}, pages = {41-47}, abstract = {Precipitation (PPT) is a primary climatic determinant of plant growth and aboveground net primary production (ANPP) over much of the globe. Thus, PPT{\textendash}ANPP relationships are important both ecologically and to land{\textendash}atmosphere models that couple terrestrial vegetation to the global carbon cycle. Empirical PPT{\textendash}ANPP relationships derived from long-term site-based data are almost always portrayed as linear, but recent evidence has accumulated that is inconsistent with an underlying linear relationship. We review, and then reconcile, these inconsistencies with a nonlinear model that incorporates observed asymmetries in PPT{\textendash}ANPP relationships. Although data are currently lacking for parameterization, this new model highlights research needs that, when met, will improve our understanding of carbon cycle dynamics, as well as forecasts of ecosystem responses to climate change.}, keywords = {LTER-KNZ}, doi = {10.1111/nph.14381}, url = {https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.14381}, author = {Alan K. Knapp and Ciais, P. and M.D. Smith} } @article {KNZ001744, title = {Altered rainfall patterns increase forb abundance and richness in native tallgrass prairie}, journal = {Scientific Reports}, year = {2016}, abstract = {

Models predict that precipitation variability will increase with climate change. We used a 15-year precipitation manipulation experiment to determine if altering the timing and amount of growing season rainfall will impact plant community structure in annually burned, native tallgrass prairie. The altered precipitation treatment maintained the same total growing season precipitation as the ambient precipitation treatment, but received a rainfall regime of fewer, larger rain events, and longer intervals between events each growing season. Although this change in precipitation regime significantly lowered mean soil water content, overall this plant community was remarkably resistant to altered precipitation with species composition relatively stable over time. However, we found significantly higher forb cover and richness and slightly lower grass cover on average with altered precipitation, but the forb responses were manifest only after a ten-year lag period. Thus, although community structure in this grassland is relatively resistant to this type of altered precipitation regime, forb abundance in native tallgrass prairie may increase in a future characterized by increased growing season precipitation variability.

}, keywords = {LTER-KNZ}, doi = {10.1038/srep20120}, url = {https://www.nature.com/articles/srep20120}, author = {Jones, Sydney K. and Scott. L. Collins and John M. Blair and M.D. Smith and Alan K. Knapp} } @article {KNZ001764, title = {Does ecosystem sensitivity to precipitation at the site-level conform to regional-scale predictions?}, journal = {Ecology}, volume = {97}, year = {2016}, pages = {561-568}, abstract = {

Central to understanding global C cycle dynamics is the functional relationship between precipitation and net primary production (NPP). At large spatial (regional) scales, the responsiveness of aboveground NPP (ANPP) to inter-annual variation in annual precipitation (AP; ANPPsensitivity) is inversely related to site-level ANPP, coinciding with turnover of plant communities along precipitation gradients. Within ecosystems experiencing chronic alterations in water availability, plant community change will also occur with unknown consequences for ANPPsensitivity. To examine the role plant community shifts may play in determining alterations in site-level ANPPsensitivity, we experimentally increased precipitation by ~35\% for two decades in a native Central US grassland. Consistent with regional models, ANPPsensitivity decreased initially as water availability and ANPP increased. However, ANPPsensitivity shifted back to ambient levels when mesic species increased in abundance in the plant community. Similarly, in grassland sites with distinct mesic and xeric plant communities and corresponding 50\% differences in ANPP, ANPPsensitivity did not differ over almost three decades. We conclude that responses in ANPPsensitivity to chronic alterations in water availability within an ecosystem may not conform to regional AP-ANPP patterns, despite expected changes in ANPP and plant communities. The result is unanticipated functional resistance to climate change at the site scale.

}, keywords = {LTER-KNZ}, doi = {10.1890/15-1437.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/15-1437.1}, author = {K.R. Wilcox and John M. Blair and M.D. Smith and Alan K. Knapp} } @article {KNZ001780, title = {The immediate and prolonged effects of climate extremes on soil respiration in a mesic grassland}, journal = {Journal of Geophysical Research: Biogeosciences}, volume = {121}, year = {2016}, pages = {1034 - 1044}, abstract = {

The predicted increase in the frequency and intensity of climate extremes is expected to impact terrestrial carbon fluxes to the atmosphere, potentially changing ecosystems from carbon sinks to sources, with positive feedbacks to climate change. As the second largest terrestrial carbon flux, soil CO2 efflux or soil respiration (Rs) is strongly influenced by soil temperature and moisture. Thus, climate extremes such as heat waves and extreme drought should have substantial impacts on Rs. We investigated the effects of such climate extremes on growing season Rs in a mesic grassland by experimentally imposing 2\ years of extreme drought combined with midsummer heat waves. After this 2\ year period, we continued to measure Rs during a recovery year. Two consecutive drought years reduced Rs by about 25\% each growing season; however, when normal rainfall returned during the recovery year, formerly droughted plots had higher rates of Rs than control plots (up to +17\%). The heat wave treatments had no effect on Rs, alone or when combined with drought, and during the growing season, soil moisture was the primary driver of Rs with little evidence for Rs temperature sensitivity. When compared to aboveground net primary production, growing season Rs was much less sensitive to drought but was more responsive postdrought. These results are consistent with the hypothesis that ecosystems become sources of CO2 during drought because carbon inputs (production) are decreased relatively more than outputs (respiration). Moreover, stimulation of Rs postdrought may lengthen the time required for net carbon exchange to return to predrought levels.

}, keywords = {LTER-KNZ}, doi = {10.1002/2015JG003256}, url = {https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JG003256}, author = {Hoover, D.L. and Alan K. Knapp and M.D. Smith} } @article {KNZ001781, title = {Rangeland responses to predicted increases in drought extremity}, journal = {Rangelands }, volume = {38}, year = {2016}, pages = {191-196}, keywords = {LTER-KNZ, Drought, grassland, savanna, shrubland, state and transition, woodland}, url = {http://dx.doi.org/10.1016/j.rala.2016.06.009}, author = {Breshears, D.D. and Alan K. Knapp and Law, D.J. and M.D. Smith and Twidwell, D. and Wonkka, C.L.} } @article {KNZ001748, title = {Shared drivers but divergent ecological responses: Insights from long-term experiments in mesic savanna grasslands}, journal = {BioScience}, volume = {66}, year = {2016}, pages = {666 - 682}, type = {Journal Articles}, abstract = {

Fire and grazing, key determinants of structure and function of savanna grasslands worldwide, have been extensively altered by humans. We used existing long-term manipulations of fire and grazing in North American and South African mesic savanna grasslands, as well as new experiments, to determine whether the impacts of fire and grazing by large herbivores differed between these systems. We found that despite a body of literature suggesting that these savanna grasslands respond uniquely to fire and grazing, their ecosystem responses (aboveground productivity) were generally similar. In contrast, plant-community responses to fire and herbivores diverged strongly between systems. The differences in plant-community responses, as well as convergence in ecosystem function, were underpinned by a common mechanism: the response of grass dominance to changing fire and grazing regimes. As a result, divergent responses of plant communities to altered fire and grazing regimes did not preclude convergence in ecosystem function.

}, keywords = {LTER-KNZ, Aboveground net primary productivity, fire, grassland, Grazing, plant community}, doi = {10.1093/biosci/biw077}, url = {https://academic.oup.com/bioscience/article/66/8/666/2464141}, author = {M.D. Smith and Alan K. Knapp and Scott. L. Collins and Burkepile, D.E. and Kirkman, K.P. and Koerner, S.E. and Thompson, D.I. and John M. Blair and Burns, C.E. and Eby, S. and Forrestel, E.J. and Fynn, R.W.S. and Govender, N. and Hagenah, N. and Hoover, D.L. and K.R. Wilcox} } @article {KNZ001763, title = {Stability of grassland soil C and N pools despite 25 years of an extreme climatic and disturbance regime}, journal = {Journal of Geophysical Research: Biogeosciences}, volume = {121}, year = {2016}, pages = {1934 - 1945}, abstract = {

Global changes are altering many important drivers of ecosystem functioning, with precipitation amount and disturbance frequency being especially important. Carbon (C) and nitrogen (N) pools are key contemporary attributes of ecosystems that can also influence future C uptake via plant growth. Thus, understanding the impacts of altered precipitation amounts (through controls of primary production inputs) and disturbance regimes (through losses of C and N in biomass) is important to project how ecosystem services will respond to future global changes. A major difficulty inherent within this task is that drivers of ecosystem function and processes often interact, resulting in novel ecosystem responses. To examine how changes in precipitation affect grassland ecosystem responses under a frequent disturbance regime (annual fire), we assessed the biogeochemical and ecological consequences of more than two decades of irrigation in an annually burned mesic grassland in the central United States. In this experiment, precipitation amount was increased by 31\% relative to ambient and 1 in 3\ years were statistically extreme relative to the long-term historical record. Despite evidence that irrigation decreased root:shoot ratios and increased rates of N cycling\—each expected to reduce soil C and N with annual burning\—we detected no changes in these biogeochemical pools. This surprising biogeochemical resistance highlights the need to explore additional mechanisms within long-term experiments concerning the consequences of global change impacts on ecosystems.

}, keywords = {LTER-KNZ}, doi = {10.1002/2016JG003370}, url = {https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JG003370}, author = {K.R. Wilcox and John M. Blair and Alan K. Knapp} } @phdthesis {KNZ001699, title = {Assessing grassland sensitivity to global change}, volume = {PhD. Dissertation}, year = {2015}, school = {Colorado State University}, type = {Ph.D. Thesis}, address = {Fort Collins, CO}, keywords = {LTER-KNZ}, url = {https://mountainscholar.org/handle/10217/167147}, author = {K.R. Wilcox} } @article {KNZ001684, title = {Characterizing differences in precipitation regimes of extreme wet and dry years: Implications for climate change experiments}, journal = {Global Change Biology}, volume = {21}, year = {2015}, pages = {2624 -2633}, abstract = {

Climate change is intensifying the hydrologic cycle and is expected to increase the frequency of extreme wet and dry years. Beyond precipitation amount, extreme wet and dry years may differ in other ways, such as the number of precipitation events, event size, and the time between events. We assessed 1614 long-term (100 year) precipitation records from around the world to identify key attributes of precipitation regimes, besides amount, that distinguish statistically extreme wet from extreme dry years. In general, in regions where mean annual precipitation (MAP) exceeded 1000 mm, precipitation amounts in extreme wet and dry years differed from average years by ~40\% and 30\%, respectively. The magnitude of these deviations increased to \>60\% for dry years and to \>150\% for wet years in arid regions (MAP\<500 mm). Extreme wet years were primarily distinguished from average and extreme dry years by the presence of multiple extreme (large) daily precipitation events (events \>99th percentile of all events); these occurred twice as often in extreme wet years compared to average years. In contrast, these large precipitation events were rare in extreme dry years. Less important for distinguishing extreme wet from dry years were mean event size and frequency, or the number of dry days between events. However, extreme dry years were distinguished from average years by an increase in the number of dry days between events. These precipitation regime attributes consistently differed between extreme wet and dry years across 12 major terrestrial ecoregions from around the world, from deserts to the tropics. Thus, we recommend that climate change experiments and model simulations incorporate these differences in key precipitation regime attributes, as well as amount into treatments. This will allow experiments to more realistically simulate extreme precipitation years and more accurately assess the ecological consequences.

}, keywords = {LTER-KNZ, rainfall patterns}, doi = {10.1111/gcb.12888}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.12888}, author = {Alan K. Knapp and D.L. Hoover and K.R. Wilcox and M.L. Avolio and Koerner, S.E. and Kimberly J. La Pierre and Loik, M.E. and Luo, Y. and Sala, O.E. and M.D. Smith} } @article {KNZ001683, title = {Contrasting above- and belowground sensitivity of three Great Plains grasslands to altered rainfall regimes}, journal = {Global Change Biology}, volume = {21}, year = {2015}, pages = {335 -344}, abstract = {

Intensification of the global hydrological cycle with atmospheric warming is expected to increase interannual variation in precipitation amount and the frequency of extreme precipitation events. Although studies in grasslands have shown sensitivity of aboveground net primary productivity (ANPP) to both precipitation amount and event size, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and NPP. We conducted a 2-year experiment in three US Great Plains grasslands \– the C4-dominated shortgrass prairie (SGP; low ANPP) and tallgrass prairie (TGP; high ANPP), and the C3-dominated northern mixed grass prairie (NMP; intermediate ANPP) \– to test three predictions: (i) both ANPP and BNPP responses to increased precipitation amount would vary inversely with mean annual precipitation (MAP) and site productivity; (ii) increased numbers of extreme rainfall events during high-rainfall years would affect high and low MAP sites differently; and (iii) responses belowground would mirror those aboveground. We increased growing season precipitation by as much as 50\% by augmenting natural rainfall via (i) many (11\–13) small or (ii) fewer (3\–5) large watering events, with the latter coinciding with naturally occurring large storms. Both ANPP and BNPP increased with water addition in the two C4 grasslands, with greater ANPP sensitivity in TGP, but greater BNPP and NPP sensitivity in SGP. ANPP and BNPP did not respond to any rainfall manipulations in the C3-dominated NMP. Consistent with previous studies, fewer larger (extreme) rainfall events increased ANPP relative to many small events in SGP, but event size had no effect in TGP. Neither system responded consistently above- and belowground to event size; consequently, total NPP was insensitive to event size. The diversity of responses observed in these three grassland types underscores the challenge of predicting responses relevant to C cycling to forecast changes in precipitation regimes even within relatively homogeneous biomes such as grasslands.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.12673}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.12673}, author = {K.R. Wilcox and Von Fischer, J.C. and Muscha, J.M. and Petersen, M.K. and Alan K. Knapp} } @article {KNZ001631, title = {Differential sensitivity to regional-scale drought in six central US grasslands}, journal = {Oecologia}, volume = {177}, year = {2015}, pages = {949 -957}, abstract = {

Terrestrial ecosystems often vary dramatically in their responses to drought, but the reasons for this are unclear. With climate change forecasts for more frequent and extensive drought in the future, a more complete understanding of the mechanisms that determine differential ecosystem sensitivity to drought is needed. In 2012, the Central US experienced the fourth largest drought in a century, with a regional-scale 40 \% reduction in growing season precipitation affecting ecosystems ranging from desert grassland to mesic tallgrass prairie. This provided an opportunity to assess ecosystem sensitivity to a drought of common magnitude in six native grasslands. We tested the prediction that drought sensitivity is inversely related to mean annual precipitation (MAP) by quantifying reductions in aboveground net primary production (ANPP). Long-term ANPP data available for each site (mean length = 16 years) were used as a baseline for calculating reductions in ANPP, and drought sensitivity was estimated as the reduction in ANPP per millimeter reduction in precipitation. Arid grasslands were the most sensitive to drought, but drought responses and sensitivity varied by more than twofold among the six grasslands, despite all sites experiencing 40 \% reductions in growing season precipitation. Although drought sensitivity generally decreased with increasing MAP as predicted, there was evidence that the identity and traits of the dominant species, as well as plant functional diversity, influenced sensitivity. A more comprehensive understanding of the mechanisms leading to differences in drought sensitivity will require multi-site manipulative experiments designed to assess both biotic and abiotic determinants of ecosystem sensitivity.

}, keywords = {LTER-KNZ, Climate change, Functional diversity, Long-term ecological research, Precipitation, Primary production}, doi = {10.1007/s00442-015-3233-6}, url = {https://link.springer.com/article/10.1007\%2Fs00442-015-3233-6}, author = {Alan K. Knapp and Carroll, C.J.W. and Denton, E.M. and Kimberly J. La Pierre and Scott. L. Collins and M.D. Smith} } @article {KNZ001689, title = {Global environmental change and the nature of aboveground net primary productivity responses: insights from long-term experiments}, journal = {Oecologia}, volume = {177}, year = {2015}, month = {Jan-04-2015}, pages = {935 - 947}, abstract = {

Many global change drivers chronically alter resource availability in terrestrial ecosystems. Such resource alterations are known to affect aboveground net primary production (ANPP) in the short term; however, it is unknown if patterns of response change through time. We examined the magnitude, direction, and pattern of ANPP responses to a wide range of global change drivers by compiling 73 datasets from long-term (\>5 years) experiments that varied by ecosystem type, length of manipulation, and the type of manipulation. Chronic resource alterations resulted in a significant change in ANPP irrespective of ecosystem type, the length of the experiment, and the resource manipulated. However, the pattern of ecosystem response over time varied with ecosystem type and manipulation length. Continuous directional responses were the most common pattern observed in herbaceous-dominated ecosystems. Continuous directional responses also were frequently observed in longer-term experiments (\>11 years) and were, in some cases, accompanied by large shifts in community composition. In contrast, stepped responses were common in forests and other ecosystems (salt marshes and dry valleys) and with nutrient manipulations. Our results suggest that the response of ANPP to chronic resource manipulations can be quite variable; however, responses persist once they occur, as few transient responses were observed. Shifts in plant community composition over time could be important determinants of patterns of terrestrial ecosystem sensitivity, but comparative, long-term studies are required to understand how and why ecosystems differ in their sensitivity to chronic resource alterations.

}, keywords = {LTER-KNZ, Aboveground productivity, Hierarchical response framework (HRF), Long-Term Ecological Research (LTER) Network, Nutrient addition, Precipitation manipulation}, issn = {0029-8549}, doi = {10.1007/s00442-015-3230-9}, url = {https://link.springer.com/article/10.1007\%2Fs00442-015-3230-9}, author = {M.D. Smith and Kimberly J. La Pierre and Scott. L. Collins and Alan K. Knapp and Gross, K.L. and Barrett, J.E. and Frey, S.D. and Gough, L. and Miller, R.J. and Morris, J.T. and Rustad, L.E. and Yarie, J.} } @article {KNZ001691, title = {Stoichiometric homeostasis predicts plant species dominance, temporal stability and responses to global change}, journal = {Ecology}, volume = {96}, year = {2015}, month = {09/2015}, pages = {2335}, chapter = {2328}, abstract = {

Why some species are consistently more abundant than others, and predicting how species will respond to global change, are fundamental questions in ecology. Long-term observations indicate that plant species with high stoichiometric homeostasis for nitrogen (HN), i.e., the ability to decouple foliar N levels from variation in soil N availability, were more common and stable through time than low HN species in a central US grassland. However, with 9-yrs of nitrogen addition, species with high HN decreased in abundance, while those with low HN increased in abundance. In contrast, in climate change experiments simulating a range of forecast hydrologic changes - extreme drought (2-yrs), increased rainfall variability (14-yrs), and chronic increases in rainfall (21-yrs) - plant species with the highest HN were least responsive to changes in soil water availability. These results suggest that HN may be predictive of plant species success and stability, and how plant species and ecosystems will respond to global-change driven alterations in resource availability.

}, keywords = {LTER-KNZ}, doi = {10.1890/14-1897.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/14-1897.1}, author = {Yu, Q. and K.R. Wilcox and Kimberly J. La Pierre and Alan K. Knapp and Han, X. and M.D. Smith} } @article {KNZ001633, title = {Differential effects of extreme drought on production and respiration: Synthesis and modeling analysis}, journal = {Biogeosciences}, volume = {11}, year = {2014}, pages = {621 -633}, abstract = {

Extremes in climate may severely impact ecosystem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis of the effects of extreme drought on two key ecosystem processes, production and respiration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as proportional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differential sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was designed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consistently showed that net primary productivity (NPP) was reduced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67\% reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was directly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content \– a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further reductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by reducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magnitude, occur on different timescales, and are affected by different mechanisms under extreme, prolonged drought.

}, keywords = {LTER-KNZ}, doi = {10.5194/bg-11-621-2014}, url = {https://www.biogeosciences.net/11/621/2014/}, author = {Shi, Z. and Thomey, M.L. and Mowll, M. and Litvak, M.E. and N. Brunsell and Scott. L. Collins and Pockman, W.T. and M.D. Smith and Alan K. Knapp and Luo, Y.} } @article {KNZ001553, title = {Geographic variation in growth and phenology of two dominant Central US grasses: Consequences for climate change}, journal = {Journal of Plant Ecology}, volume = {7}, year = {2014}, pages = {211 -221}, abstract = {

Aims The rate of climate change may exceed many plant species\’ migration rates, particularly for long-lived perennial species that dominate most ecosystems. If bioclimatic envelopes shift more rapidly than dominant species can migrate, individuals located peripheral to biomes or in adjacent biomes may become a significant source of traits for future dominant populations (DPs). Thus, traits of individuals from peripheral populations (PPs) may affect future ecosystem functioning more than those of today\’s DPs. Methods We assessed key traits of individuals collected from populations that currently dominate two central US grasslands, the shortgrass steppe (Bouteloua gracilis) and the tallgrass prairie (Andropogon gerardii). We compared these to individuals from PPs in a reciprocal-transplant common garden experiment with gardens at the Shortgrass Steppe Long Term Ecological Research site in Colorado and the Konza Prairie Biological Station Long Term Ecological Research site in Kansas. DPs and PPs were subjected to high and reduced water availability in common gardens located in each biome. Traits measured included the following: individual plant biomass, reproductive allocation, specific leaf area (SLA) and plant\–water relations. We focused on the climate-change relevant comparisons of traits from PPs versus DPs expressed under the climate of DPs. Important Findings PPs of B. gracilis differed from DPs primarily in phenological traits. Under a semiarid shortgrass steppe climate, PPs initiated flowering later in the season, produced fewer reproductive tillers and were more sensitive to water stress. Biomass differences between populations were minimal. For A. gerardii, biomass in PPs was 50\% lower than in DPs under the mesic tallgrass prairie climate and reproductive tillers were considerably smaller, despite higher SLA in PPs. Biomass of PPs was less sensitive to water stress, however. From these results, we conclude that key traits of PPs differed from DPs in both grassland types, but potential effects on reproductive phenology were greater for the bioclimatic shift in which a mesic biome becomes arid, whereas aboveground productivity may be affected more when a semiarid biome becomes more mesic.

}, keywords = {LTER-KNZ, grassland, phenology, productivity, traits, water stress}, doi = {10.1093/jpe/rtt036}, url = {https://academic.oup.com/jpe/article/7/3/211/905746}, author = {Giuliani, A.L. and Kelly, E.F. and Alan K. Knapp} } @article {KNZ001588, title = {Loss of a large grazer impacts savanna grassland plant communities similarly in North America and South Africa}, journal = {Oecologia}, volume = {175}, year = {2014}, pages = {293 -303}, abstract = {

Large herbivore grazing is a widespread disturbance in mesic savanna grasslands which increases herbaceous plant community richness and diversity. However, humans are modifying the impacts of grazing on these ecosystems by removing grazers. A more general understanding of how grazer loss will impact these ecosystems is hampered by differences in the diversity of large herbivore assemblages among savanna grasslands, which can affect the way that grazing influences plant communities. To avoid this we used two unique enclosures each containing a single, functionally similar large herbivore species. Specifically, we studied a bison (Bos bison) enclosure at Konza Prairie Biological Station, USA and an African buffalo (Syncerus caffer) enclosure in Kruger National Park, South Africa. Within these enclosures we erected exclosures in annually burned and unburned sites to determine how grazer loss would impact herbaceous plant communities, while controlling for potential fire-grazing interactions. At both sites, removal of the only grazer decreased grass and forb richness, evenness and diversity, over time. However, in Kruger these changes only occurred with burning. At both sites, changes in plant communities were driven by increased dominance with herbivore exclusion. At Konza, this was caused by increased abundance of one grass species, Andropogon gerardii, while at Kruger, three grasses, Themeda triandra, Panicum coloratum, and Digitaria eriantha increased in abundance.

}, keywords = {LTER-KNZ, disturbance, fire, Grazing, Plant community richness, Species diversity}, doi = {10.1007/s00442-014-2895-9}, url = {https://link.springer.com/article/10.1007\%2Fs00442-014-2895-9}, author = {Eby, S. and Burkepile, D.E. and Fynn, R.W.S. and Burns, C.E. and Govender, N. and Hagenah, N. and Koerner, S.E. and Matchett, K.J. and Thompson, D.I. and K.R. Wilcox and Scott. L. Collins and Kirkman, K.P. and Alan K. Knapp and M.D. Smith} } @article {KNZ001590, title = {Plant community response to loss of large herbivores differs between North American and South African savanna grasslands}, journal = {Ecology}, volume = {95}, year = {2014}, pages = {808 -816}, abstract = {

Herbivory and fire shape plant community structure in grass-dominated ecosystems, but these disturbance regimes are being altered around the world. To assess the consequences of such alterations, we excluded large herbivores for seven years from mesic savanna grasslands sites burned at different frequencies in North America (Konza Prairie Biological Station, Kansas, USA) and South Africa (Kruger National Park). We hypothesized that the removal of a single grass-feeding herbivore from Konza would decrease plant community richness and shift community composition due to increased dominance by grasses. Similarly, we expected grass dominance to increase at Kruger when removing large herbivores, but because large herbivores are more diverse, targeting both grasses and forbs, at this study site, the changes due to herbivore removal would be muted. After seven years of large-herbivore exclusion, richness strongly decreased and community composition changed at Konza, whereas little change was evident at Kruger. We found that this divergence in response was largely due to differences in the traits and numbers of dominant grasses between the study sites rather than the predicted differences in herbivore assemblages. Thus, the diversity of large herbivores lost may be less important in determining plant community dynamics than the functional traits of the grasses that dominate mesic, disturbance-maintained savanna grasslands.

}, keywords = {LTER-KNZ}, doi = {10.1890/13-1828.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/13-1828.1}, author = {Koerner, S.E. and Burkepile, D.E. and Fynn, R.W.S. and Burns, C.E. and Eby, S. and Govender, N. and Hagenah, N. and Matchett, K.J. and Thompson, D.I. and K.R. Wilcox and Scott. L. Collins and Kirkman, K.P. and Alan K. Knapp and M.D. Smith} } @article {KNZ001532, title = {Rainfall variability has minimal effects on grassland recovery from repeated grazing}, journal = {Journal of Vegetation Science}, volume = {25}, year = {2014}, pages = {36 -44}, abstract = {

Question Mesic grasslands experience a complex disturbance regime including frequent fire, grazing by large ungulates and strong inter-annual climate variability. As a result of climate change, growing season precipitation regimes are predicted to become more variable, with larger event sizes and longer dry periods resulting in more temporally dynamic soil moisture regimes. Increased climate variability is likely to interact with other disturbances, such as grazing, in grassland ecosystems. We investigated the individual and combined effects of increased rainfall variability and grazing on plant community composition, structure and function in an annually burned, native tallgrass prairie. Our overarching question was: are grazing impacts modified under a more variable precipitation regime? Location Konza Prairie, Kansas, USA. Methods Plots were established within a long-term rainfall manipulation experiment in which larger, but less frequent, rain events were imposed during the growing season without altering the total rain amount. We then simulated intense grazing pressure during one growing season by repeatedly clipping all graminoids to 5 cm and monitored recovery over 3 yr. Results Neither grazing nor rainfall treatments affected species richness; however, grazing decreased total and grass above-ground net primary production (ANPP) and increased forb ANPP relative to ungrazed plots. Grass stem density recovered from intense grazing under ambient rainfall but did not fully recover, even after 2 yr in the altered rainfall treatment. Conclusions We found that increased rainfall variability had little effect on tallgrass prairie structure and function, while grazing had large effects. Grazing and increased rainfall variability interacted to suppress grass stem density and delayed recovery relative to controls. Although stem density was reduced, individual stem size increased, resulting in no net change in ANPP. This suggests that ANPP in grazed and ungrazed North American tallgrass prairie may be relatively resilient under more temporally variable precipitation regimes.

}, keywords = {LTER-KNZ, Annual net primary production, Climate change, Clipping, disturbance, diversity, Grass{\textendash}forb interaction, Konza, Precipitation, Stem density}, doi = {10.1111/jvs.12065}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12065}, author = {Koerner, S.E. and Scott. L. Collins and John M. Blair and Alan K. Knapp and M.D. Smith} } @article {KNZ001613, title = {Resistance and resilience of a grassland ecosystem to climate extremes}, journal = {Ecology}, volume = {95}, year = {2014}, pages = {2646 -2656}, abstract = {

Climate change forecasts of more frequent climate extremes suggest that such events will become increasingly important drivers of future ecosystem dynamics and function. Because the rarity and unpredictability of naturally occurring climate extremes limits assessment of their ecological impacts, we experimentally imposed extreme drought and a mid-summer heat wave over two years in a central U.S. grassland. While the ecosystem was resistant to heat waves, it was not resistant to extreme drought, which reduced aboveground net primary productivity (ANPP) below the lowest level measured in this grassland in almost 30 years. This extreme reduction in ecosystem function was a consequence of reduced productivity of both C4 grasses and C3 forbs. However, the dominant forb was negatively impacted by the drought more than the dominant grass, and this led to a reordering of species abundances within the plant community. Although this change in community composition persisted post-drought, ANPP recovered completely the year after drought due to rapid demographic responses by the dominant grass, compensating for loss of the dominant forb. Overall, these results show that an extreme reduction in ecosystem function attributable to climate extremes (e.g., low resistance) does not preclude rapid ecosystem recovery. Given that dominance by a few species is characteristic of most ecosystems, knowledge of the traits of these species and their responses to climate extremes will be key for predicting future ecosystem dynamics and function.

}, keywords = {LTER-KNZ}, doi = {10.1890/13-2186.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/13-2186.1}, author = {D.L. Hoover and Alan K. Knapp and M.D. Smith} } @article {KNZ001587, title = {Responses to fire differ between South African and North American grassland communities}, journal = {Journal of Vegetation Science}, volume = {25}, year = {2014}, pages = {793 -804}, abstract = {

Question Does fire frequency affect mesic grassland plant community structure and composition similarly in North America and South Africa? Location Konza Prairie Biological Station (KNZ), Kansas, USA, and Ukulinga Research Farm (URF), KwaZulu-Natal, South Africa. Methods Plant community structure and composition were compared among annually burned, unburned and intermediate treatments within two long-term fire frequency manipulation experiments in native grasslands in North America and South Africa using comparable methodology over a 5-yr period. Because fire may reduce soil nitrogen (N) availability and thus affect plant community structure, N additions were superimposed on the fire treatments as a means of assessing direct vs indirect mechanisms driving responses to fire. Results The total number of species was higher at URF (183) than at KNZ (57). Overall divergence in plant community response to fire frequency occurred despite similar responses to nutrient additions. At KNZ, more frequent fire resulted in dominance by a few, tall, deep-rooted rhizomatous grasses (e.g. Andropogon gerardii). On unburned sites, shorter, more shade-tolerant species such as Poa pratensis increased in abundance, although A. gerardii remained dominant. Species richness increased with decreasing fire frequency at KNZ. At URF, frequent fire resulted in short, diverse grassland weakly dominated by a range of grass species, including Themeda triandra, Tristachya leucothrix and Hyparrhenia hirta. Decreasing fire frequency reduced species richness and resulted in dominance by a few, relatively tall caespitose grasses such as Aristida junciformis. There was a complete turnover of dominant species between annually burned and unburned treatments at URF, while at KNZ A. gerardii and Sorghastrum nutans occurred across the range of treatments. N addition reduced species richness in both sites. Conclusions Different responses to fire frequency between KNZ and URF are likely linked to the dominant species and their characteristic traits, including height and method of clonal reproduction, with the rhizomatous growth form of A. gerardii dominating the North American grassland. South Africa does not have an equivalent grass species; instead, a range of tufted, non-rhizomatous species dominate across the fire frequency treatments at URF. Reductions in soil N due to frequent fire did not appear to be a common mechanism driving responses in community composition in these two grasslands.

}, keywords = {LTER-KNZ, Community ecology, Divergence, Fire frequency, Konza Prairie Biological Station, Mesic grassland, nitrogen, Nutrient addition, Richness, tallgrass prairie, Ukulinga Research Farm}, doi = {10.1111/jvs.12130}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12130}, author = {Kirkman, K. and Scott. L. Collins and M.D. Smith and Alan K. Knapp and Burkepile, D.E. and Burns, C.E. and Fynn, R.W.S. and Hagenah, N. and Koerner, S.E. and Matchett, K.J. and Thompson, D.I. and K.R. Wilcox and Wragg, P.D.} } @phdthesis {KNZ001646, title = {The timing of growing season drought and its effects on above- and bekowground production in a mesic grassland}, volume = {PhD Dissertation}, year = {2014}, school = {Colorado State University}, type = {Ph.D. Thesis}, address = {Fort Collins, CO}, keywords = {LTER-KNZ}, url = {https://sites.biology.colostate.edu/knapplab/People/Profiles/technicians/ElsieDentonpub3.pdf}, author = {Denton, E.} } @article {KNZ001538, title = {Coordinated distributed experiments: an emerging tool for testing global hypotheses in ecology and environmental science}, journal = {Frontiers in Ecology and the Environment}, volume = {11}, year = {2013}, pages = {147 -155}, abstract = {

There is a growing realization among scientists and policy makers that an increased understanding of today\&$\#$39;s environmental issues requires international collaboration and data synthesis. Meta-analyses have served this role in ecology for more than a decade, but the different experimental methodologies researchers use can limit the strength of the meta-analytic approach. Considering the global nature of many environmental issues, a new collaborative approach, which we call coordinated distributed experiments (CDEs), is needed that will control for both spatial and temporal scale, and that encompasses large geographic ranges. Ecological CDEs, involving standardized, controlled protocols, have the potential to advance our understanding of general principles in ecology and environmental science.

}, keywords = {LTER-KNZ}, doi = {10.1890/110279}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/110279}, author = {Fraser, L.H. and Henry, H.A. and Carlyle, C.N. and White, S.R. and Beierkuhnlein, C. and Cahill, J.F. and Casper, B.B. and Cleland, E.E. and Scott. L. Collins and Dukes, J.S. and Alan K. Knapp and Lind, E. and Long, R. and Luo, Y. and P.B. Reich and M.D. Smith and Sternberg, M. and Turkington, R.} } @phdthesis {KNZ001647, title = {Ecological responses to climate extremes in a mesic grassland}, volume = {PhD. Dissertation}, year = {2013}, school = {Colorado State University}, type = {Ph.D. Thesis}, address = {Fort Collins, CO}, abstract = {

Climate change threatens ecosystems through altered climate means and by increasing the frequency and intensity of extreme climatic events. Such events may have greater impacts on ecosystems than shifting means alone because they can push organisms beyond critical thresholds. Thus, there is an urgent need to assess the response of ecosystems to climate extremes as well as elucidate the mechanisms underlying the observed responses. My dissertation examined the ecological impacts of two years of experimentally imposed climate extremes (heat waves and drought) followed by a recovery year, on a mesic tallgrass prairie grassland ecosystem. The broad objectives of this research were (1) to assess the resistance and resilience of this ecosystem to the individual and combined effects of heat waves and drought, and (2) to identify the ecological mechanisms driving the responses and (3) to evaluate the sensitivities of key carbon cycling process to heat waves and drought. I measured a range of biotic responses to these treatments including: ecophysiology, community dynamics, primary production, and soil respiration in order to gain a comprehensive understanding how this ecosystem responds to such extremes. During the first year of the experiment, I examined the ecophysiological and productivity responses of the dominant C4 grasses to a growing season-long drought and a midsummer, two-week heat wave. Although differential sensitivities were apparent, the independent effects of drought dominated the ecological responses for both species, with only minor direct effects of heat were observed. However, the heat wave treatments had indirect effects via enhanced soil drying, making it difficult to separate the effects of the heat wave and precipitation treatments on biotic responses. Therefore in the second year of the experiment, I controlled for heat-induced water losses during the heat wave and examined the independent effects of heat on net photosynthesis in both grass species under contrasting soil moisture regimes. Under low soil moisture, heat had no effect on net photosynthesis, while increasing temperatures moderately reduced photosynthesis under high soil moisture. Next I examined the resistance and resilience in ecosystem function (aboveground primary production) of this tallgrass prairie to the two years of extreme treatments and for one subsequent recovery year. I observed high resistance to heat but not drought, as aboveground production dropped below historic levels during the second year of the drought. Despite this extreme ecological response, productivity fully recovered in just one year post-drought due to rapid demographic compensation by the dominant grass offsetting the loss of the dominant forb. Finally, I examined the response of soil respiration to heat and drought across the three years of the experiment. As with aboveground net primary production, soil respiration was more sensitive to drought than heat, but it was less sensitive overall to drought than production. There are three main conclusions from my dissertation research. First, this tallgrass prairie ecosystem has low resistance but high resilience to extreme short-term drought, which may be an important characteristic for long-term stability in ecosystems with histories of drought. Secondly, the two most abundant species governed both community and ecosystem-level dynamics across this three-year experiment, providing evidence for the central role of dominant species during these short-term events. Finally, my results suggest that three key carbon cycling processes in this mesic grassland - photosynthesis, plant productivity and soil respiration - are all significantly more sensitive to the independent effects of an extreme drought than heat waves and there were little to no combined effects of heat waves and drought. Overall, these results suggest that in a future with more frequent and extreme heat waves and drought, this mesic grassland will be most vulnerable to water stress, either directly through precipitation deficits or indirectly through warming-induced drying, while the direct ecological effects of midsummer heat waves will be minor.

}, keywords = {LTER-KNZ}, url = {http://hdl.handle.net/10217/82474}, author = {Hoover, D.L.} } @article {KNZ001514, title = {Community stability does not preclude ecosystem sensitivity to chronic resource alteration}, journal = {Functional Ecology}, volume = {26}, year = {2012}, pages = {1231 -1233}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2435.2012.02053.x}, url = {https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2435.2012.02053.x}, author = {Alan K. Knapp and J. M. Briggs and M.D. Smith} } @booklet {KNZ001539, title = {Long-term trends in ecological systems: a basis for understanding responses to global change. USDA Agriculture Research Service Publication, Technical Bulletin 1931. Washington, D.C}, year = {2012}, abstract = {

Peters, D.P.C., C.M. Laney, A.E. Lugo, et al. 2013. Long-Term Trends in Ecological Systems: A Basis for Understanding Responses to Global Change. U.S. Department of Agriculture, Technical Bulletin Number 1931. The EcoTrends Editorial Committee sorted through vast amounts of historical and ongoing data from 50 ecological sites in the continental United States including Alaska, several islands, and Antarctica to present in a logical format the variables commonly collected. This report presents a subset of data and variables from these sites and illustrates through detailed examples the value of comparing longterm data from different ecosystem types. This work provides cross-site comparisons of ecological responses to global change drivers, as well as longterm trends in global change drivers and responses at site and continental scales. Site descriptions and detailed data also are provided in the appendix section.

}, keywords = {LTER-KNZ, atmospheric chemistry, Climate change, cross-site comparisons, disturbance, ecological response, ecology, ecosystem, EcoTrends, experimental forests, global change, human demography, human population growth, Long Term Ecological Research (LTER), long-term datasets, Precipitation, rangeland, rangeland research stations, surface water chemistry}, author = {Peters, D.P.C. and Laney, C.M. and Lugo, A.E. and Scott. L. Collins and Driscoll, C.T. and Groffman, P.M. and Grove, J.M. and Alan K. Knapp and Kratz, T.K. and Ohman, M.D. and Waide, R.B. and Yao, J.} } @article {KNZ001496, title = {Past, present, and future roles of long-term experiments in the LTER Network}, journal = {Bioscience}, volume = {62}, year = {2012}, pages = {377 -389}, abstract = {

The US National Science Foundation\–funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after many years of study; (b) influenced management and policy decisions; and (c) evolved into research platforms supporting studies and involving investigators who were not part of the original design. Furthermore, this suite of long-term experiments addresses, at the site level, all of the US National Research Council\&$\#$39;s Grand Challenges in Environmental Sciences. Despite these contributions, we argue that the scale and scope of global environmental change requires a more-coordinated multisite approach to long-term experiments. Ideally, such an approach would include a network of spatially extensive multifactor experiments, designed in collaboration with ecological modelers that would build on and extend the unique context provided by the LTER Network.

}, keywords = {LTER-KNZ, Climate change, global change, long-term research, LTER Network, multifactor experiments}, doi = {10.1525/bio.2012.62.4.9}, url = {https://academic.oup.com/bioscience/article/62/4/377/243762}, author = {Alan K. Knapp and M.D. Smith and Hobbie, S.E. and Scott. L. Collins and Fahey, T.J. and Hansen, G.J.A. and Landis, D.A. and Kimberly J. La Pierre and Melillo, J.M. and Seastedt, T.R. and Shaver, G.R. and Webster, J.R.} } @article {KNZ001524, title = {A test of two mechanisms proposed to optimize grassland aboveground primary productivity in response to grazing}, journal = {Journal of Plant Ecology}, volume = {5}, year = {2012}, pages = {357 -365}, abstract = {

Aims Mesic grasslands have a long evolutionary history of grazing by large herbivores and as a consequence, grassland species have numerous adaptations allowing them to respond favourably to grazing. Although empirical evidence has been equivocal, theory predicts that such adaptations combined with alterations in resources can lead to grazing-induced overcompensation in aboveground net primary production (ANPP; grazed ANPP \> ungrazed ANPP) under certain conditions. We tested two specific predictions from theory. First, overcompensation is more likely to occur in annually burned grasslands because limiting nutrients that would be lost with frequent fires are recycled through grazers and stimulate ANPP. Second, overcompensation of biomass lost to grazers is more likely to occur in unburned sites where grazing has the greatest effect on increasing light availability through alterations in canopy structure. Methods We tested these nutrient versus light-based predictions in grazed grasslands that had been annually burned or protected from fire for \>20 years. We assessed responses in ANPP to grazing by large ungulates using both permanent and moveable grazing exclosures (252 exclosures from which biomass was harvested from 3192 quadrats) in a 2-year study. Study sites were located at the Konza Prairie Biological Station (KPBS) in North America and at Kruger National Park (KNP) in South Africa. At KPBS, sites were grazed by North American bison whereas in KNP sites were grazed either by a diverse suite of herbivores (e.g. blue wildebeest, Burchell\’s zebra, African buffalo) or by a single large ungulate (African buffalo). Important Findings We found no evidence for overcompensation in either burned or unburned sites, regardless of grazer type. Thus, there was no support for either mechanism leading to overcompensation. Instead, complete compensation of total biomass lost to grazers was the most common response characterizing grazing\–ANPP relationships with, in some cases, undercompensation of grass ANPP being offset by increased ANPP of forbs likely due to competitive release. The capability of these very different grass-dominated systems to maintain ANPP while being grazed has important implications for energy flow, ecosystem function and the trophic dynamics of grasslands.

}, keywords = {LTER-KNZ, aboveground net primary production, fire, forbs, herbivores, savanna}, doi = {10.1093/jpe/rts020}, url = {https://academic.oup.com/jpe/article/5/4/357/908695}, author = {Alan K. Knapp and D.L. Hoover and John M. Blair and Buis, G. and Burkepile, D.E. and Chamberlain, A.J. and Scott. L. Collins and Fynn, R.W.S. and Kirkman, K.P. and M.D. Smith and Blake, D. and Govender , N. and O{\textquoteright}Neal, P. and Schreck, T. and Zinn, A.} } @article {KNZ001454, title = {An integrated conceptual framework for long-term social-ecological research}, journal = {Frontiers in Ecology and the Environment}, volume = {9}, year = {2011}, pages = {351 -357}, abstract = {

The global reach of human activities affects all natural ecosystems, so that the environment is best viewed as a social\–ecological system. Consequently, a more integrative approach to environmental science, one that bridges the biophysical and social domains, is sorely needed. Although models and frameworks for social\–ecological systems exist, few are explicitly designed to guide a long-term interdisciplinary research program. Here, we present an iterative framework, \“Press\–Pulse Dynamics\” (PPD), that integrates the biophysical and social sciences through an understanding of how human behaviors affect \“press\” and \“pulse\” dynamics and ecosystem processes. Such dynamics and processes, in turn, influence ecosystem services \–thereby altering human behaviors and initiating feedbacks that impact the original dynamics and processes. We believe that research guided by the PPD framework will lead to a more thorough understanding of social\–ecological systems and generate the knowledge needed to address pervasive environmental problems.

}, keywords = {LTER-KNZ}, doi = {10.1890/100068}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/100068}, author = {Scott. L. Collins and Carpenter, S.R. and Swinton, S.M. and Orenstein, D.E. and Childers, D.L. and Gragson, T.L. and Grimm, N.B. and Grove, J.M. and Harlan, S.L. and Kaye, J.P. and Alan K. Knapp and Kofinas, G.P. and Magnuson, J.J. and W.H. McDowell and Melack, J.M. and Ogden, L.A. and Robertson, G.R. and M.D. Smith and Whitmer, A.C.} } @article {KNZ001449, title = {Relative effects of precipitation variability and warming on tallgrass prairie ecosystem function}, journal = {Biogeosciences}, volume = {8}, year = {2011}, pages = {3053 -3068}, abstract = {

Precipitation and temperature drive many aspects of terrestrial ecosystem function. Climate change scenarios predict increasing precipitation variability and temperature, and long term experiments are required to evaluate the ecosystem consequences of interannual climate variation, increased growing season (intra-annual) rainfall variability, and warming. We present results from an experiment applying increased growing season rainfall variability and year round warming in native tallgrass prairie. During ten years of study, total growing season rainfall varied 2-fold, and we found ~50\–200\% interannual variability in plant growth and aboveground net primary productivity (ANPP), leaf carbon assimilation (ACO2), and soil CO2 efflux (JCO2) despite only ~40\% variation in mean volumetric soil water content (0\–15 cm, Θ15). Interannual variation in soil moisture was thus amplified in most measures of ecosystem response. Differences between years in Θ15 explained the greatest portion (14\–52\%) of the variation in these processes. Experimentally increased intra-annual season rainfall variability doubled the amplitude of intra-annual soil moisture variation and reduced Θ15 by 15\%, causing most ecosystem processes to decrease 8\–40\% in some or all years with increased rainfall variability compared to ambient rainfall timing, suggesting reduced ecosystem rainfall use efficiency. Warming treatments increased soil temperature at 5 cm depth, particularly during spring, fall, and winter. Warming advanced canopy green up in spring, increased winter JCO2, and reduced summer JCO2 and forb ANPP, suggesting that the effects of warming differed in cooler versus warmer parts of the year. We conclude that (1) major ecosystem processes in this grassland may be substantially altered by predicted changes in interannual climate variability, intra-annual rainfall variability, and temperature, (2) interannual climate variation was a larger source of variation in ecosystem function than intra-annual rainfall variability and warming, and (3) effects of increased growing season rainfall variability and warming were small, but ecologically important. The relative effects of these climate drivers are likely to vary for different ecosystem processes and in wetter or drier ecosystems.

}, keywords = {LTER-KNZ}, doi = {10.5194/bg-8-3053-2011}, url = {https://www.biogeosciences.net/8/3053/2011/}, author = {Fay, P.A. and John M. Blair and M.D. Smith and Jesse B. Nippert and Carlisle, J.D. and Alan K. Knapp} } @article {KNZ001438, title = {Woody plant proliferation in North American drylands: A synthesis of impacts on ecosystem carbon balance}, journal = {Journal of Geophysical Research}, volume = {116}, year = {2011}, abstract = {

[1] Changes in the magnitude and direction of ecosystem carbon (C) balance accompanying woody plant encroachment are among the largest contributors to the uncertainty in the North American C budget. In this synthesis we identify the important species contributing to woody encroachment, summarize our current knowledge of aboveground and belowground C storage change with woody encroachment, and evaluate the range of human and natural disturbance factors that alter the course of C gains and losses within ecosystems experiencing woody encroachment. Available data indicate that relative to the historic vegetation, aboveground net primary production (ANPP) decreases with woody plant encroachment in arid regions (mean annual precipitation (MAP) \< 336 mm), but increases in semiarid and subhumid regions (on the order of 0.7 g C m\−2 yr\−1 per mm of MAP over 336 mm). Soil organic carbon response to woody plant encroachment ranged from losses of 6200 g C m\−2 to gains of 2700 g C m\−2 with an average accumulation of 385 g C m\−2 across all studies and did not appear to be closely coupled to ANPP. Taken together, in the absence of disturbance, woody encroachment appears to result in a net ecosystem C gain across most species and ecoregions. However, disturbance associated with wildfire, land management practices, and drought may quickly and significantly offset these gains and should be explicitly factored into regional-scale C balance estimates. Our findings may be used to better constrain future estimates of woody plant encroachment influences on the North American C budget.

}, keywords = {LTER-KNZ}, doi = {10.1029/2010JG001506}, url = {https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JG001506}, author = {Barger, N.N. and S.R. Archer and Campbell, J.L. and C. Huang and Morton, J.A. and Alan K. Knapp} } @article {KNZ001265, title = {Fire and grazing impacts on silica production and storage in grass dominated ecosystems}, journal = {Biogeochemistry}, volume = {97}, year = {2010}, pages = {263 -278}, abstract = {

Grassland ecosystems are an important terrestrial component of the global biogeochemical silicon cycle. Although the structure and ecological functioning of grasslands are strongly influenced by fire and grazing, the role of these key ecological drivers in the production and storage of silicon represents a significant knowledge gap, particularly since they are being altered worldwide by human activities. We evaluated the effects of fire and grazing on the range and variability of plant derived biogenic silica stored in plant biomass and soils by sampling plants and soils from long-term experimental plots with known fire and grazing histories. Overall, plants and soils from grazed sites in the South African ecosystems had up to 76 and 54\% greater biogenic silica totals (kg ha\−1), respectively, than grazed North American sites. In North American soils, the combination of grazing and annual fire resulted in the greatest abundance of biogenic silica, whereas South African soils had the highest biogenic silica content where grazed regardless of burn frequency. These results as well as those that show greater Si concentrations in grazed South African plants indicate that South African plants and soils responded somewhat differently to fire and grazing with respect to silicon cycling, which may be linked to differences in the evolutionary history and in the grazer diversity and grazing intensity of these ecosystems. We conclude that although fire and grazing (as interactive and/or independent factors) do not affect the concentration of Si taken up by plants, they do promote increased silicon storage in aboveground biomass and soil as a result of directly affecting other site factors such as aboveground net primary productivity. Therefore, as management practices, fire and grazing have important implications for assessing global change impacts on the terrestrial biogeochemical cycling of silicon.

}, keywords = {LTER-KNZ, Biogenic silica, North American grasslands, Soil South African savannas, Terrestrial plants}, doi = {10.1007/s10533-009-9371-3}, url = {https://link.springer.com/article/10.1007\%2Fs10533-009-9371-3}, author = {Melzer, S.E. and Alan K. Knapp and Fynn, R.W.S. and Kirkman, K.P. and M.D. Smith and John M. Blair and Kelly, E.F.} } @article {KNZ001293, title = {Variation in gene expression of Andropogon gerardii in response to altered environmental conditions associated with climate change}, journal = {Journal of Ecology}, volume = {98}, year = {2010}, pages = {374 -383}, abstract = {

1.\ If we are to understand the mechanisms underlying species responses to climate change in natural systems, studies are needed that focus on responses of non-model species under field conditions. We measured transcriptional profiles of individuals of Andropogon gerardii, a C4 grass native to North American grasslands, in a field experiment in which both temperature and precipitation were manipulated to simulate key aspects of forecasted climate change. 2.\ By using microarrays developed for a closely related model species, Zea mays, we were able to compare the relative influence of warming versus altered soil moisture availability on expression levels of over 7000 genes, identify responsive functional groups of genes and correlate changes in gene transcription with physiological responses. 3.\ We observed more statistically significant shifts in transcription levels of genes in response to thermal stress than in response to water stress. We also identified candidate genes that demonstrated transcription levels closely associated with physiological variables, in particular chlorophyll fluorescence. 4.Synthesis. These results suggest that an ecologically important species responds differently to different environmental aspects of forecast climate change. These translational changes have the potential to influence phenotypic characters and ultimately adaptive responses.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2745.2009.01618.x}, url = {https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2745.2009.01618.x}, author = {Travers, S.E. and Tang, Z. and Caragea, D. and Garrett, K.A. and Hulbert, S.H. and Leach, J.E. and Bai, J. and Saleh, A. and Alan K. Knapp and Fay, P.A. and Jesse B. Nippert and Schnable, P.S. and M.D. Smith} } @inbook {KNZ001204, title = {Approaches to predicting broad-scale regime shifts using changing pattern-process relationships across scales}, booktitle = {Real World Ecology: Design and Analyses of Large-Scale and Long-Term Research}, year = {2009}, publisher = {Springer-Verlag, NY}, organization = {Springer-Verlag, NY}, abstract = {Understanding and predicting the occurrence of alternative ecosystem states (i.e., regime shifts) at broad scales is a pressing challenge for ecologists given the scope and nature of global change. In many cases, regime shifts at broad-scales are affected by pattern-process relationships across a range of finer scales. However, experimental and analytical methods to examine state changes, such as from perennial grasslands to woodlands, have not been fully developed. We first define and describe the expansion of woody plants into perennial grasslands. Woody plant encroachment has well-documented consequences for local, regional, and global ecology, and takes several forms, from shrub invasion in arid/semiarid grasslands and arctic ecosystems to tree invasion in mesic and alpine grasslands. We then outline a multi-scale experimental approach to examining the key processes influencing woody plant encroachment from fine to broad scales, and describe the application of this approach to the Jornada Basin Agricultural Research Service-Long Term Ecological Research site as a case study. Several questions can be addressed with this approach, such as: how and under what conditions do dynamics and decisions made at fine scales influence dynamics at broader scales? How and under what conditions do broad-scale dynamics overwhelm fine-scale processes to influence landscape patterns? Finally, we discuss analytical techniques for predicting regime shifts and their associated thresholds.}, keywords = {LTER-KNZ, broad-scale regime, pattern-process}, url = {http://www.springer.com/us/book/9780387779416}, author = {Peters, D.P.C. and B.T. Bestelmeyer and Alan K. Knapp and Herrick, J.E. and Monger, H.C. and Havstad, K.M.}, editor = {Miao, S. and Carstenn, S. and Nungesser, M.} } @article {KNZ001239, title = {Contingent productivity responses to more extreme rainfall regimes across a grassland biome}, journal = {Global Change Biology}, volume = {15}, year = {2009}, pages = {2894 -2904}, abstract = {

Climate models predict, and empirical evidence confirms, that more extreme precipitation regimes are occurring in tandem with warmer atmospheric temperatures. These more extreme rainfall patterns are characterized by increased event size separated by longer within season drought periods and represent novel climatic conditions whose consequences for different ecosystem types are largely unknown. Here, we present results from an experiment in which more extreme rainfall patterns were imposed in three native grassland sites in the Central Plains Region of North America, USA. Along this 600 km precipitation\–productivity gradient, there was strong sensitivity of temperate grasslands to more extreme growing season rainfall regimes, with responses of aboveground net primary productivity (ANPP) contingent on mean soil water levels for different grassland types. At the mesic end of the gradient (tallgrass prairie), longer dry intervals between events led to extended periods of below-average soil water content, increased plant water stress and reduced ANPP by 18\%. The opposite response occurred at the dry end (semiarid steppe), where a shift to fewer, but larger, events increased periods of above-average soil water content, reduced seasonal plant water stress and resulted in a 30\% increase in ANPP. At an intermediate mixed grass prairie site with high plant species richness, ANPP was most sensitive to more extreme rainfall regimes (70\% increase). These results highlight the inherent complexity in predicting how terrestrial ecosystems will respond to forecast novel climate conditions as well as the difficulties in extending inferences from single site experiments across biomes. Even with no change in annual precipitation amount, ANPP responses in a relatively uniform physiographic region differed in both magnitude and direction in response to within season changes in rainfall event size/frequency.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2009.01961.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2009.01961.x}, author = {Heisler-White, J.L. and John M. Blair and Kelly, E.F. and Harmoney, K. and Alan K. Knapp} } @article {KNZ001259, title = {Controls of aboveground net primary production in mesic savanna grasslands: An inter-hemispheric comparison}, journal = {Ecosystems}, volume = {12}, year = {2009}, pages = {982 -995}, abstract = {

Patterns and controls of annual aboveground net primary productivity (ANPP) are fundamental metrics of ecosystem functioning. It is generally assumed, but rarely tested, that determinants of ANPP in one region within a biome will operate similarly throughout that biome, as long as physiognomy and climate are broadly consistent. We tested this assumption by quantifying ANPP responses to fire, grazing history, and nitrogen (N) addition in North American (NA) and South African (SA) savanna grasslands. We found that total ANPP responded in generally consistent ways to fire, grazing history, and N addition on both continents. Annual fire in both NA and SA consistently stimulated total ANPP (28\–100\%) relative to unburned treatments at sites with deep soils, and had no effect on ANPP in sites with shallow soils. Fire did not affect total ANPP in sites with a recent history of grazing, regardless of whether a single or a diverse suite of large herbivores was present. N addition interacted strongly and consistently with fire regime in both NA and SA. In annually burned sites that were not grazed, total ANPP was stimulated by N addition (29\–39\%), but there was no effect of N fertilization in the absence of fire. In contrast, responses in forb ANPP to fire and grazing were somewhat divergent across this biome. Annual fire in NA reduced forb ANPP, whereas grazing increased forb ANPP, but neither response was evident in SA. Thus, despite a consistent response in total ANPP, divergent responses in forb ANPP suggest that other aspects of community structure and ecosystem functioning differ in important ways between these mesic savanna grasslands.

}, keywords = {LTER-KNZ, ANPP, fire, Grasslands, Grazing, nitrogen, Savannas}, doi = {10.1007/s10021-009-9273-1}, url = {https://link.springer.com/article/10.1007\%2Fs10021-009-9273-1}, author = {Buis, G.M. and John M. Blair and Burkepile, D.E. and Burns, C.E. and Chamberlain, A.J. and Chapman, P. and Scott. L. Collins and Fynn, R.W.S. and Govender, N. and Kirkman, K. and M.D. Smith and Alan K. Knapp} } @article {KNZ001238, title = {Ecophysiological responses of two dominant grasses to altered temperature and precipitation regimes}, journal = {Acta Oecologia}, volume = {35}, year = {2009}, pages = {400 -408}, abstract = {

Ecosystem responses to climate change will largely be driven by responses of the dominant species. However, if co-dominant species have traits that lead them to differential responses, then predicting how ecosystem structure and function will be altered is more challenging. We assessed differences in response to climate change factors for the two dominant C4 grass species in tallgrass prairie, Andropogon gerardii and Sorghastrum nutans, by measuring changes in a suite of plant ecophysiological traits in response to experimentally elevated air temperatures and increased precipitation variability over two growing seasons. Maximum photosynthetic rates, stomatal conductance, water-use efficiency, chlorophyll fluorescence, and leaf water potential varied with leaf and canopy temperature as well as with volumetric soil water content (0\–15 cm). Both species had similar responses to imposed changes in temperature and water availability, but when differences occurred, responses by A. gerardii were more closely linked with changes in air temperature whereas S. nutans was more sensitive to changes in water availability. Moreover, S. nutans was more responsive overall than A. gerardii to climate alterations. These results indicate both grass species are responsive to forecast changes in temperature and precipitation, but their differential sensitivity to temperature and water availability suggest that future population and community structure may vary based on the magnitude and scope of an altered climate.

}, keywords = {LTER-KNZ, Andropogon gerardii, Climate variability, Leaf gas exchange, RaMPs, Sensitivity, Sorghastrum nutans}, doi = {10.1016/j.actao.2009.01.010}, url = {https://www.sciencedirect.com/science/article/pii/S1146609X09000204?via\%3Dihub}, author = {Jesse B. Nippert and Fay, P.A. and Carlisle, J.D. and Alan K. Knapp and M.D. Smith} } @article {KNZ001252, title = {A framework for assessing ecosystem dynamics in response to chronic resource alterations induced by global change}, journal = {Ecology}, volume = {90}, year = {2009}, pages = {3279 -3289}, abstract = {

In contrast to pulses in resource availability following disturbance events, many of the most pressing global changes, such as elevated atmospheric carbon dioxide concentrations and nitrogen deposition, lead to chronic and often cumulative alterations in available resources. Therefore, predicting ecological responses to these chronic resource alterations will require the modification of existing disturbance-based frameworks. Here, we present a conceptual framework for assessing the nature and pace of ecological change under chronic resource alterations. The \“hierarchical-response framework\” (HRF) links well-documented, ecological mechanisms of change to provide a theoretical basis for testing hypotheses to explain the dynamics and differential sensitivity of ecosystems to chronic resource alterations. The HRF is based on a temporal hierarchy of mechanisms and responses beginning with individual (physiological/metabolic) responses, followed by species reordering within communities, and finally species loss and immigration. Each mechanism is hypothesized to differ in the magnitude and rate of its effects on ecosystem structure and function, with this variation depending on ecosystem attributes, such as longevity of dominant species, rates of biogeochemical cycling, levels of biodiversity, and trophic complexity. Overall, the HRF predicts nonlinear changes in ecosystem dynamics, with the expectation that interactions with natural disturbances and other global-change drivers will further alter the nature and pace of change. The HRF is explicitly comparative to better understand differential sensitivities of ecosystems, and it can be used to guide the design of coordinated, cross-site experiments to enable more robust forecasts of contemporary and future ecosystem dynamics.

}, keywords = {LTER-KNZ}, doi = {10.1890/08-1815.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/08-1815.1}, author = {M.D. Smith and Alan K. Knapp and Scott. L. Collins} } @article {KNZ001186, title = {Tracking the rhythm of the seasons in the face of global change: phenological research in the 21st century}, journal = {Frontiers in Ecology and the Environment}, volume = {7}, year = {2009}, pages = {253 -260}, abstract = {

Phenology is the study of recurring life-cycle events, classic examples being the flowering of plants and animal migration. Phenological responses are increasingly relevant for addressing applied environmental issues. Yet, challenges remain with respect to spanning scales of observation, integrating observations across taxa, and modeling phenological sequences to enable ecological forecasts in light of future climate change. Recent advances that are helping to address these questions include refined landscape-scale phenology estimates from satellite data, advanced, instrument-based approaches for field measurements, and new cyberinfrastructure for archiving and distribution of products. These breakthroughs are improving our understanding in diverse areas, including modeling land-surface exchange, evaluating climate\–phenology relationships, and making land-management decisions.

}, keywords = {LTER-KNZ}, doi = {10.1890/070217}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/070217}, author = {Morisette, J.T. and Richardson, A.D. and Alan K. Knapp and Fisher, J.I. and Graham, E.A. and Abatzoglou, J. and Wilson, B.E. and Breshears, D.D. and Henebry, G.M. and Hanes, J.M. and Liang, L.} } @article {KNZ001187, title = {Consequences of more extreme precipitation regimes for terrestrial ecosystems}, journal = {BioScience}, volume = {58}, year = {2008}, pages = {811 -821}, abstract = {

mplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers.

}, keywords = {LTER-KNZ, Climate change, Drought, Ecosystems, Precipitation, soil water}, doi = {10.1641/B580908}, url = {https://academic.oup.com/bioscience/article/58/9/811/250853}, author = {Alan K. Knapp and Beier, C. and Briske, D.D. and Classen, A.T. and Luo, Y. and Reichstein, M. and M.D. Smith and Smith, S.D. and Bell, J.E. and Fay, P.A. and Heisler, J.L. and Leavitt, S.W and Sherry, R. and Smith, B. and Weng, E.} } @article {KNZ001185, title = {Defoliation synchronizes above-ground growth of co-occurring C4 grass species}, journal = {Ecology}, volume = {89}, year = {2008}, pages = {2860 -2867}, abstract = {

The aboveground net primary productivity (ANPP) of grass communities in grasslands and savannas is primarily determined by precipitation quantity. Recent research, motivated by predictions of changes in the distribution of rainfall events by global climate change models, indicates that ANPP may be affected by rainfall distribution as much as by annual totals. Grazing and community composition are also known to affect grassland ANPP. The manner in which interactions between rainfall distribution, grazing, and community composition affect the relationship between precipitation and ANPP represents a critical knowledge gap. The effects of community composition and grazing on aboveground growth responses to intraseasonal variation in water availability were investigated at seven grassland sites with a nonselective clipping experiment. The aboveground growth of the dominant C4 species at each site was measured at regular intervals for 2\–3 growing seasons in the presence or absence of regular defoliation. In the absence of defoliation, there was a general lack of synchrony of intraseasonal growth among co-occurring species. Variation in growth rates was high and was only partially explained by variation in rainfall. Regular defoliation increased growth synchrony at all sites, but changes in growth responses to rainfall varied between sites. These results suggest that community composition will be important in determining ANPP\–precipitation relationships under conditions of altered rainfall distribution. However this effect appears to be a result of species responding differently to soil water or other resources rather than to rainfall per se. Grazing may override the effects of community composition by reducing differences in growth patterns between species and has the potential to weaken precipitation controls on ANPP.

}, keywords = {LTER-KNZ}, doi = {10.1890/07-1434.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/07-1434.1}, author = {Swemmer, A.M. and Alan K. Knapp} } @article {KNZ001007, title = {Development of water usage coefficients for the fully-watered tallgrass prairie}, journal = {Transactions of the American Society of Agricultural Engineers}, volume = {51}, year = {2008}, pages = {153 -159}, keywords = {LTER-KNZ}, author = {Hutchinson, S.L. and Koelliker, J.K. and Alan K. Knapp} } @inbook {KNZ001114, title = {Ecological consequences of the replacement of native grassland by Juniperus virginiana and other woody plants}, booktitle = {Ecological Studies Vol. 196, Western North American Juniperus communities: A dynamic vegetation type}, year = {2008}, pages = {156 -169}, publisher = {Springer-Verlag, NY}, organization = {Springer-Verlag, NY}, abstract = {

Although grasslands have been altered by humans for thousands of years (Wedel 1961; Bond et al. 2003), the loss of grassland as a result of anthropogenic activities has increased dramatically over the past 150 years. When Europeans first settled the Midwest and Great Plains, the greatest threat to native grasslands was the conversion of the most highly productive of these ecosystems to row-crop agriculture (Samson and Knopf 1994). Later, with improvements in soil moisture management and irrigation technology, even low-productivity grasslands were plowed. Today, those remnants of the most productive grasslands that escaped the plow are threatened, as are most of Earth\’s ecosystems, by a variety of global change phenomena (Vitousek et al. 1997), with the invasion and expansion of woody species into grasslands one of the greatest of these threats. The replacement of grasslands by shrubland, woodland, and forest is a concern not only in the United States but worldwide (Archer et al. 1988; Van Auken 2000; Roques et al. 2001; Silva et al. 2001). Species of woody plants that invade grasslands may include both native plants which previously existed as more minor components of the ecosystem as well as alien species (Bragg and Hulbert 1976; Harcombe et al. 1993).

}, keywords = {LTER-KNZ}, doi = {10.1007/978-0-387-34003-6_8}, url = {https://link.springer.com/chapter/10.1007\%2F978-0-387-34003-6_8}, author = {Alan K. Knapp and McCarron, J.K. and Silletti, A.M. and Hoch, G.A. and Heisler, J.L. and Lett, M.S. and John M. Blair and J. M. Briggs and M.D. Smith}, editor = {Van Auken, O.W.} } @article {KNZ001203, title = {Increased precipitation event size increases aboveground net primary productivity in a semi-arid grassland}, journal = {Oecologia}, volume = {158}, year = {2008}, pages = {129 -140}, abstract = {

Water availability is the primary constraint to aboveground net primary productivity (ANPP) in many terrestrial biomes, and it is an ecosystem driver that will be strongly altered by future climate change. Global circulation models predict a shift in precipitation patterns to growing season rainfall events that are larger in size but fewer in number. This \“repackaging\” of rainfall into large events with long intervening dry intervals could be particularly important in semi-arid grasslands because it is in marked contrast to the frequent but small events that have historically defined this ecosystem. We investigated the effect of more extreme rainfall patterns on ANPP via the use of rainout shelters and paired this experimental manipulation with an investigation of long-term data for ANPP and precipitation. Experimental plots (n = 15) received the long-term (30-year) mean growing season precipitation quantity; however, this amount was distributed as 12, six, or four events applied manually according to seasonal patterns for May\–September. The long-term mean (1940\–2005) number of rain events in this shortgrass steppe was 14 events, with a minimum of nine events in years of average precipitation. Thus, our experimental treatments pushed this system beyond its recent historical range of variability. Plots receiving fewer, but larger rain events had the highest rates of ANPP (184 \± 38 g m\−2), compared to plots receiving more frequent rainfall (105 \± 24 g m\−2). ANPP in all experimental plots was greater than long-term mean ANPP for this system (97 g m\−2), which may be explained in part by the more even distribution of applied rain events. Soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile. These results indicate that semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns.

}, keywords = {LTER-KNZ, Climate change, Grasslands, Precipitation variability, Pulse-reserve paradigm, Rain event size}, doi = {10.1007/s00442-008-1116-9}, url = {https://link.springer.com/article/10.1007\%2Fs00442-008-1116-9}, author = {Heisler-White, J.L. and Alan K. Knapp and Kelly, E.F.} } @article {KNZ001184, title = {Modeled interactive effects of precipitation, temperature, and CO2 on ecosystem carbon and water dynamics in different climatic zones}, journal = {Global Change Biology}, volume = {14}, year = {2008}, pages = {1986 -1999}, abstract = {

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO2] (C) on net primary production (NPP), heterotrophic respiration (Rh), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three-way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two-way interactive effects on NPP, Rh, and NEP were generally positive (i.e. amplification of one factor\&$\#$39;s effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor\&$\#$39;s effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two-way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, Rh, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2008.01629.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01629.x}, author = {Luo, Y. and Gerten, D. and Le Maire, G. and Parton, W.J. and Weng, E. and Zhou, X. and Keough, C. and Beier, C. and Ciais, P. and Cramer, W. and Dukes, J.S. and Emmett, B. and Hanson, P.J. and Alan K. Knapp and Linder, S. and Nepstad, D. and Rustad, L.} } @article {KNZ001183, title = {Modelled effects of precipitation on ecosystem carbon and water dynamics in different climatic zones}, journal = {Global Change Biology}, volume = {14}, year = {2008}, pages = {1 -15}, abstract = {

The ongoing changes in the global climate expose the world\&$\#$39;s ecosystems not only to increasing CO2 concentrations and temperatures but also to altered precipitation (P) regimes. Using four well-established process-based ecosystem models (LPJ, DayCent, ORCHIDEE, TECO), we explored effects of potential P changes on water limitation and net primary production (NPP) in seven terrestrial ecosystems with distinctive vegetation types in different hydroclimatic zones. We found that NPP responses to P changes differed not only among sites but also within a year at a given site. The magnitudes of NPP change were basically determined by the degree of ecosystem water limitation, which was quantified here using the ratio between atmospheric transpirational demand and soil water supply. Humid sites and/or periods were least responsive to any change in P as compared with moderately humid or dry sites/periods. We also found that NPP responded more strongly to doubling or halving of P amount and a seasonal shift in P occurrence than that to altered P frequency and intensity at constant annual amounts. The findings were highly robust across the four models especially in terms of the direction of changes and largely consistent with earlier P manipulation experiments and modelling results. Overall, this study underscores the widespread importance of P as a driver of change in ecosystems, although the ultimate response of a particular site will depend on the detailed nature and seasonal timing of P change.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2008.01651.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01651.x}, author = {Gerten, D. and Luo, Y. and Le Maire, G. and Parton, W.J. and Keough, C. and Weng, E. and Beier, C. and Ciais, P. and Cramer, W. and Dukes, J.S. and Emmett, B. and Hanson, P.J. and Alan K. Knapp and Linder, S. and Nepstad, D. and Rustad, L. and Sowerby, A.} } @article {KNZ001149, title = {Shrub encroachment in North American grasslands: Shifts in growth form dominance rapidly alters control of ecosystem carbon inputs}, journal = {Global Change Biology}, volume = {14}, year = {2008}, pages = {615 -623}, abstract = {

Shrub encroachment into grass-dominated biomes is occurring globally due to a variety of anthropogenic activities, but the consequences for carbon (C) inputs, storage and cycling remain unclear. We studied eight North American graminoid-dominated ecosystems invaded by shrubs, from arctic tundra to Atlantic coastal dunes, to quantify patterns and controls of C inputs via aboveground net primary production (ANPP). Across a fourfold range in mean annual precipitation (MAP), a key regulator of ecosystem C input at the continental scale, shrub invasion decreased ANPP in xeric sites, but dramatically increased ANPP (\>1000 g m\−2) at high MAP, where shrub patches maintained extraordinarily high leaf area. Concurrently, the relationship between MAP and ANPP shifted from being nonlinear in grasslands to linear in shrublands. Thus, relatively abrupt (\<50 years) shifts in growth form dominance, without changes in resource quantity, can fundamentally alter continental-scale pattern of C inputs and their control by MAP in ways that exceed the direct effects of climate change alone.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2007.01512.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2007.01512.x}, author = {Alan K. Knapp and J. M. Briggs and Scott. L. Collins and S.R. Archer and Bret-Harte, M.S. and Ewers, B.E. and Peters, D.P. and Young, D.R. and Shaver, G.R. and Pendall, E. and Cleary, M.B.} } @article {KNZ001182, title = {Temporal coherence of aboveground net primary productivity in mesic grasslands}, journal = {Ecography}, volume = {31}, year = {2008}, pages = {408 -416}, abstract = {

Synchrony in ecological variables over wide geographic areas suggests that large-scale environmental factors drive the structure and function of ecosystems and override more local-scale environmental variation. Described also as coherence, this phenomenon has been documented broadly in the ecological literature and has recently received increasing attention as scientists attempt to quantify the impacts of global changes on organisms and their habitats. Using a mesic grassland site in North America, we assessed coherence in ecosystem function by quantifying similarity in aboveground net primary production (ANPP) dynamics in 48 permanent sampling locations (PSLs) over a 16-yr period. Our primary objective was to characterize coherence across a broad geographic region (with similar ecosystem structure and function), and we hypothesized that precipitation and a similar fire frequency would strengthen coherence between PSLs. All 48 PSLs at our site (Konza Prairie Biological Station, Manhattan, KS, USA; KPBS) were exposed to a similar regional driver of ANPP (precipitation); however, local drivers (including differences in fire frequency and soil depth at different topographic positions) varied strongly among individual PSLs. For the purpose of this assessment, the watershed-level experimental design of KPBS was considered a model, which represented different fire management strategies across the Great Plains Region. Our analyses revealed a site-level (KPBS) coherence in ANPP dynamics of 0.53 for the period of 1984\–1999. Annual fire enhanced coherence among PSLs to 0.76, whereas less frequent fire (fire exclusion or a 4-yr fire return interval) failed to further increase coherence beyond that of the KPBS site level. Soil depth also strongly influenced coherence among PSLs with shallow soils at upland sites showing strong coherence across fire regimes and annually burned uplands closely linked to annual precipitation dynamics. The lack of coherence in ecosystem function in PSLs with deep soils and low fire frequencies suggests that conservation and management efforts will need to be more location specific in such areas where biotic interactions may be more important than regional abiotic drivers.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.0906-7590.2008.05351.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.0906-7590.2008.05351.x}, author = {Heisler, J.L. and Alan K. Knapp} } @article {KNZ001083, title = {Ecological genomics: making the leap from model systems in the lab to native populations in the field. Frontiers in Ecology and the Environment}, journal = {Frontiers in Ecology and the Environment}, volume = {5}, year = {2007}, pages = {19 -24}, abstract = {Recent reviews have emphasized the need to incorporate genomics into ecological field studies to further understand how species respond to changing environmental conditions. Genomic tools, such as cDNA (complementary DNA) microarrays, allow for the simultaneous analysis of gene expression of thousands of genes from all or part of an organism{\textquoteright}s genome (the transcription profile), thereby revealing the genetic mechanisms that underlie species{\textquoteright} responses to environmental change. However, despite their potential, two major limitations have hindered the incorporation of microarrays and other genomic tools into field studies: (1) the limited availability of microarrays for ecologically relevant, non-model species and limited financial resources for developing new microarrays; and (2) concern that high sensitivity of gene expression to even subtle alterations in environmental conditions will hinder detection of relevant changes in field measures of transcription profiles. Here, we show that with cross-species hybridizations of microarrays developed for a closely related model organism, an appropriate experimental design, and sufficient replication, transcriptional profiling can successfully be incorporated into field studies. In this way, relevant changes in gene expression with changing environmental conditions can be detected.}, keywords = {LTER-KNZ}, doi = {10.1890/1540-9295(2007)5[19:EGMTLF]2.0.CO;2}, author = {Travers, S.E. and M.D. Smith and Bai, J. and Hulbert, S.H. and Leach, J.E. and Schnable, P.S. and Alan K. Knapp and Milliken, G.A. and Fay, P.A. and Saleh, A. and Garrett, K.A.} } @inbook {KNZ001125, title = {Estimating aboveground net primary production in grassland and herbaceous dominated ecosystems}, booktitle = {Principles and Standards for Measuring Net Primary Production}, year = {2007}, pages = {27 -48}, publisher = {Oxford University Press, NY}, organization = {Oxford University Press, NY}, abstract = {This paper reviews past and currently accepted methods of estimating aboveground net primary production (NPP) in grass and herb-dominated ecosystems, provides some guiding principles and recommendations to facilitate accurate determination of aboveground NPP, and discusses biases and errors and sampling adequacy. }, keywords = {LTER-KNZ}, author = {Alan K. Knapp and J. M. Briggs and Childers, D.L. and Sala, O.E.} } @proceedings {KNZ001208, title = {Fire and resource availability influence carbon and water relations of the C3 shrub Cornus drummondii in a mesic grassland}, journal = {Proceedings of the 23rd Tall Timbers Fire Ecology Conference: Fire in Grassland and Shrubland Ecosystems}, year = {2007}, pages = {86 -93}, address = {Tall Timbers Research Station, Tallahassee, Florida, USA}, abstract = {

A dramatic increase in cover by woody vegetation has been observed in grasslands worldwide, due to independent and interacting global changes. In the C4-dominated mesic tallgrass prairie of North America, the most proximate factor driving this shift in growth-form dominance is fire exclusion. In 2001, we reintroduced annual fire into a C4-dominated mesic grassland where it had been excluded for 10 y, and evaluated changes in stem density and ecophysiology of an increasingly abundant C3 woody plant, roughleaf dogwood(Cornus drummondii). Our primary objective was to quantify the impacts of fire-induced aboveground mortality and resource constraints on the persistence of roughleaf dogwood in this ecosystem.

In both years of the study (2001\–2002), burned shrubs suffered a complete loss of aboveground biomass due to top-kill from spring fire but persisted via resprouting. A considerable reduction in total growing-season precipitation in 2002, as compared to 2001, revealed that resprouts had lower mortality with reduced soil moisture, were less vulnerable to reductions in soil moisture availability, and maintained higher predawn xylem pressure potentials and rates of photosynthesis than did unburned shrubs. Additionally, enrich-ment of13Cleaf in burned shrubs in 2002 was indicative of greater water use efficiency of resprouting stems. During the drought that occurred in the second growing season, mortality of stems within unburned shrub islands increased and was nearly 9 times more than in burned shrub islands. Thus, resprouting shrubs\—with their characteristics of reduced leaf tissue, taller stems, and greater stem densities\—are efficient in rapidly reoccupying post-fire environments, even with the added stress of below-average precipitation during the growing season. The results of this study suggest that within this grassland, reintroduction of fire will not eliminate rough leaf dogwood in the short term because of its ability to resprout vigorously and persist, even under stressful moisture conditions. Given that North American grasslands are important both economically and ecologically, fire management plans should therefore emphasize frequent fire to prevent shrub establishment

}, keywords = {LTER-KNZ}, url = {http://talltimbers.org/wp-content/uploads/2014/03/Heisleretal2007_op.pdf}, author = {Heisler, J.L. and Alan K. Knapp and J. M. Briggs} } @article {KNZ001122, title = {Intra-seasonal precipitation patterns and aboveground productivity in three perennial grasslands}, journal = {Journal of Ecology}, volume = {95}, year = {2007}, pages = {780 -788}, abstract = {1 Relationships between above-ground net primary productivity (ANPP) of grasslands and annual precipitation are often weak at the site level, with much of the inter-annual variation in ANPP left unexplained. A potential reason for this is that the distribution of precipitation within a growing season affects productivity in addition to the total amount. 2 We analysed long-term ANPP data for three southern African temperate grasslands (mean annual precipitation ranging from 538 mm to 798 mm) to determine the effects of precipitation event size, number and spacing relative to seasonal totals. 3 Ungrazed, non-manipulated treatments at each site showed contrasting results despite sharing a common, dominant species. At the driest site, a model combining average event size and number of events per growing season provided a substantially better fit to the ANPP data than precipitation amount (seasonal total). At the wettest site, the interval between events was the most important precipitation variable. Precipitation distribution was not important at the intermediate site where amount was the best predictor of ANPP. A limit to the size of precipitation events efficiently utilized for ANPP was evident for the driest site only. 4 At each site, experimental treatments that altered species composition and soil fertility had little effect on precipitation{\textendash}ANPP relationships. The lack of consistency in the relative importance of the precipitation variables among sites suggests that local, edaphic factors modify precipitation{\textendash}ANPP relationships. 5 This analysis demonstrates that the distribution and size of precipitation events can affect ANPP independent of precipitation amount. As altered precipitation regimes are forecast by global climate models, the sensitivity of ecosystems to precipitation distribution should be considered when predicting responses to climate change. 6 While mean values of precipitation, and other ecosystem drivers, are typically used to predict function at the level of whole ecosystems, our results show that more complex measures of environmental variability may be required to understand ecosystem function, and to increase the accuracy of predictions of ecosystem responses to global change.}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2745.2007.01237.x}, author = {Swemmer, A.M. and Alan K. Knapp and Snyman, H.A.} } @article {KNZ001120, title = {Linking water uptake with rooting patterns in grassland species}, journal = {Oecologia}, volume = {153}, year = {2007}, pages = {261 -272}, abstract = {Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004{\textendash}2005) in which we measured the isotopic signature of plant xylem water from seven species (including C3 forbs and shrubs and C4 grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (δ18O) values were compared to soil water δ18O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C3 species used proportionally more water from deeper portions of the soil profile relative to the C4 grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth.}, keywords = {LTER-KNZ, C3 plants, C4 plants, Mixing model, Stable oxygen isotope ratio, tallgrass prairie}, doi = {10.1007/s00442-007-0745-8}, author = {Jesse B. Nippert and Alan K. Knapp} } @article {KNZ001119, title = {Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments}, journal = {Environmental and Experimental Botany}, volume = {60}, year = {2007}, pages = {412 -420}, abstract = {The North American tallgrass prairie is composed of a diverse mix of C3 and C4 plant species that are subject to multiple resource limitations. C4 grasses dominate this ecosystem, purportedly due to greater photosynthetic capacity and resource-use efficiency associated with C4 photosynthesis. We tested the hypothesis that intrinsic physiological differences between C3 and C4 species are consistent with C4 grass dominance by comparing leaf gas exchange and chlorophyll fluorescence variables for seven C4 and C3 herbaceous species (legumes and non-legumes) in two different settings: experimental mesocosms and natural grassland sites. In the mesocosms, C4 grasses had higher photosynthetic rates, water potentials and water-use efficiency than the C3 species. These differences were absent in the field, where photosynthetic rates declined similarly among non-leguminous species. Thus, intrinsic photosynthetic advantages for C4 species measured in resource-rich mesocosms could not explain the dominance of C4 species in the field. Instead, C4 dominance in this ecosystem may depend more on the ability of the grasses to grow rapidly when resources are plentiful and to tolerate multiple limitations when resources are scarce.}, keywords = {LTER-KNZ, A:Ci curves, C3, C4, Chlorophyll fluorescence, gas exchange, Konza, tallgrass prairie}, doi = {10.1016/j.envexpbot.2006.12.012}, author = {Jesse B. Nippert and Fay, P.A. and Alan K. Knapp} } @inbook {KNZ001124, title = {Primary production: Guiding principles and standards for measurement}, booktitle = {Principles and Standards for Measuring Net Primary Production}, year = {2007}, pages = {3 -11}, publisher = {Oxford University Press, NY}, organization = {Oxford University Press, NY}, abstract = {This chapter serves as an introduction to the book, outlining the general considerations in the measurement of net primary production (NPP) in particular biomes or groups of biomes (e.g. savannas, grasslands and forests), the temporal and spatial variation and principles of site selection, field sampling and laboratory procedures for NPP measurement, and statistical analysis and quality assurance/quality control.}, keywords = {LTER-KNZ}, url = {http://www.cabdirect.org/abstracts/20073139615.html?freeview=true}, author = {Fahey, T.J. and Alan K. Knapp}, editor = {Fahey, T.J. and Alan K. Knapp} } @book {KNZ001123, title = {Principles and Standards for Measuring Net Primary Production}, series = {Principles and Standards for Measuring Net Primary Production}, year = {2007}, pages = {268 -}, publisher = {Oxford University Press, NY}, organization = {Oxford University Press, NY}, abstract = {Standardized approaches for the measurement of primary production {\textemdash} the rate of energy storage in the organic matter of plants {\textemdash} are essential to facilitate scientific comparisons and syntheses as well as policy and management on global climate change and the carbon cycle. This book provides an overview of the principles that should underlie every program of measurement of primary production in the Earth{\textquoteright}s major biomes. Each of seven biome-based chapters provides an overview of essential features of primary production processes in the biome and detailed descriptions of the procedures used to quantify primary production in grasslands, shrublands, forests, peatlands and tundra, salt marshes, marine pelagic, and freshwater ecosystems. Recent advances in the measurement of belowground production in terrestrial biomes are described. The book also provides detailed guidelines for information management based upon current experiences of the US Long-Term Ecological Research network. Advanced techniques are described for scaling up empirical measurements of primary production using remotely-sensed information. Finally, the principles and practices for quantifying uncertainty in primary production measurements are explored using examples from various biomes.}, keywords = {LTER-KNZ, belowground, biomes, carbon cycle, Ecosystems, remote sensing, uncertainty}, author = {Fahey, T.J. and Alan K. Knapp}, editor = {Fahey, T.J. and Alan K. Knapp} } @article {KNZ001118, title = {Soil water partitioning contributes to species coexistence in tallgrass prairie}, journal = {Oikos}, volume = {116}, year = {2007}, pages = {1017 -1029}, abstract = {The majority of tallgrass prairie root biomass is located in the upper soil layers (0{\textendash}25 cm), but species differences exist in reliance on soil water at varying depths. These differences have led to the hypothesis that resource partitioning belowground facilitates species co-existence in this mesic grassland. To determine if plant water relations can be linked to soil water partitioning as a potential mechanism allowing C3 species to persist among the more dominant C4 grasses, we measured differences in the source of water-use using the isotopic signature of xylem water, volumetric soil water content at 4 depths, and leaf water potentials. Data were collected for seven species representing C4 grasses, C3 forbs and C3 shrubs over three growing seasons at the Konza Prairie (Kansas, USA) to encompass a range of natural climatic conditions. C4 grasses relied on shallow soil water (5 cm) across the growing season and had midday leaf water potentials that were highly correlated with shallow soil water regardless of soil water availability at other portions of the soil profile (20, 40 and 90 cm). In contrast, C3 species only used shallow soil water when plentiful at this depth; these species increased their dependence on soil water from greater depths as the upper soil layers dried. Structural equation models describing plant water relations were very similar for the three C4 species, whereas a unique set of models and drivers were identified for each of the C3 species. These results support soil water partitioning as a mechanism for species coexistence, as C4 species in this grassland have relatively consistent dependence on water in shallow soil layers, whereas C3 species show niche differentiation in water use strategies to avoid competition with C4 grasses for water in shallow soil layers when this resource is limiting and leaf water stress is high.}, keywords = {LTER-KNZ}, doi = {10.1111/j.0030-1299.2007.15630.x}, author = {Jesse B. Nippert and Alan K. Knapp} } @article {KNZ001082, title = {Convergence and contingency in production-precipitation relationships in North American and South African C4 grasslands}, journal = {Oecologia}, volume = {149}, year = {2006}, pages = {456 -464}, abstract = {Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C4 grasses and had similar growing season climates; thus, we expected convergence in precipitation{\textendash}ANPP relationships. Lack of convergence, however, would support an alternative hypothesis{\textemdash}that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m2 respectively) and growing season precipitation{\textendash}ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of the growing season strongly affected annual ANPP in South Africa (P < 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal course of available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation in these two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site.}, keywords = {LTER-KNZ, climate, Evolutionary history, Net primary production, Precipitation, soil moisture}, doi = {10.1007/s00442-006-0468-2}, author = {Alan K. Knapp and Burns, C.E. and Fynn, R.W.S. and Kirkman, K.P. and Morris, C.D. and M.D. Smith} } @article {KNZ001070, title = {Growth responses of twodominant C4 grass species to altered water availability}, journal = {InternationalJournal of Plant Sciences}, volume = {167}, year = {2006}, pages = {1001 -1010}, abstract = {Identifying key ecophysiological traits that differ among dominant plant species and can be linked to species-specific responses to drought would improve our ability to forecast community and ecosystem responses to global climate change. The mesic grasslands of the central plains of North America are dominated by two C4 grass species, Andropogon gerardii and Sorghastrum nutans, which purportedly differ in their tolerance of water stress. Individuals of these two species were grown in the field under rain-out shelters and subjected to wet (watered every 2{\textendash}3 d) or dry (repeatedly subjected to wilting before watering) soil moisture regimes. A range of ecophysiological traits potentially important for tolerating water stress were concurrently measured. Although few traits differed between the species in the wet treatment, several traits were identified in the dry treatment that may enable A. gerardii to better tolerate drought. These were greater allocation to roots, reduced allocation to flowering, more rapid leaf turnover, and more rapid recovery of photosynthesis after wilting. The latter two traits may be particularly important for coping with increased variability in rainfall regimes in the future and are consistent with recently documented responses of A. gerardii to experimental increases in soil moisture variability.}, keywords = {LTER-KNZ, Andropogon gerardii, biomass allocation, Climate change, leaf turnover, photosynthesis, Sorghastrum nutans, water stress}, doi = {10.1086/505611}, author = {Swemmer, A.M. and Alan K. Knapp and M.D. Smith} } @article {KNZ00993, title = {Intra-annual rainfallvariability and grassland productivity: can the past predictthe future}, journal = {Plant Ecology}, volume = {184}, year = {2006}, pages = {65 -74}, abstract = {Precipitation quantity has been shown to influence grassland aboveground net primary productivity (ANPP) positively whereas experimental increases in of temporal variability in water availability commonly exhibit a negative relationship with ANPP. We evaluated long term ANPP datasets from the Konza Prairie Long Term Ecological Research (LTER) program (1984{\textendash}1999) to determine if similar relationships could be identified based on patterns of natural variability (magnitude and timing) in precipitation. ANPP data were analyzed from annually burned sites in native mesic grassland and productivity was partitioned into graminoid (principally C4 grasses) and forb (C3 herbaceous) components. Although growing season precipitation amount was the best single predictor of total and grass ANPP (r 2=0.62), several measures of precipitation variability were also significantly and positively correlated with productivity, independent of precipitation amount. These included soil moisture variability, expressed as CV, for June (r 2=0.45) and the mean change in soil moisture between weekly sampling periods in June and August (\%wv) (r 2=0.27 and 0.32). In contrast, no significant relationships were found between forb productivity and any of the precipitation variables (p>0.05). A multiple regression model combining precipitation amount and both measures of soil moisture variability substantially increased the fit with productivity (r 2=0.82). These results were not entirely consistent with those of short-term manipulative experiments in the same grassland, however, because soil moisture variability was often positively, not negatively related to ANPP. Differences in results between long and short term experiments may be due to low variability in the historic precipitation record compared to that imposed experimentally as experimental levels of variability exceeded the natural variability of this dataset by a factor of two. Thus, forecasts of ecosystem responses to climate change (i.e. increased climatic variability), based on data constrained by natural and recent historical rainfall patterns may be inadequate for assessing climate change scenarios if precipitation variability in the future is expected to exceed current levels.}, keywords = {LTER-KNZ, ANPP, Climate change, grassland, Precipitation variability, soil moisture, tallgrass prairie}, doi = {10.1007/s11258-005-9052-9}, author = {Jesse B. Nippert and Alan K. Knapp and J. M. Briggs} } @article {KNZ00960, title = {An ecosystem in transition: causes and consequences of the conversion of mesic grassland to shrubland}, journal = {BioScience}, volume = {55}, year = {2005}, pages = {243 -254}, abstract = {Woody plant expansion is one of the greatest contemporary threats to mesic grasslands of the central United States. In this article, we synthesize more than 20 years of research to elucidate the causes and consequences of the ongoing transition of C4-dominated grasslands to savanna-like ecosystems codominated by grasses and woody plants. This transition is contingent on fire-free intervals, which provide the opportunity for recruitment both of new individuals and of additional shrub and tree species into this grassland. Once shrubs establish, their cover increases regardless of fire frequency, and infrequent fires accelerate the spread of some shrub species. This process has resulted in a new dynamic state of shrub{\textendash}grass coexistence in the mesic grasslands of North America. Important consequences of this shift in plant life-form abundance include alterations in plant productivity, species diversity, and carbon storage. Without drastic measures such as mechanical removal of shrubs, it is unlikely that management of fire and grazing regimes alone will be sufficient to restore historic grass dominance in these ecosystems.}, keywords = {LTER-KNZ, fire, Grasslands, Grazing, tallgrass prairie, woody vegetation}, doi = {10.1641/0006-3568(2005)055[0243:AEITCA]2.0.CO;2}, author = {J. M. Briggs and Alan K. Knapp and John M. Blair and Heisler, J.L. and Hoch, G.A. and Lett, M.S. and McCarron, J.K.} } @article {KNZ00939, title = {Increased rainfall variability and reduced rainfall amount decreases soil CO2 flux in a grassland ecosystem}, journal = {Global Change Biology}, volume = {11}, year = {2005}, pages = {322 -344}, abstract = {Predicted climate changes in the US Central Plains include altered precipitation regimes with increased occurrence of growing season droughts and higher frequencies of extreme rainfall events. Changes in the amounts and timing of rainfall events will likely affect ecosystem processes, including those that control C cycling and storage. Soil carbon dioxide (CO2) flux is an important component of C cycling in terrestrial ecosystems, and is strongly influenced by climate. While many studies have assessed the influence of soil water content on soil CO2 flux, few have included experimental manipulation of rainfall amounts in intact ecosystems, and we know of no studies that have explicitly addressed the influence of the timing of rainfall events. In order to determine the responses of soil CO2 flux to altered rainfall timing and amounts, we manipulated rainfall inputs to plots of native tallgrass prairie (Konza Prairie, Kansas, USA) over four growing seasons (1998{\textendash}2001). Specifically, we altered the amounts and/or timing of growing season rainfall in a factorial combination that included two levels of rainfall amount (100\% or 70\% of naturally occurring rainfall quantity) and two temporal patterns of rain events (ambient timing or a 50\% increase in length of dry intervals between events). The size of individual rain events in the altered timing treatment was adjusted so that the quantity of total growing season rainfall in the ambient and altered timing treatments was the same (i.e. fewer, but larger rainfall events characterized the altered timing treatment). Seasonal mean soil CO2 flux decreased by 8\% under reduced rainfall amounts, by 13\% under altered rainfall timing, and by 20\% when both were combined (P<0.01). These changes in soil CO2 flux were consistent with observed changes in plant productivity, which was also reduced by both reduced rainfall quantity and altered rainfall timing. Soil CO2 flux was related to both soil temperature and soil water content in regression analyses; together they explained as much as 64\% of the variability in CO2 flux across dates under ambient rainfall timing, but only 38{\textendash}48\% of the variability under altered rainfall timing, suggesting that other factors (e.g. substrate availability, plant or microbial stress) may limit CO2 flux under a climate regime that includes fewer, larger rainfall events. An analysis of the temperature sensitivity of soil CO2 flux indicated that temperature had a reduced effect (lower correlation and lower Q10 values) under the reduced quantity and altered timing treatments. Recognition that changes in the timing of rainfall events may be as, or more, important than changes in rainfall amount in affecting soil CO2 flux and other components of the carbon cycle highlights the complex nature of ecosystem responses to climate change in North American grasslands.}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2005.00899.x}, author = {Harper, C.W. and John M. Blair and Fay, P.A. and Alan K. Knapp and Carlisle, J.D.} } @article {KNZ001024, title = {Indicators of plant species richness in AVIRIS spectra of a mesic grassland}, journal = {Remote Sensing of Environment}, volume = {98}, year = {2005}, pages = {304 -316}, abstract = {Hyperspectral imagery of the Konza Prairie Biological Station in northeastern Kansas was used to evaluate upwelling spectral radiance, prairie spectral reflectance and band ratios of each as potential indicators of vascular plant species richness in a mesic grassland. The extent to which spatial variability in these parameters related to plant species richness also was investigated. A 224 channel hyperspectral data cube acquired in June 2000 by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) provided complete coverage of the 400{\textendash}2500 nm range at approximately 10 nm per channel. After band deletions accounted for detector overlap and strong atmospheric attenuation features, 176 bands were retained for analysis and spanned the 404{\textendash}2400 nm range. Prairie reflectance was estimated via radiative transfer modeling and scaling to a library spectrum of highway construction material. Data were sampled from pixels having a 19 m ground sample distance (GSD) to represent each of 93 vegetation sampling transects. Reflectance and radiance at mid-infrared wavelengths (e.g., 1553 nm), and band ratios that were based on atmospheric windows in the red, near-infrared and mid-infrared spectra estimated species richness to within 6 to 7 species per transect. The 856 to 780 nm radiance or reflectance ratio yielded maximum adjusted coefficients of determination (r2) of approximately 0.4 in regressions with richness when data from bison-grazed and ungrazed areas were combined. These regressions remained significant (p <= 0.001) when only ungrazed areas were assessed although r2 reduced to approximately 0.2. Richness was related significantly also to the 433 to 674 nm reflectance ratio for grazed-plus-ungrazed and ungrazed-only areas. In contrast, the effectiveness of the 433 to 674 nm radiance ratio was reduced by atmospheric backscatter. Species richness did not correlate strongly or consistently with transect spatial variability (coefficient of variation or range) in radiance, reflectance or band ratio value, apparently as a consequence of the relatively small area sampled for each transect (approximately 0.5 ha). Relationships between richness and prairie spectral features were explained by the influence of soil exposure on both parameters. Richness and estimated soil exposure tended to increase from ungrazed lowlands, to ungrazed slopes, to ungrazed uplands to grazed areas. Remotely sensed estimates of soil exposure may be particularly useful in addressing plant species richness on grazed grasslands owing to an overall similarity in spectral reflectance among dominant plant species.}, keywords = {LTER-KNZ, AVIRIS, Biodiversity, Bison grazing, grassland, Hyperspectral, Plant species richness, Radiance, Reflectance, Soil exposure, Spatial variability, Spectral mixing}, doi = {10.1016/j.rse.2005.08.001}, author = {Carter, G.A. and Alan K. Knapp and Anderson, J.E. and Hoch, G.A. and M.D. Smith} } @article {KNZ00893, title = {Regional patterns in carbon cycling across the Great Plains of North America}, journal = {Ecosystems}, volume = {8}, year = {2005}, pages = {106 -121}, abstract = {The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m-2 y-1) to tallgrass prairie (180, 1221, and 208 g C m-2 y-1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.}, keywords = {LTER-KNZ, grassland, Great Plains, litter decomposition, Net primary production, Precipitation, Regional trends, soil respiration}, doi = {10.1007/s10021-004-0117-8}, author = {McCulley, R.L. and Burke, I.C. and Nelson, J.A. and Lauenroth, W.K. and Alan K. Knapp and Kelly, E.F.} } @article {KNZ00978, title = {Soil heterogeneity effects on tallgrass prairie community heterogeneity: anapplication of ecological theory to restoration ecology}, journal = {Restoration Ecology}, volume = {13}, year = {2005}, pages = {413 -424}, abstract = {Spatial heterogeneity of resources can influence plant community composition and diversity in natural communities. We manipulated soil depth (two levels) and nutrient availability (three levels) to create four heterogeneity treatments (no heterogeneity, depth heterogeneity, nutrient heterogeneity, and depth + nutrient heterogeneity) replicated in an agricultural field seeded to native prairie species. Our objective was to determine whether resource heterogeneity influences species diversity and the trajectory of community development during grassland restoration. The treatments significantly increased heterogeneity of available inorganic nitrogen (N), soil water content, and light penetration. Plant diversity was indirectly related to resource heterogeneity through positive relationships with variability in productivity and cover established by the belowground manipulations. Diversity was inversely correlated with the average cover of the dominant grass, Switchgrass (Panicum virgatum), which increased over time in all heterogeneity treatments and resulted in community convergence among the heterogeneity treatments over time. The success of this cultivar across the wide range of resource availability was attributed to net photosynthesis rates equivalent to or higher than those of the native prairie plants in the presence of lower foliar N content. Our results suggest that resource heterogeneity alone may not increase diversity in restorations where a dominant species can successfully establish across the range of resource availability. This is consistent with theory regarding the role of ecological filters on community assembly in that the establishment of one species best adapted for the physical and biological conditions can play an inordinately important role in determining community structure.}, keywords = {LTER-KNZ, grassland, Panicum virgatum, restoration, soil heterogeneity, switchgrass, tallgrass prairie}, doi = {10.1111/j.1526-100X.2005.00051.x}, author = {S.G. Baer and Scott. L. Collins and John M. Blair and Fiedler, A. and Alan K. Knapp} } @article {KNZ00944, title = {Woody plant encroachment and removal in mesic grassland: production and composition responses of herbaceous vegetation}, journal = {American Midland Naturalist}, volume = {153}, year = {2005}, pages = {217 -231}, abstract = {The clonal shrub Cornus drummondii is rapidly displacing mesic grassland in the central U.S. due to fire suppression and changes in land use. Once established, this shrub is not readily eliminated by the return of frequent fire, leading to significant and perhaps irreversible shifts in tallgrass prairie structure and function. We assessed the impacts of C. drummondii encroachment on herbaceous aboveground net primary production (ANPP) and plant community structure in tallgrass prairie and the role this species plays in the conversion of grassland ecosystems to shrub/woodlands. We also removed established C. drummondii clones to assess the potential of the grassland ecosystem to recover after shrub dominance. Aboveground net primary production, vegetative cover and species richness and diversity (exclusive of C. drummondii) were significantly reduced beneath shrub islands relative to open grassland, with reductions in ANPP and richness of up to 94 and 45\%, respectively. Forbs were the dominant growth form in the C. drummondii understory, and 10 species primarily associated with woodland habitats occurred only within shrub islands. Upon removal of C. drummondii, ANPP, richness and diversity recovered to grassland values within 2 y; however, forbs remained the dominant growth form, comprising 73\% of total cover. These results indicate that C. drummondii exerts strong control over the structure and function of unburned mesic grassland ecosystems and that this shrub may be key in the conversion of grasslands to woodlands. Removal of C. drummondii resulted in some aspects of recovery, but the return of graminoid dominance was not attained after 2 y. This represents a legacy effect of C. drummondii of unknown duration. Prevention of woody species encroachment through frequent burning is a preferred management option for this ecosystem.}, keywords = {LTER-KNZ}, doi = {10.1674/0003-0031(2005)153[0217:WPEARI]2.0.CO;2}, author = {Lett, M.S. and Alan K. Knapp} } @article {KNZ00894, title = {Competition and coexistence in grassland co-dominants: responses to neighbor removal and resource availability}, journal = {Canadian Journal of Botany}, volume = {82}, year = {2004}, pages = {450 -460}, abstract = {We examined the role of interspecific competition in the regulation of abundance and coexistence of the dominant grasses in tallgrass prairie using a removal experiment with Andropogon gerardii Vitman and Sorghastrum nutans L. Nash, two of the most abundant grasses in tallgrass prairie. Plant removal treatments (using foliar herbicide), applied to 0.3-m2 plots at the Konza Prairie Biological Station (northeast Kansas, USA), included removal of all A. gerardii, removal of all S. nutans, and no removal. To determine whether soil fertility altered the outcome, we included a fertilizer addition treatment (10 g N{\textperiodcentered}m–2) fully crossed with the removal treatments. Andropogon gerardii removal resulted in significantly increased net photosynthesis, stomatal conductance, and tiller mass in S. nutans. Sorghastrum nutans removal had little effect on A. gerardii, suggesting asymmetric competition. Fertilizer significantly increased tiller mass and flowering stalk production in S. nutans, but had little effect on A. gerardii. The ability of A. gerardii to suppress the performance of S. nutans is consistent with the greater abundance of A. gerardii over much of the tallgrass prairie, while the ability of S. nutans to take advantage of increased resources may be one mechanism by which it avoids competitive exclusion. Because of the greater variability in the performance of S. nutans than in that of A. gerardii, any natural or anthropogenic alterations to this grassland that lead to shifts in dominance between these species may affect ecosystem productivity and stability.Key words: Andropogon gerardii, competition, grassland, neighbour removal, photosynthesis, Sorghastrum nutans, tallgrass prairie.}, keywords = {LTER-KNZ}, doi = {10.1139/b04-016}, author = {Silletti, A.M. and Alan K. Knapp and John M. Blair} } @article {KNZ00917, title = {Convergence across biomes to a common rain-use efficiency}, journal = {Nature}, volume = {429}, year = {2004}, pages = {651 -654}, keywords = {LTER-KNZ}, doi = {10.1038/nature02561}, author = {Huxman, T.E. and M.D. Smith and Fay, P.A. and Alan K. Knapp and Shaw, M.R. and Loik, M.E. and Smith, S.D. and Tissue, D.T. and Zak, J.C. and Weltzin, J.F. and Pockman, W.T. and Sala, O.E. and Haddad, B.M. and Harte, J. and Koch, G.W. and Schwinning, S. and Small, E.E. and Williams, D.G.} } @article {KNZ00891, title = {Direct and indirect effects of fire on shrub density and aboveground productivity in a mesic grassland}, journal = {Ecology}, volume = {85}, year = {2004}, pages = {2245 -2257}, abstract = {Determinants of the balance between grass and woody vegetation in grasslands and savannas have received considerable attention because of the potential for dramatic shifts in ecosystem structure and function as one growth form replaces the other. We studied a mesic grassland where recently established {\textquotedblleft}shrub islands{\textquotedblright} are increasing in abundance due to fire suppression. Our objective was to assess the role of the direct effects of fire vs. indirect alterations in resource availability (N and light), as mechanisms that may constrain/facilitate shrub (Cornus drummondii) encroachment. The direct effects of fire in 2001 and 2002 were 100\% aboveground mortality of C. drummondii shoots and removal of the detrital layer. Post-fire resprouting resulted in \~{}600\% increase in stem density compared to a 200\% increase in shrub islands protected from fire. In burned shrub islands with an added detrital layer, temperature and light penetration to the soil surface were reduced (by 6.5{\textdegree}C and to <3\% of full sunlight), but stem density still increased by \~{}400\%. Thus, both the direct effects of fire and the indirect effect on the energy environment increased C. drummondii stem densities. In contrast, N additions did not influence new stem production or aboveground net primary productivity (ANPP; grams per square meter per year), suggesting that N availability did not constrain shrub growth during this study. While fire did not impact total ANPP, it did shift the relative abundance of growth forms. Grass productivity (360.7 {\textpm} 20.1 g/m2 [mean {\textpm} 1 se]) was stimulated (an increase of \~{}30\%) by the high light conditions of the post-fire environment, while C. drummondii ANPP (34.2 {\textpm} 2.4 g/m2) was reduced by \~{}30\%. In shrub islands protected from fire, C. drummondii ANPP was greatest (50.4 {\textpm} 2.2 g/m2), whereas lower graminoid ANPP (282.5 {\textpm} 19.9 g/m2) was observed. The persistence of woody vegetation, despite two successive fires, along with a significant reduction in grass ANPP (\~{}30\%) suggests that once established, C. drummondii can persist and exclude C4 grasses. Thus, restoring fire to mesic grasslands may prevent further conversion to shrub/woodland, but the abundance of shrubs is likely to remain unchanged with community structure co-dominated by multiple growth forms.}, keywords = {LTER-KNZ}, doi = {10.1890/03-0574}, author = {Heisler, J.L. and J. M. Briggs and Alan K. Knapp and John M. Blair and Seery, A.} } @article {KNZ00902, title = {Dominance not diversity determines invasibility of tallgrass prairie}, journal = {Oikos}, volume = {106}, year = {2004}, pages = {253 -262}, abstract = {Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume, Melilotus officinalis, in intact, native Kansas grassland. We manipulated dominance of C4 grasses by reducing their abundance (i.e. ramet densities) by \~{}25 and 50\%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001{\textendash}2002), invasibility, measured as peak establishment of Melilotus, was not affected by a 3-fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space-filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.}, keywords = {LTER-KNZ}, doi = {10.1111/j.0030-1299.2004.13057.x}, author = {M.D. Smith and Wilcox, J. and Kelly, T. and Alan K. Knapp} } @article {KNZ00943, title = {Generality in ecology: testing North American grassland rules in South African savannas}, journal = {Frontiers in Ecology and the Environment}, volume = {9}, year = {2004}, pages = {483 -491}, abstract = {Ecology has emerged as a global science, and there is a pressing need to identify ecological rules {\textendash} general principles that will improve its predictive capability for scientists and its usefulness for managers and policy makers. Ideally, the generality and limits of these ecological rules should be assessed using extensive, coordinated experiments that ensure consistency in design and comparability of data. To improve the design of these large-scale efforts, existing data should be used to test prospective ecological rules and to identify their limits and contingencies. As an example of this approach, we describe prospective rules for grassland responses to fire and rainfall gradients, identified from long-term studies of North American grasslands and tested with existing data from long-term experiments in South African savanna grasslands. Analyses indicated consistent effects of fire on the abundance of the dominant (grasses) and subdominant (forbs) flora on both continents, but no common response of grass or forb abundance across a rainfall gradient. Such analyses can inform future research designs to refine and more explicitly test ecological rules.}, keywords = {LTER-KNZ}, doi = {10.1890/1540-9295(2004)002[0483:GIETNA]2.0.CO;2}, author = {Alan K. Knapp and M.D. Smith and Scott. L. Collins and Zambatis, N. and Peel, M. and Emery, S.M. and Wojdak, J. and Horner-Devine, M.C. and Biggs, H. and Kruger, J. and Andelman, S.J.} } @article {KNZ00892, title = {Influence of shrub encroachment on aboveground net primary productivity and carbon and nitrogen pools in a mesic grassland}, journal = {Canadian Journal of Botany}, volume = {82}, year = {2004}, pages = {1363 -1370}, abstract = {The clonal shrub Cornus drummondii C.A. Mey. is rapidly increasing in cover and displacing mesic grassland species in the central USA as a consequence of fire suppression. We assessed the impact of C. drummondii on carbon (C) and nitrogen (N) pools and C fluxes in a tallgrass prairie in eastern Kansas, USA, through a comparison of both burned and unburned C. drummondii islands with open grassland areas. Allometric equations relating C. drum mondii foliage and wood biomass to basal stem diameter were developed to estimate aboveground biomass and net primary productivity (ANPP) of C. drummondii. Within C. drummondii islands, ANPP was 496 {\textpm} 45 g C{\textperiodcentered}m–2{\textperiodcentered}year–1, nearly three times that within open grassland (167 {\textpm} 13 g C{\textperiodcentered}m–2{\textperiodcentered}year–1). As a result of greater aboveground biomass, aboveground C and N storage within shrub islands (3270 {\textpm} 466 g C{\textperiodcentered}m–2, 37.9 {\textpm} 5.3 g N{\textperiodcentered}m–2) was substantially greater than that within open grassland (241 {\textpm} 33 g C{\textperiodcentered}m–2, 6.1 {\textpm} 0.8 g N{\textperiodcentered}m–2). No change in soil organic C or total N to 10-cm depth was evident; however, soil CO2 flux was significantly reduced in C. drummondii islands relative to the open grassland. The storage of C in aboveground biomass of C. drummondii represents a significant short-term increase in C storage relative to open grassland. However, potential alterations in belowground processes must be quantified before the long-term net effect of shrub encroachment on C and N pools within this mesic grassland can be determined.Key words: aboveground biomass, Cornus drummondii, net primary productivity, shrub encroachment, tallgrass prairie.}, keywords = {LTER-KNZ}, doi = {10.1139/b04-088}, author = {Lett, M.S. and Alan K. Knapp and J. M. Briggs and John M. Blair} } @article {KNZ00889, title = {Plant community responses to resource availability and heterogeneity during restoration}, journal = {Oecologia}, volume = {139}, year = {2004}, pages = {617 -629}, abstract = {Availability and heterogeneity of resources have a strong influence on plant community structure in undisturbed systems, as well as those recovering from disturbance. Less is known about the role of resource availability and heterogeneity in restored communities, although restoration provides a valuable opportunity to test our understanding of factors that influence plant community assembly. We altered soil nitrogen (N) availability and soil depth during a prairie restoration to determine if the availability and/or heterogeneity of soil resources influenced plant community composition in restored grassland communities. Plant community responses to three levels of N availability (ambient, enriched by fertilization, and reduced by carbon amendment) and two levels of soil depth (deep and shallow) were evaluated. In addition, we evaluated plant community responses to four whole plot heterogeneity treatments created from the six possible combinations of soil N availability and soil depth. The soil depth treatment had little influence on community structure during the first 3 years of restoration. Total diversity and richness declined over time under annual N enrichment, whereas diversity was maintained and richness increased over time in soil with reduced N availability. Non-native species establishment was lowest in reduced-N soil in the initial year, but their presence was negligible in all of the soil N treatments by the second year of restoration. Panicum virgatum, a native perennial C4 grass, was the dominant species in all soil N treatments by year three, but the magnitude of its dominance was lowest in the reduced-N soil and highest in enriched-N soil. Consequently, the relative cover of P. virgatum was strongly correlated with community dominance and inversely related to diversity. The differential growth response of P. virgatum to soil N availability led to a higher degree of community similarity to native prairie in the reduced-N treatment than in the enriched-N treatment. There were no differences in plant community structure among the four whole plot-level heterogeneity treatments, which all exhibited the same degree of similarity to native prairie. Diversity and community heterogeneity in the whole-plot treatments appeared to be regulated by the dominant species{\textquoteright} effect on light availability, rather than soil N heterogeneity per se. Our results indicate that a strong differential response of a dominant species to resource availability in a restored community can regulate community structure, diversity, and similarity to the native (or target) community, but the importance of resource heterogeneity in restoring diversity may be dampened in systems where a dominant species can successfully establish across a range of resource availability.}, keywords = {LTER-KNZ, diversity, grassland, nitrogen, tallgrass prairie}, doi = {10.1007/s00442-004-1541-3}, author = {S.G. Baer and John M. Blair and Scott. L. Collins and Alan K. Knapp} } @article {KNZ00920, title = {Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2}, journal = {Oecologia}, volume = {140}, year = {2004}, pages = {11 -25}, abstract = {

Atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations. Increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years. In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO2 effects on photosynthesis and those indirect CO2 effects via soil moisture as documented here.

}, keywords = {LTER-KNZ, biomass, Carbon dioxide enrichment, Landscape predictions, soil water, Stomata}, doi = {10.1007/s00442-004-1550-2}, author = {Morgan, J.A. and Pataki, D.E. and Korner, C. and Clark, H. and Del Grosso, S.J. and Grunzweig, J.M. and Alan K. Knapp and Mosier, A.R. and Newton, P.C.D. and Niklaus, P.A. and Jesse B. Nippert and Nowak, R.S. and Parton, W.J. and Polley, H.W. and Shaw, M.R.} } @article {KNZ00844, title = {Abiotic constraints on the establishment of Quercus seedlings in grasslands}, journal = {Global Change Biology}, volume = {9}, year = {2003}, pages = {266 -275}, abstract = {High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first-year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland-forest ecotone (shade). In the 2000 growing season, precipitation was 35\% below the long-term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10\% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late-season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present-day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue.}, keywords = {LTER-KNZ}, doi = {10.1046/j.1365-2486.2003.00574.x}, author = {Danner, B.T. and Alan K. Knapp} } @article {KNZ00881, title = {Assessing the response of terrestrial ecosystems to potential changes in precipitation}, journal = {BioScience}, volume = {53}, year = {2003}, pages = {941 -952}, abstract = {Changes in Earth{\textquoteright}s surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.}, keywords = {LTER-KNZ, Community, ecosystem, global change, Precipitation, soil moisture}, doi = {doi: 10.1641/0006-3568(2003)053[0941:ATROTE]2.0.CO;2}, author = {Weltzin, J.F. and Loik, M.E. and Schwinning, S. and Williams, D.G. and Fay, P.A. and Haddad, B. and Harte, J. and Huxman, T.E. and Alan K. Knapp and Lin, G. and Pockman, W.T. and Shaw, M.R. and Small, E. and M.D. Smith and Tissue, D.T. and Zak, J.C.} } @article {KNZ00867, title = {C3 shrub expansion in a C4 grassland: positive post-fire responses in resources and shoot growth}, journal = {American Journal of Botany}, volume = {90}, year = {2003}, pages = {1496 -1501}, abstract = {Changes in land management and reductions in fire frequency have enabled woody species to increase in grasslands worldwide. Nevertheless, fire is rarely eliminated from grasslands, and for shrubs to survive, they must be able cope with fire and replace aboveground structures. Because new shoots may have more available solar radiation, greater root : shoot ratios, and thus more resources available belowground after fire compared to undisturbed shrub communities, we hypothesized that carbon, nutrient, and water relations may be enhanced in stems compared to those in an undisturbed grassland. However, this same post-fire resource pulse stimulates the grasses and may intensify competitive interactions between shrubs and grasses. To test these predictions, we measured seasonal patterns in net photosynthesis (A), predawn xylem pressure potentials (XPP), leaf nitrogen (N) content, and productivity of Cornus drummondii shoots from shrub patches (islands) of different sizes in mesic grasslands burned annually, burned infrequently, and protected from fire. Seasonal average A was 20\% higher (P = 0.016) in burned than in unburned shrubs, regardless of island size. Shrubs in burned sites also produced shoots with higher leaf N than unburned shrubs, and N content was higher in leaves from small islands compared to large islands (P < 0.0001). Burning caused a decrease in late summer predawn XPP in small islands (-3.1 MPa), whereas burned large islands did not differ from unburned shrubs. Post-fire productivity of new shoots was significantly greater compared to shoots in unburned sites. These results indicate that a transient period of high resource availability after fire allows for increased growth and rapid recovery of grassland shrubs. Thus, although fire has a negative effect on aboveground biomass of shrubs, the post-fire increases in resource availability, which enhance growth in the dominant grasses, are also important for recovery of woody species.}, keywords = {LTER-KNZ, C3, C4, Cornus drummondii, fire, shrub, tallgrass prairie, transient maxima hypothesis, woody expansion}, doi = {10.3732/ajb.90.10.1496}, author = {McCarron, J.K. and Alan K. Knapp} } @article {KNZ00873, title = {A comparative assessment of potential mechanisms influencing plant species richness in grazed grasslands}, journal = {Oecologia}, volume = {137}, year = {2003}, pages = {385 -391}, abstract = {Grazing by large ungulates often increases plant species richness in grasslands of moderate to high productivity. In a mesic North American grassland with and without the presence of bison (Bos bison), a native ungulate grazer, three non-exclusive hypotheses for increased plant species richness in grazed grasslands were evaluated: (1) bison grazing enhances levels of resource (light and N) availability, enabling species that depend on higher resource availability to co-occur; (2) spatial heterogeneity in resource availability is enhanced by bison, enabling coexistence of a greater number of plant species; (3) increased species turnover (i.e. increased species colonization and establishment) in grazed grassland is associated with enhanced plant species richness. We measured availability and spatial heterogeneity in light, water and N, and calculated species turnover from long-term data in grazed and ungrazed sites in a North American tallgrass prairie. Both regression and path analyses were performed to evaluate the potential of the three hypothesized mechanisms to explain observed patterns of plant species richness under field conditions. Experimental grazing by bison increased plant species richness by 25\% over an 8-year period. Neither heterogeneity nor absolute levels of soil water or available N were related to patterns of species richness in grazed and ungrazed sites. However, high spatial heterogeneity in light and higher rates of species turnover were both strongly related to increases in plant species richness in grazed areas. This suggests that creation of a mosaic of patches with high and low biomass (the primary determinant of light availability in mesic grasslands) and promotion of a dynamic species pool are the most important mechanisms by which grazers affect species richness in high productivity grasslands.}, keywords = {LTER-KNZ, colonization, Grazing, heterogeneity, Species richness, tallgrass prairie}, doi = {10.1007/s00442-003-1360-y}, author = {Bakker, C. and John M. Blair and Alan K. Knapp} } @article {KNZ00838, title = {Consequences of shrub expansion in mesic grassland: resource alterations and graminoid responses}, journal = {Journal of Vegetation Science}, volume = {14}, year = {2003}, pages = {487 -496}, abstract = {In the mesic grasslands of the central United States, the shrub Cornus drummondii has undergone widespread expansion in the absence of recurrent fire. We quantified alterations in light, water and N caused by C. drummondii expansion in tall-grass prairie and assessed the hypothesis that these alterations are consistent with models of resource enrichment by woody plants. Responses in graminoid species, particularly the dominant C4 grass Andropogon gerardii, were concurrently evaluated. We also removed established shrub islands to quantify their legacy effect on resource availability and assess the capability of this grassland to recover in sites formerly dominated by woody plants. The primary effect of shrub expansion on resource availability was an 87\% reduction in light available to the herbaceous understorey. This reduced C uptake and N use efficiency in A. gerardii and lowered graminoid cover and ANPP at the grass-shrub ecotone relative to undisturbed grassland. Shrub removal created a pulse in light and N availability, eliciting high C gain in A. gerardii in the first year after removal. By year two, light and N availability within shrub removal areas returned to levels typical of grassland, as had graminoid cover and ANPP were similar to those in open grassland. Recovery within central areas of shrub removal sites lagged behind that at the former grass-shrub ecotone. These results indicate that the apparent alternative stable state of C. drummondii dominance in tall-grass prairie is biotically maintained and driven by reductions in light, rather than resource enrichment. Within areas of shrub removal, the legacy effect of C. drummondii dominance is manifest primarily through the loss of rhizomes of the dominant grasses, rather than any long-term changes in resource availability. C. drummondii removal facilitates grassland recovery, but the effort required to initiate this transition is a significant cost of woody plant expansion in mesic grasslands. Prevention of woody plant expansion in remnant tall-grass prairies is, therefore, a preferred management option.}, keywords = {LTER-KNZ, Cornus drummondii, Grassland recovery, land cover change, Tall-grass prairie, Woody plant expansion}, doi = {10.1111/j.1654-1103.2003.tb02175.x}, author = {Lett, M.S. and Alan K. Knapp} } @article {KNZ00868, title = {Dominant species maintain ecosystem function with non-random species loss}, journal = {Ecology Letters}, volume = {6}, year = {2003}, pages = {509 -517}, abstract = {Loss of species caused by widespread stressors, such as drought and fragmentation, is likely to be non-random depending on species abundance in the community. We experimentally reduced the number of rare and uncommon plant species while independently reducing only the abundance of dominant grass species in intact, native grassland. This allowed us to simulate a non-random pattern of species loss, based on species abundances, from communities shaped by natural ecological interactions and characterized by uneven species abundance distributions. Over two growing seasons, total above-ground net primary productivity (ANPP) declined with reductions in abundance of the dominant species but was unaffected by a threefold decline in richness of less common species. In contrast, productivity of the remaining rare and uncommon species decreased with declining richness, in part due to loss of complementary interactions among these species. However, increased production of the dominant grasses offset the negative effects of species loss. We conclude that the dominant species, as controllers of ecosystem function, can provide short-term resistance to reductions in ecosystem function when species loss is nonrandom. However, the concurrent loss of complementary interactions among rare and uncommon species, the most diverse component of communities, may contribute to additional species loss and portends erosion of ecosystem function in the long term.}, keywords = {LTER-KNZ}, doi = {10.1046/j.1461-0248.2003.00454.x}, author = {M.D. Smith and Alan K. Knapp} } @article {KNZ00845, title = {The importance of land-use legacies to ecology and conservation}, journal = {BioScience}, volume = {53}, year = {2003}, pages = {77 -88}, abstract = {Recognition of the importance of land-use history and its legacies in most ecological systems has been a major factor driving the recent focus on human activity as a legitimate and essential subject of environmental science. Ecologists, conservationists, and natural resource policymakers now recognize that the legacies of land-use activities continue to influence ecosystem structure and function for decades or centuries{\textemdash}or even longer{\textemdash}after those activities have ceased. Consequently, recognition of these historical legacies adds explanatory power to our understanding of modern conditions at scales from organisms to the globe and reduces missteps in anticipating or managing for future conditions. As a result, environmental history emerges as an integral part of ecological science and conservation planning. By considering diverse ecological phenomena, ranging from biodiversity and biogeochemical cycles to ecosystem resilience to anthropogenic stress, and by examining terrestrial and aquatic ecosystems in temperate to tropical biomes, this article demonstrates the ubiquity and importance of land-use legacies to environmental science and management.}, keywords = {LTER-KNZ, conservation, disturbance, ecosystem process, Land use, natural resource management}, doi = {10.1641/0006-3568(2003)053[0077:TIOLUL]2.0.CO;2}, author = {D.R. Foster and Swanson, F. and Aber, J. and I. Burke and Brokaw, N. and Tilman, D. and Alan K. Knapp} } @article {KNZ00820, title = {The influence of disturbance from military training activities on terrestrial-aquatic linkages in a tallgrass prairie ecosystem}, journal = {Ecological Applications}, volume = {13}, year = {2003}, pages = {432 -442}, abstract = {The role of allochthonous organic matter in lotic ecosystems has been an important research topic among aquatic ecologists since the seminal work by Lindeman was published in 1942. Since 1986, studies on organic matter budgets, ecosystem metabolism, and decomposition published in J-NABS have made significant contributions to the overall understanding of organic matter dynamics in streams. In this review, we summarize the utility of organic matter budgets, cover the major advances in research on ecosystem metabolism, and describe the intrinsic and extrinsic factors influencing organic matter decomposition. We also discuss future directions and current applications of research and highlight the need for additional studies on the role of land use and climate change, as well as continued use of organic matter processing as a functional metric in biomonitoring studies. We emphasize the need for continued data synthesis into comprehensive organic matter budgets. Such comparative studies can elucidate important drivers of organic matter dynamics and can assist in the understanding of large continental/global changes that might be occurring now and in the near future. In general, continued emphasis on synthesizing information into a larger framework for streams and rivers will improve our overall understanding of the importance of organic matter in lotic ecosystems.}, keywords = {LTER-KNZ, carbon cycling, decomposition, litter breakdown, metabolism, organic matter budgets, organic matter dynamics, streams}, doi = {10.1899/08-170.1}, author = {Quist, M.C. and P. Fay and Guy, C.S. and Alan K. Knapp and Rubenstein, B.N.} } @article {KNZ00830, title = {Long-term patterns of shrub expansion in a C4-dominated grassland: fire frequency and the dynamics of shrub cover and abundance}, journal = {American Journal of Botany}, volume = {90}, year = {2003}, pages = {423 -428}, abstract = {Worldwide, grassland ecosystems have experienced a major shift in growth-form dominance as woody plant species have expanded and replaced native grasses. In the C4-dominated grasslands of central North America, a reduction in fire frequency is the most cited cause of this shift in growth forms as fire both enhances grass productivity and constrains the establishment and expansion of native woody vegetation. Using an 18-yr plant species composition data set, we quantified patterns of change in shrub cover, frequency, and species richness associated with three distinct fire regimes. During the study period (1983{\textendash}2000), shrub cover increased most dramatically in sites in which the frequency of fire was once every 4 yr (intermediate frequency; 28.6\%) followed by sites in which fire occurred only once during the 18-yr period (low frequency; 23.7\%). Annual fire effectively prevented the recruitment of new woody species, but even with this high fire frequency, shrub cover increased slightly (3.7\%). Comparatively, shrub species richness increased by three and six, respectively, in the intermediate- and low-frequency fire sites. These data indicate that within this grassland, periods without fire are necessary for recruitment of both new individuals and additional shrub species; however, once established, shrub cover will increase regardless of fire frequency and even annual fire will not reduce shrub abundance.}, keywords = {LTER-KNZ, C4 grassland, Fire frequency, growth form substitution, Shrubs, woody encroachment}, doi = {10.3732/ajb.90.3.423}, author = {Heisler, J.L. and J. M. Briggs and Alan K. Knapp} } @article {KNZ00863, title = {Productivity responses to altered rainfall patterns in a C4-dominated grassland}, journal = {Oecologia}, volume = {137}, year = {2003}, pages = {245 -251}, abstract = {Rainfall variability is a key driver of ecosystem structure and function in grasslands worldwide. Changes in rainfall patterns predicted by global climate models for the central United States are expected to cause lower and increasingly variable soil water availability, which may impact net primary production and plant species composition in native Great Plains grasslands. We experimentally altered the timing and quantity of growing season rainfall inputs by lengthening inter-rainfall dry intervals by 50\%, reducing rainfall quantities by 30\%, or both, compared to the ambient rainfall regime in a native tallgrass prairie ecosystem in northeastern Kansas. Over three growing seasons, increased rainfall variability caused by altered rainfall timing with no change in total rainfall quantity led to lower and more variable soil water content (0{\textendash}30 cm depth), a ~10\% reduction in aboveground net primary productivity (ANPP), increased root to shoot ratios, and greater canopy photon flux density at 30 cm above the soil surface. Lower total ANPP primarily resulted from reduced growth, biomass and flowering of subdominant warm-season C4 grasses while productivity of the dominant C4 grass Andropogon gerardii was relatively unresponsive. In general, vegetation responses to increased soil water content variability were at least equal to those caused by imposing a 30\% reduction in rainfall quantity without altering the timing of rainfall inputs. Reduced ANPP most likely resulted from direct effects of soil moisture deficits on root activity, plant water status, and photosynthesis. Altered rainfall regimes are likely to be an important element of climate change scenarios in this grassland, and the nature of interactions with other climate change elements remains a significant challenge for predicting ecosystem responses to climate change.}, keywords = {LTER-KNZ, Climate change, Konza Prairie, Net primary productivity, Precipitation, soil moisture}, doi = {10.1007/s00442-003-1331-3}, author = {P. Fay and Carlisle, J.D. and Alan K. Knapp and John M. Blair and Scott. L. Collins} } @inbook {KNZ00847, title = {Rainfall timing, soil moisture dynamics, and plant responses in a mesic tallgrass prairie ecosystem}, booktitle = {Precipitation Regimes and Terrestrial Ecosystems. A North American Perspective}, year = {2003}, pages = {147 -163}, publisher = {University of Arizona Press}, organization = {University of Arizona Press}, address = {Tucson, AZ}, keywords = {LTER-KNZ, tallgrass prairie}, author = {P. Fay and Alan K. Knapp and John M. Blair and Carlisle, J.D. and Danner, B.T. and McCarron, J.K.}, editor = {Weltzin, J.F. and McPherson, G.R.} } @article {KNZ00862, title = {Soil C and N responses to woody plant expansion in a mesic grassland}, journal = {Plant and Soil}, volume = {257}, year = {2003}, pages = {183 -192}, abstract = {Changes in land management and reductions in fire frequency have contributed to increased cover of woody species in grasslands worldwide. These shifts in plant community composition have the potential to alter ecosystem function, particularly through changes in soil processes and properties. In semi-arid grasslands, the invasion of shrubs and trees is often accompanied by increases in soil resources and more rapid N and C cycling. We assessed the effects of shrub encroachment in a mesic grassland in Kansas (USA) on soil CO2 flux, extractable inorganic N, and N mineralization beneath shrub communities (Cornus drummondii) and surrounding undisturbed grassland sites. In this study, a shift in plant community composition from grassland to shrubland resulted in a 16\% decrease in annual soil CO2 flux(4.78 kg CO2 m-2 year-1 for shrub dominated sites versus 5.84 kg CO2 m-2 year-1 for grassland sites) with no differences in total soil C or N or inorganic N. There was considerable variability in N mineralization rates within sites, which resulted in no overall difference in cumulative N mineralized during this study (4.09 g N m-2 for grassland sites and 3.03 g N m-2 for shrub islands). These results indicate that shrub encroachment into mesic grasslands does not significantly alter N availability (at least initially), but does alter C cycling by decreasing soil CO2 flux.}, keywords = {LTER-KNZ, N availability, N mineralization, shrub invasion, soil CO2 flux, soil properties}, doi = {10.1023/A:1026255214393}, author = {McCarron, J.K. and Alan K. Knapp and John M. Blair} } @article {KNZ00822, title = {Soil resources regulate productivity and diversity in newly established tallgrass prairie}, journal = {Ecology}, volume = {84}, year = {2003}, pages = {724 -735}, abstract = {In native tallgrass prairie, soil depth and nitrogen (N) availability strongly influence aboveground net primary productivity (ANPP) and plant species composition. We manipulated these factors in a newly restored grassland to determine if these resources similarly constrain productivity and diversity during the initial three years of grassland establishment. Four types of experimental plots with six treatment combinations of deep and shallow soil at reduced-, ambient-, and enriched-N availability formed the basis of this study. The soil responses to the experimental treatments were examined over three years, and patterns in diversity and productivity were examined in year 3. The soil depth treatment did not significantly affect soil carbon (C) and N pools or ANPP and diversity. A pulse amendment of C added to the soil prior to planting increased soil microbial biomass and decreased potential net N mineralization rates to effectively reduce N availability throughout the study. Nitrogen availability declined over time in nonamended soils as a result of plant establishment, but adding fertilizer N alleviated the increasing immobilization potential of the soil. The level of ANPP was lowest and diversity highest in the reduced-N treatment, whereas the enriched-N treatment resulted in high productivity, but low diversity. As a result, diversity was inversely correlated with productivity in these newly established communities. The same mechanism invoked to explain decreased diversity under nutrient enrichment in old-field ecosystems and native grasslands (e.g., reduced light availability with increased production) was supported in the restored prairie by the positive relationship between ANPP and intercepted light, and a strong correlation between light availability and diversity. The effects of nutrient availability on plant community composition (diversity and richness) were due primarily to the responses of prairie species, as the productivity of early successional, nonprairie species was less than 1\% of total ANPP after three years of establishment. These results show that the effects of resource availability on productivity and diversity are similar in young and mature grasslands, and that manipulation of a limiting nutrient during grassland establishment can influence floristic composition, with consequences for long-term patterns of diversity in restored ecosystems.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1890/0012-9658(2003)084[0724:SRRPAD]2.0.CO;2}, author = {S.G. Baer and John M. Blair and Alan K. Knapp and Scott. L. Collins} } @article {KNZ005, title = {Altered rainfall patterns, gas exchange and growth in C3 and C4 grassland species}, journal = {International Journal of Plant Sciences}, volume = {163}, year = {2002}, pages = {549 -557}, keywords = {LTER-KNZ}, author = {Fay, P.A. and Carlisle, J.D. and Danner, B.T. and Lett, M.S. and McCarron, J.K. and Stewart, C. and Alan K. Knapp and John M. Blair and Scott. L. Collins} } @article {KNZ002, title = {Expansion of woody plants in tallgrass prairie: a 15 year study of fire and fire-grazing interactions}, journal = {The American Midland Naturalist}, volume = {147}, year = {2002}, pages = {287 -294}, abstract = {Temporal changes in the abundance of trees and a common shrub, Cornus drummondii, were quantified for 15 y (1981{\textendash}1996) in seven tallgrass prairie watersheds in Northeast Kansas. Woody plant responses to different fire frequencies and grazing were assessed with a data set that included >9000 individuals. Although 15 tree species were included in this data set, only four (Juniperus virginiana, Celtis occidentalis, Gleditsia triacanthos and Ulmus americana) were sufficiently abundant for detailed analysis. Over the 15 y study tree density increased by two- to 10-fold, except in watersheds burned annually where woody plants remained almost completely absent throughout the study. Although increased woody plant abundance was expected in watersheds protected from fire, tree and shrub density also increased substantially in watersheds burned only once in 4 y. An intermediate fire frequency (burned every 3{\textendash}5 y) actually increased the abundance of C. drummondii relative to a low fire frequency (burned only once in 15 y). Moreover, a severe wildfire in 1991, which affected all watersheds, did not markedly reverse this pattern of increase in abundance in most tree species. Four years after the addition of native herbivores (Bos bison) to three of the long-term experimental watersheds (infrequently and annually burned) woody plant abundance increased by four- and 40-fold, respectively, compared to corresponding ungrazed watersheds. Thus, the presence of large ungulate grazers in tallgrass prairie resulted in a significant increase in woody plant abundance. The most parsimonious explanation for this phenomenon is that fire intensity and extent was reduced in grazed grasslands allowing greater success of woody species.}, keywords = {LTER-KNZ, tallgrass prairie, woody plant}, doi = {10.1674/0003-0031(2002)147[0287:EOWPIT]2.0.CO;2}, author = {J. M. Briggs and Alan K. Knapp and B. Brock} } @article {KNZ00826, title = {Long-term responses of the grassland co-dominants Andropogon gerardii and Sorghastrum nutans to changes in climate and management}, journal = {Plant Ecology}, volume = {163}, year = {2002}, pages = {15 -22}, abstract = {Understanding responses of both dominant and sub-dominant plant speciesto management regimes and climatic variables is a critical component ofunderstanding an ecosystem{\textquoteright}s potential response to local or regional changes insuch parameters. The dominant vegetation of much of the North Americantallgrassprairie is composed of C4 grasses. As the most abundant species,Andropogon gerardii has been well characterizedecologically. The second most abundant, co-dominant species,Sorghastrum nutans, has received less attention, mainlybecause it has been considered the {\textquotedblleft}ecological equivalent{\textquotedblright} ofA. gerardii and is assumed to have responses similar tothose of A. gerardii. The objective of this study was toassess the validity of this assumption by examining long-term abundancepatternsand responses to climate, fire, and grazing for both species. Based on 15 years(1983{\textendash}1997) of plant species composition and cover data collected atKonzaPrairie Biological Station in northeastern Kansas, USA, we were able to detectimportant differences between these two grasses. Regression analysis showedthatlong-term reductions in the cover of A. gerardii werecorrelated with decreasing summer maximum temperatures, whereas inter-annualvariations in the cover of S. nutans were related toannualprecipitation. Additionally, the cover of S. nutansincreased with increasing fire frequency, while the cover of A.gerardii was not significantly affected by fire frequency. Bisongrazing did not significantly affect the cover of either species studied. Themean cover of A. gerardii decreased significantly over the15 years, but only a slight change was noted in the cover of S.nutans. The decrease in A. gerardiicorrespondedto increased plant species diversity and evenness and has the potential toimpact the tallgrass prairie ecosystem from population through ecosystemlevels.The observed differences in these co-dominant species may prove important underclimate change scenarios predicted for the tallgrass prairie of North America.}, keywords = {LTER-KNZ, C4 grasses, Cover, diversity, Dominance, fire, prairie, tallgrass}, doi = {10.1023/A:1020320214750}, author = {Silletti, A.M. and Alan K. Knapp} } @article {KNZ00827, title = {Rainfall variability, carbon cycling and plant species diversity in a mesic grassland}, journal = {Science}, volume = {298}, year = {2002}, pages = {2202 -2205}, abstract = {

Ecosystem responses to increased variability in rainfall, a prediction of general circulation models, were assessed in native grassland by reducing storm frequency and increasing rainfall quantity per storm during a 4-year experiment. More extreme rainfall patterns, without concurrent changes in total rainfall quantity, increased temporal variability in soil moisture and plant species diversity. However, carbon cycling processes such as soil CO2 flux, CO2 uptake by the dominant grasses, and aboveground net primary productivity (ANPP) were reduced, and ANPP was more responsive to soil moisture variability than to mean soil water content. Our results show that projected increases in rainfall variability can rapidly alter key carbon cycling processes and plant community composition, independent of changes in total precipitation.

}, keywords = {LTER-KNZ}, doi = {10.1126/science.1076347}, url = {https://science.sciencemag.org/content/298/5601/2202}, author = {Alan K. Knapp and Fay, P.A. and John M. Blair and Scott. L. Collins and M.D. Smith and Carlisle, J.D. and Harper, C.W. and Danner, B.T. and Lett, M.S. and McCarron, J.K.} } @article {KNZ00801, title = {White-tailed deer browsing on six shrub species of tallgrass prairie}, journal = {Great Plains Research}, volume = {12}, year = {2002}, pages = {141 -156}, abstract = {Native grasses and forbs were once dominant in tallgrass prairies, but nonnative plants have largely replaced natives in most grassland of Illinois. Ample evidence indicates that white-tailed deer (Odocoileus virginianus) have a strong impact on native forests and prairie, but we know little about their impact on nonnative grasslands. This study tested the hypothesis that foraging by deer on native forbs and woody plants increases the dominance of introduced grasses as succession proceeds in old fields of central Illinois. We tested that hypothesis using fencing that excluded deer, but not other herbivores, from replicated plots in three old fields at different successional stages. The composition of the plant community changed rapidly in early succession and showed relatively little effect of deer, other than slowing the invasion by woody plants. In mid- and late successional fields, however, fenced plots had higher relative abundance of native forbs, particularly goldenrod (Solidago spp.), and lower relative abundance of introduced perennial grasses than did control plots. Thus, deer facilitated introduced perennial grasses and inhibited native forbs and woody plants in old fields, thereby delaying succession to deciduous forest.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1674/0003-0031-170.2.323}, author = {Van Der Hoek, D.J. and Alan K. Knapp and J. M. Briggs and Bokdam, J.} } @article {KNZ00779, title = {C3 woody plant expansion in a C4 grassland: are grasses and shrubs functionally distinct?}, journal = {American Journal of Botany}, volume = {88}, year = {2001}, pages = {1818 -1823}, abstract = {

The expansion of C3 shrubs into C4-dominated tallgrass prairies represents a fundamental shift in growth-form dominance accompanied by changes in resource acquisition and use. We assessed these changes by comparing the ecophysiological traits of the dominant C4 grass Andropogon gerardii, with traits of three C3 invasive shrub species, Cornus drummondii, Prunus americana, and Rhus glabra. We tested the hypothesis that ecophysiological traits of the shrubs would be similar within this growth form but distinct from grasses and that these species would conform to the two-layer soil water model. Photosynthetic rates in R. glabra were similar to A. gerardii and higher than in the other two shrubs, while water use efficiency was markedly greater in A. gerardii. Among all species, midday xylem pressure potentials (XPP) were distinctly lower (70\%) for P. americana, but were similar among the other species. Predawn XPP was related to soil water at shallow depths for A. gerardii (r2 = 0.59) and P. americana (r2 = 0.62), and to deeper soil moisture for R. glabra (r2 = 0.63); there was no relationship for C. drummondii at any soil depth. Thus, a simple two-layer soil water model for partitioning shrub/grass resource acquisition was not appropriate for this grassland. We conclude that these shrubs could not be considered functional equivalents from an ecophysiological perspective, nor were they, as a group, distinct from A. gerardii in resource acquisition and use

}, keywords = {LTER-KNZ}, url = {http://www.amjbot.org/cgi/content/abstract/88/10/1818}, author = {McCarron, J.K. and Alan K. Knapp} } @article {KNZ00864, title = {Carbon and water relations of juvenile Quercus species in tallgrass prairie}, journal = {Journal of Vegetation Science}, volume = {12}, year = {2001}, pages = {807 -816}, abstract = {In ecosystems where environments are extreme, such as deserts, adult plant species may facilitate the establishment and growth of seedlings and juveniles. Because high temperatures and evaporative demand characterize tall-grass prairies of the central United States (relative to forests), we predicted that the grassland-forest ecotone, by minimizing temperature extremes and moderating water stress, may function to facilitate the expansion of Quercus species into undisturbed tall-grass prairie. We assessed the carbon and water relations of juvenile Quercus macrocarpa and Q. muhlenbergii, the dominant tree species in gallery forests of northeast Kansas, in ecotone and prairie sites. To evaluate the potentially competitive effects of neighboring herbaceous biomass on these oaks, juveniles (< 0.5 m tall) of both species also were subjected to either: (1) removal of surrounding above-ground herbaceous biomass, or (2) control (prairie community intact) treatments. Herbaceous biomass removal had no significant effect on gas exchange or water relations in these oak species in either the prairie or the ecotone environment. Although the ecotone did alleviate some environmental extremes, photosynthetic rates and stomatal conductance were ca. 20 \% higher (p < 0.05) in both oaks in prairie sites vs. the ecotone. Moreover, although leaf temperatures on average were higher in oaks in the prairie, high leaf temperatures in the ecotone had a greater negative effect on photosynthesis. These data suggest that the grassland-forest ecotone did not facilitate the growth of Quercus juveniles expanding into this grassland. Moreover, the carbon and water relations of juvenile oaks in the prairie appeared to be unaffected by the presence of the dominant C4 grasses.}, keywords = {LTER-KNZ, Ecotone, Facilitation, forest expansion, Quercus, tallgrass prairie}, doi = {10.2307/3236868}, author = {Fay, P.A. and Carlisle, J.D. and Danner, B.T. and Lett, M.S. and McCarron, J.K. and Stewart, C. and Alan K. Knapp and John M. Blair and Scott. L. Collins} } @article {KNZ00756, title = {Determinants of C3 forb growth and production in a C4 dominated grassland}, journal = {Plant Ecology}, volume = {152}, year = {2001}, pages = {93 -100}, abstract = {Forbs are the most abundant species within the vascular flora of tallgrass prairie and they make the greatest contribution to biodiversity of all growth forms. However, little is known about the factors that determine their productivity and growth rates. The objective of this study was to assess the controls of forb growth (absolute and relative) and production in tallgrass prairie from a long-term burning experiment at the Konza Prairie in NE Kansas. Over the 14-year study, forbs comprised 9\% of the total biomass production on sites with a high fire frequency vs. 29\% on the low fire frequency site, with gramminoids accounting for the remainder. Although interannual variations in peak biomass of the grasses was strongly correlated with environmental variables related to water availability, there were no similar relationships for forbs, suggesting that production of forbs and grasses responded to interannual variations in climate in different ways. Multivariate analysis of climatic controls on growth rates of grasses and forbs yielded similar results. Although forbs had low biomass and absolute growth per unit ground area in frequently burned prairie, their relative growth rates were highest in such sites. Thus, it appears that reduced growth rates of individual forbs per se do not limit forb success in annually burned prairie. Instead, direct negative effects of fire on forbs (increased mortality) may be responsible. Determinants of forb growth and productivity in unburned prairie remain unresolved.}, keywords = {LTER-KNZ, aboveground net primary production, Biodiversity, fire, forbs, Graminoids, Relative growth rates, tallgrass prairie}, doi = {10.1023/A:1011400101014}, author = {J. M. Briggs and Alan K. Knapp} } @article {KNZ00774, title = {Frequency and extent of water limitation to primary production in a mesic temperate grassland}, journal = {Ecosystems}, volume = {4}, year = {2001}, pages = {19 -28}, abstract = {The frequency and extent of water limitation to aboveground net primary production (ANPP) in a mesic grassland in NE Kansas (Konza Prairie, USA) was assessed with an 8-year irrigation experiment. Since 1991, transects spanning upland and lowland sites in annually burned, ungrazed tallgrass prairie were provided with supplemental water to satisfy evapotranspirational demands. This protocol minimized water limitations during the growing season, as well as interannual variability in water stress. Irrigation of this mesic grassland increased ANPP in 6 of 8 years by an average of 26\% when compared to control transects. Although interannual variation in ANPP was greater in uplands than lowlands at nominal levels of precipitation, reducing interannual variability in water availability via irrigation eliminated topographic differences; the irrigation protocol also reduced interannual variability in ANPP by as much as 40\%. The addition of supplemental water enabled us to extend the relationship between annual precipitation and ANPP in grasslands to precipitation levels (average, 1153 mm; maximum, 1346 mm) similar to those experienced by more mesic grasslands that today exist only as remnants several hundred kilometers east of Kansas. This relationship was linear (r 2= 0.81), with maximum ANPP (738 g/m2) similar to values reported for sites in Illinois and Wisconsin. After 8 years of irrigation, production of the C3 forb component was twice that in control sites. These results indicate that water limitations in grasslands at the western edge of the presettlement extent of tallgrass prairie affect ANPP in most years and that this high frequency of water limitation may lead to greater dominance of the C4 grasses than is seen in more eastern grassland sites.}, keywords = {LTER-KNZ, Andropogon gerardii, C 4 grasses, forbs, grassland, irrigation, leaf water potential, Primary production, tallgrass prairie, water limitation}, doi = {10.1007/s100210000057}, author = {Alan K. Knapp and J. M. Briggs and Koelliker, J.K.} } @article {KNZ00763, title = {Growth dynamics of gallery forest oak seedlings (Quercus macrocarpa Michx. and Quercus muhlenbergii Engelm.) from gallery forests: implications for forest expansion into grasslands}, journal = {Trees}, volume = {15}, year = {2001}, pages = {271 -277}, abstract = {Seedling growth dynamics of Quercus macrocarpa Michx. and Quercus muhlenbergii Engelm. were compared over a 3-month period under optimal growth conditions. These two species are the dominant trees at the western limit of the eastern deciduous forest, and are typically confined to gallery forests along stream beds in tallgrass prairie. Since tallgrass prairie is characterized by a highly variable climate and is prone to periodic drought, we hypothesized that these oaks would have rapid root growth and produce deep taproots as seedlings, enabling them to avoid drought stress and persist in this region. These traits may also facilitate forest expansion into the more xeric tallgrass prairie if fires are suppressed. Taproots of Q. macrocarpa and Q. muhlenbergii grew to approximately 140 cm and 100 cm in length, respectively, after 104 days. In both species, 65\% or more of seedling biomass was allocated below ground, and root/total biomass was significantly greater in Q. muhlenbergii at 0{\textendash}20 and 21{\textendash}40 days after germination. The seedling taproot elongation rates reported here are much greater than rates reported in other eastern deciduous forest trees. Long-term precipitation data and soil moisture patterns from tallgrass prairie, when combined with rapid taproot elongation rates, suggest that soil moisture may not limit oak establishment or growth in tallgrass prairie in most years, although water uptake by roots was not measured in this study. Other factors, such as fire, herbivory, and seed predation and dispersal may be equally important in constraining the distribution of these species to gallery forests.}, keywords = {LTER-KNZ, Gallery forests, Quercus, Seedling, Taproot}, doi = {10.1007/s004680100103}, author = {Danner, B.T. and Alan K. Knapp} } @article {KNZ00759, title = {Leaf optical properties in higher plants: linking spectral characteristics to stress and chlorophyll concentration}, journal = {American Journal of Botany}, volume = {88}, year = {2001}, pages = {677 -684}, keywords = {LTER-KNZ}, author = {Carter, G.A. and Alan K. Knapp} } @article {KNZ00788, title = {Physiological and morphological traits of exotic, invasive exotic, and native plant species in tallgrass prairie}, journal = {International Journal of Plant Sciences}, volume = {162}, year = {2001}, pages = {785 -792}, abstract = {We compared 13 traits of invasive exotic, noninvasive exotic, and ecologically similar native species to determine if there are generalizable differences among these groups that relate to persistence and spread of exotic species in tallgrass prairie plant communities. When species were grouped as invasive (two species), noninvasive (five species), and native (six species), no differences were found for the suite of traits examined, likely because of the high variability within and between groups. However, when exotic species, regardless of invasiveness, were compared with the native species, specific leaf area was ca. 40\% higher for the exotic species, a result that is consistent with that of other studies. This pattern was also observed for five of seven pairwise comparisons of exotic and native species with similar life history traits. In contrast, total end-of-season biomass was as much as three times higher for the native species in five of seven of the native-exotic species pairs. For other traits, differences between exotic and native species were species-specific and were generally more numerous for noninvasive than for invasive exotic species pair-wise comparisons. Thus, contrary to predictions, exotic species capable of successfully invading tallgrass prairie did not differ considerably from native species in most traits related to resource utilization and carbon gain. Moreover, invasive exotic species, those capable of displacing native species and dominating a community, were not distinct for the observed traits from their native counterparts. These results indicate that other traits, such as the ability to respond to resource pulses or herbivory, may explain more effectively why certain invasive species are able to invade these communities aggressively.}, keywords = {LTER-KNZ, biomass allocation, grassland, invasive plants, photosynthesis, specific leaf area}, doi = {10.1086/320774}, author = {M.D. Smith and Alan K. Knapp} } @article {KNZ00787, title = {Responses of the co-dominant grassland species Andropogon gerardii and Sorghastrum nutans to long-term manipulations of nitrogen and water}, journal = {American Midland Naturalist}, volume = {145}, year = {2001}, pages = {159 -167}, abstract = {The availability and distribution of essential resources such as water and nitrogen are changing on a global scale and it is important to determine the effects of such alterations on dominant and subdominant species in ecosystems. We examined the effects of long-term additions of water and nitrogen on leaf-level ecophysiological characteristics of the tallgrass prairie codominants Andropogon gerardii (big bluestem) and Sorghastrum nutans (Indian grass). Both species responded to irrigation with increased water potentials and to fertilization with decreased water potentials. Only in S. nutans, however, were net photosynthesis and stomatal conductance greater in response to both water and nitrogen addition. In contrast, A. gerardii, which currently is more abundant than S. nutans, was relatively unresponsive to resource manipulations. If net photosynthesis is positively related to plant productivity, as has been shown for tallgrass prairie species, the differential photosynthetic responses of these two grasses suggests that their patterns of abundance may change in the future if resource availability is altered. These results are also inconsistent with the historical assumption that these two grasses are {\textquotedblleft}ecological equivalents.{\textquotedblright}}, keywords = {LTER-KNZ}, doi = {10.1674/0003-0031(2001)145[0159:ROTCGS]2.0.CO;2}, author = {Silletti, A.M. and Alan K. Knapp} } @article {KNZ00789, title = {Size of the local species pool determines invasibility of a C4-dominated grassland}, journal = {Oikos}, volume = {92}, year = {2001}, pages = {55 -61}, abstract = {The size of the local species pool (i.e., species surrounding a community capable of dispersal into that community) and other dispersal limitations strongly influence native plant community composition. However, the role that the local species pool plays in determining the invasibility of communities by exotic plants remains to be evaluated. We hypothesized that the richness and abundance of exotic species would be greater in C4-dominated grassland communities if the local species pool included a larger proportion of exotic species. We also predicted that an increase in the exotic species pool would increase the invasibility of sites thought to be resistant to invasion (annually burned grassland). To test these hypotheses, study plots were established within two long-term (>20 yr) fire experiments at a tallgrass prairie preserve in NE Kansas (USA). Study plots were surrounded by either a small pool of exotic species (small species pool (SSP) plots; six species) or a larger exotic species pool (large species pool (LSP) plots; 18 species). We found that richness and absolute cover of exotic species was significantly (P<0.001) lower (\~{}70 and 90\%, respectively) in annually burned compared to unburned plots, regardless of the size of the exotic species pool. As predicted, exotic species richness was higher (P<0.001) for LSP plots (3.9 per 250 m2) than for SSP plots (0.7 per 250 m2); however, absolute cover was unaffected by the size of the exotic species pool. In the absence of fire, plots with a LSP had four times as many exotic species than SSP plots. An increase in the local exotic species pool also increased the invasibility of annually burned grassland. Indeed, richness of exotic plant species in annually burned LSP plots did not differ from unburned plots with a SSP, indicating that a larger pool of exotic species countered the negative effects of fire. These findings have important implications for predicting how the invasion of plant communities may respond to human-induced global changes, such as habitat fragmentation. Community characteristics or factors such as frequent fires in grasslands may impart resistance to invasions by exotic species in large, intact ecosystems. However, when a large pool of exotic species is present, frequent fire may not be sufficient to limit the invasions of exotic plants in fragmented landscapes.}, keywords = {LTER-KNZ}, doi = {10.1034/j.1600-0706.2001.920107.x}, author = {M.D. Smith and Alan K. Knapp} } @article {KNZ00775, title = {Variation among biomes in temporal dynamics of aboveground primary production}, journal = {Science}, volume = {291}, year = {2001}, pages = {481 -484}, abstract = {Interannual variability in aboveground net primary production (ANPP) was assessed with long-term (mean = 12 years) data from 11 Long Term Ecological Research sites across North America. The greatest interannual variability in ANPP occurred in grasslands and old fields, with forests the least variable. At a continental scale, ANPP was strongly correlated with annual precipitation. However, interannual variability in ANPP was not related to variability in precipitation. Instead, maximum variability in ANPP occurred in biomes where high potential growth rates of herbaceous vegetation were combined with moderate variability in precipitation. In the most dynamic biomes, ANPP responded more strongly to wet than to dry years. Recognition of the fourfold range in ANPP dynamics across biomes and of the factors that constrain this variability is critical for detecting the biotic impacts of global change phenomena.}, keywords = {LTER-KNZ, Primary production}, doi = {10.1126/science.291.5503.481}, author = {Alan K. Knapp and M.D. Smith} } @article {KNZ00858, title = {Altering rainfall timing and quantity in a mesic grassland ecosystem: Design and performance of rainfall manipulation shelters}, journal = {Ecosystems}, volume = {3}, year = {2000}, pages = {308 -319}, abstract = {Global climate change is predicted to alter growing season rainfall patterns, potentially reducing total amounts of growing season precipitation and redistributing rainfall into fewer but larger individual events. Such changes may affect numerous soil, plant, and ecosystem properties in grasslands and ultimately impact their productivity and biological diversity. Rainout shelters are useful tools for experimental manipulations of rainfall patterns, and permanent fixed-location shelters were established in 1997 to conduct the Rainfall Manipulation Plot study in a mesic tallgrass prairie ecosystem in northeastern Kansas. Twelve 9 x 14{\textendash}m fixed-location rainfall manipulation shelters were constructed to impose factorial combinations of 30\% reduced rainfall quantity and 50\% greater interrainfall dry periods on 6 x 6{\textendash}m plots, to examine how altered rainfall regimes may affect plant species composition, nutrient cycling, and above- and belowground plant growth dynamics. The shelters provided complete control of growing season rainfall patterns, whereas effects on photosynthetic photon flux density, nighttime net radiation, and soil temperature generally were comparable to other similar shelter designs. Soil and plant responses to the first growing season of rainfall manipulations (1998) suggested that the interval between rainfall events may be a primary driver in grassland ecosystem responses to altered rainfall patterns. Aboveground net primary productivity, soil CO2 flux, and flowering duration were reduced by the increased interrainfall intervals and were mostly unaffected by reduced rainfall quantity. The timing of rainfall events and resulting temporal patterns of soil moisture relative to critical times for microbial activity, biomass accumulation, plant life histories, and other ecological properties may regulate longer-term responses to altered rainfall patterns.}, keywords = {LTER-KNZ, Climate change, floristic diversity, Grasslands, Konza Prairie, life histories, long-term research, Net primary production, precipitation patterns, rainout shelters, soil moisture}, doi = {10.1007/s100210000028}, author = {Fay, P.A. and Carlisle, J.D. and Alan K. Knapp and John M. Blair and Scott. L. Collins} } @article {KNZ00720, title = {Ecosystems as functional units in nature}, journal = {Natural Resources and Environment}, volume = {14}, year = {2000}, pages = {150 -155}, keywords = {LTER-KNZ}, author = {John M. Blair and Scott. L. Collins and Alan K. Knapp} } @article {KNZ00740, title = {Patterns and determinants of potential carbon gain in the C3 evergreen Yucca glauca (Liliaceae) in a C4 grassland}, journal = {American Journal of Botany}, volume = {87}, year = {2000}, pages = {230 -236}, abstract = {Yucca glauca is a C3 evergreen rosette species locally common in the C4-dominated grasslands of the central Great Plains. Most congeners of Y. glauca are found in deserts, and Y. glauca{\textquoteright}s morphological similarities to desert species (steeply angled leaves, evergreen habit) may be critical to its success in grasslands. We hypothesized that the evergreen habit of Y. glauca, coupled with its ability to remain physiologically active at cool temperatures, would allow this species to gain a substantial portion of its annual carbon budget when the C4 grasses are dormant. Leaf-level gas exchange was measured over an 18-mo period at Konza Prairie in northeast Kansas to assess the annual pattern of potential C gain. Two short-term experiments also were conducted in which nighttime temperatures were manipulated to assess the cold tolerance of this species. The annual pattern of C gain in Y. glauca was bimodal, with a spring productive period (maximum monthly photosynthetic rate = 21.1 {\textpm} 1.97 μmol{\textperiodcentered}m-2{\textperiodcentered}s-1) in March through June, a period of midseason photosynthetic depression, and a fall productive period in October (15.6 {\textpm} 1.25 μmol{\textperiodcentered}m-2{\textperiodcentered}s-1). The steeply angled leaves resulted in interception of photon flux density at levels above photosynthetic saturation throughout the year. Reduced photosynthetic rates in the summer may have been caused by low soil moisture, but temperature was strongly related (r2 = 0.37) to annual variations in photosynthesis, with nocturnal air temperatures below -5{\textdegree}C in the late fall and early spring, and high air temperatures (>32{\textdegree}C) in the summer, limiting gas exchange. Overall, 31\% of the potential annual carbon gain in Y. glauca occurred outside the {\textquotedblleft}frost-free{\textquotedblright} period (April{\textendash}October) at Konza Prairie and 43\% occurred when the dominant C4 grasses were dormant. Future climates that include warmer minimum temperatures in the spring and fall may enhance the success of Y. glauca relative to the C4 dominants in these grasslands.}, keywords = {LTER-KNZ, Climate change, cold tolerance, evergreen, Grasslands, Liliaceae, photosynthesis, tallgrass prairie, Water relations, Yucca}, url = {http://www.amjbot.org/content/87/2/230.short}, author = {Maragni, L.A. and Alan K. Knapp and McAllister, C.A.} } @article {KNZ00705, title = {Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2}, journal = {Global Change Biology}, volume = {5}, year = {1999}, pages = {497 -506}, abstract = {To determine the long-term impact of elevated CO2 on primary production of native tallgrass prairie, we compared the responses of tallgrass prairie at ambient and twice-ambient atmospheric CO2 levels over an 8-year period. Plots in open-top chambers (4.5 m diameter) were exposed continuously (24 h) to ambient and elevated CO2 from early April to late October each year. Unchambered plots were monitored also. Above-ground peak biomass was determined by clipping each year in early August, and root growth was estimated by harvesting roots from root ingrowth bags. Plant community composition was censused each year in early June. In the last 2 years of the study, subplots were clipped on 1 June or 1 July, and regrowth was harvested on 1 October. Volumetric soil water content of the 0{\textendash}100 cm soil layer was determined using neutron scattering, and was generally higher in elevated CO2 plots than ambient. Peak above-ground biomass was greater on elevated CO2 plots than ambient CO2 plots with or without chambers during years with significant plant water stress. Above-ground regrowth biomass was greater under elevated CO2 than under ambient CO2 in a year with late-season water stress, but did not differ in a wetter year. Root ingrowth biomass was also greater in elevated CO2 plots than ambient CO2 plots when water stress occurred during the growing season. The basal cover and relative amount of warm-season perennial grasses (C4) in the stand changed little during the 8-year period, but basal cover and relative amount of cool-season perennial grasses (C3) in the stand declined in the elevated CO2 plots and in ambient CO2 plots with chambers. Forbs (C3) and members of the Cyperaceae (C3) increased in basal cover and relative amount in the stand at elevated compared to ambient CO2. Greater biomass production under elevated CO2 in C4-dominated grasslands may lead to a greater carbon sequestration by those ecosystems and reduce peak atmospheric CO2 concentrations in the future.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1046/j.1365-2486.1999.00245.x}, author = {Owensby, C.E. and J.M. Ham and Alan K. Knapp and Auen, L.M.} } @article {KNZ00696, title = {Elevated CO2 and leaf longevity in the C4 grassland dominant Andropogon gerardii}, journal = {International Journal of Plant Sciences}, volume = {160}, year = {1999}, pages = {1057 -1061}, abstract = {In central U.S. grasslands, plant and ecosystem responses to elevated CO2 are most pronounced when water availability is limited. In a northeast Kansas grassland, responses to elevated CO2 in leaf area, number, development, and longevity were quantified for the tallgrass prairie dominant, Andropogon gerardii. Plants were grown in open-top chambers (OTCs) modified to limit water availability and to maximize responses to elevated CO2. In OTCs with elevated ({\texttimes}2 ambient) levels of CO2, aboveground biomass production and leaf water potentials were increased significantly compared with those of plants in OTCs with ambient CO2. There were no differences in leaf area or leaf number per tiller in A. gerardii in elevated compared with ambient OTCs. However, leaf area in adjacent unchambered plots with greater water availability was significantly higher than in the OTCs. The time required for developing leaves to achieve maximum leaf area was reduced by 29\%, and the period of time until leaves senesced was increased by 20\% for plants exposed to elevated compared with ambient CO2. Thus, leaves of this C4 grass species expanded more rapidly (6 d) and remained green longer (9 d) when exposed to elevated CO2. Such CO2-mediated increases in leaf longevity in the dominant species may allow this grassland to respond more opportunistically to temporally variable rainfall patterns in high-CO2 environments. These responses should be included in leaf-based simulation models that attempt to mechanistically link physiological alterations to predicted canopy responses to increased CO2.}, keywords = {LTER-KNZ, Ecophysiology, elevated carbon dioxide, grassland, leaf lifespan, tallgrass prairie, Water relations}, doi = {10.1086/314202}, author = {Alan K. Knapp and Bargman, N. and Maragni, L.A. and McAllister, C.A. and D. Bremer and J.M. Ham and Owensby, C.E.} } @article {KNZ00710, title = {Exotic plant species in a C4-dominated grassland: Invasibility, disturbance and community structure}, journal = {Oecologia}, volume = {120}, year = {1999}, pages = {605 -612}, abstract = {We used data from a 15-year experiment in a C4-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90\% C3 plants) comprised approximately 10\% of the flora, and their turnover was relatively high (30\%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80{\textendash}90\%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C4 grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C4-dominated grassland can also be directly related to community structure independent of disturbance.}, keywords = {LTER-KNZ, community structure, disturbance, exotic species, grassland, Invasibility}, doi = {10.1007/s004420050896}, author = {M.D. Smith and Alan K. Knapp} } @article {KNZ00697, title = {The keystone role of bison in North American tallgrass prairie}, journal = {BioScience}, volume = {49}, year = {1999}, pages = {39 -50}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/10.1525/bisi.1999.49.1.39}, author = {Alan K. Knapp and John M. Blair and J. M. Briggs and Scott. L. Collins and D.C. Hartnett and Johnson, L.C. and Towne, E.G.} } @proceedings {KNZ00672, title = {Manipulation of soil resource heterogeneity in a tallgrass prairie restoration}, year = {1999}, pages = {78 -87}, publisher = {University of Nebraska at Kearney}, address = {Kearney, NE}, abstract = {Plant species diversity in native prairie ecosystems is influenced by heterogeneity of soil resources, which results from interactions among plant communities, soil properties, topography, and disturbance. Key factors which promote plant diversity in native prairie include soil moisture, rooting depth, and nutrient availability. Most prairie restorations in the Great Plains occur on former agricultural land and the soil template for these restorations has been physically homogenized through tillage practices. The reduction in soil resource heterogeneity in restored prairies may be causally related to low plant species diversity commonly encountered in these sites. To examine the role of soil heterogeneity in restoring prairie ecosystems, replicated blocks (n=4) of 4 plots consisting of different levels of soil heterogeneity were established in an agricultural field. The 4 plot types included: control (least heterogeneous with respect to soil resources), altered plant rooting depth, altered nutrient availability, and combined variation in plant rooting depth and nutrient availability (maximum heterogeneity). Plant rooting depth was varied by burying limestone barriers at 25 cm to create alternating strips of deep and shallow soil. Variation in soil nutrient (inorganic N) availability was established via increased N (+fertilizer), decreased N (+ recalcitrant C), and no change in N. All plots were seeded with the dominant native grasses and >30 forb species. Incorporation of recalcitrant C reduced total inorganic N availability 90\% as a result of 98\% reduction in available nitrate. Plots containing strips of low N availability exhibited significantly (p < 0.05) greater spatial variability than plots without this manipulation. Thus, the experimental design and methods implemented have increased heterogeneity of soil nutrients, and this will enable us to address the role of variation in soil resources on restoring tallgrass prairie.}, keywords = {LTER-KNZ, altered plant rooting depth, altered soil nutrient availability, Great Plains, limestone barriers, recalcitrant C, soil template, Species diversity}, url = {http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC16/reference/econatres.napc16.sbaer.pdf}, author = {S.G. Baer and John M. Blair and Alan K. Knapp}, editor = {Springer, J.T.} } @inbook {KNZ00698, title = {Success of C4 photosynthesis in the field: lessons from communities dominated by C4 plants}, booktitle = {C4 Plant Biology}, year = {1999}, pages = {251 -283}, publisher = {Academic Press}, organization = {Academic Press}, address = {New York}, keywords = {LTER-KNZ}, author = {Alan K. Knapp and Medina, E.}, editor = {Sage, R.F. and Monson, R.K.} } @inbook {KNZ00649, title = {Animal populations and communities}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {113 -139}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {D.W. Kaufman and Kaufman, G.A. and Fay, P.A. and Zimmerman, J.L. and Evans, E.W.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00670, title = {Aspects of methane flow from sediment through emergent cattail (Typha latifolia) plants}, journal = {New Phytologist}, volume = {139}, year = {1998}, pages = {495 -503}, abstract = {We measured the flow of methane in Typha latifolia L. (cattail)-dominated wetlands from microbial production in anoxic sediment into, through, and out of emergent T. latifolia shoots (i.e. plant transport). The purpose was to identify key environmental and plant factors that might affect rates of methane efflux from wetlands to the Earth{\textquoteright}s atmosphere. Methane accumulated in leafy T. latifolia shoots overnight, reaching concentrations up to 10000 μl l-1 (vs. atmospheric concentrations <4 μl l-1), suggesting that lower stomatal conductance at night limits methane efflux from the plant into ambient air. Daytime light and (or) lower atmospheric humidity that induce convective gas flow through the plant coincided with (a) an increase in the rate of methane efflux from T. latifolia leaves to ambient air (from <0{\textperiodcentered}1 to >2{\textperiodcentered}0 μmol m-2 (leaf) s-1) and (b) a decrease in shoot methane concentration to <70 μl l-1. Very short fluctuation in stomatal conductance during the day did not affect the methane efflux rate unless, possibly, the rate of photosynthesis decreased. A strong relationship between the maximum daily rate of methane efflux and shoot methane concentration (measured before the onset of convective gas flow) suggests T. latifolia plants behave like a capacitor (filling with methane at night, emitting the stored methane during the day). Experimentally cutting leaves (to prevent pressurization) reduced plant capacitance for methane.}, keywords = {LTER-KNZ}, doi = {10.1046/j.1469-8137.1998.00210.x}, author = {Yavitt, J.B. and Alan K. Knapp} } @inbook {KNZ00661, title = {Belowground biology and processes}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {244 -264}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {C. W. Rice and Todd, T.C. and John M. Blair and Seastedt, T.R. and Ramundo, R.A. and G.T. Wilson}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00665, title = {Climate change, elevated CO2 and predictive modeling: Past and future climate change scenarios for the tallgrass prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {283 -300}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://www.colostate.edu/Depts/GDPE/Distinguished_Ecologists/2005/Hayden/grassland\%20dynamics\%20ch16.pdf}, author = {Seastedt, T.R. and Hayden, B.P. and Owensby, C.E. and Alan K. Knapp}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00654, title = {Determinants of soil CO2 flux from a sub-humid grassland: Effect of fire and fire history}, journal = {Ecological Applications}, volume = {8}, year = {1998}, pages = {760 -770}, abstract = {Soil CO2 flux (JCO2) was measured at midday over a 2-yr period in undisturbed tallgrass prairie (Konza Prairie, Kansas, USA) to quantify seasonal and annual budgets, to evaluate temperature and moisture as determinants of soil CO2 flux, and to assess the effect of a common land management tool, spring fire, and fire history on soil respiration. We hypothesized that: (1) maximum rates and annual estimates of soil JCO2 would be greater in more productive burned sites than in unburned sites, (2) soil JCO2 would be greater in newly burned sites with a history of fire exclusion than in annually burned sites (consistent with differences in aboveground production), and (3) soil temperature and water availability would be primary abiotic determinants of soil JCO2 in tallgrass prairie. A preliminary assessment of the effects of large herbivores on soil JCO2 was included to evaluate the hypothesis that removal of aboveground biomass would reduce soil JCO2. Results indicated that spring fire increased maximum monthly soil JCO2 by 20{\textendash}55\% relative to unburned tallgrass prairie, with greatest monthly differences measured in April (fourfold higher in burned sites). In burned sites that differed in fire history, maximum monthly JCO2 in annually burned prairie was 33\% greater than in burned sites with a history of fire exclusion. Soil JCO2 in these latter sites was still significantly higher than in unburned sites. Soil JCO2 in sites grazed by bison was reduced by as much as 30\% relative to adjacent ungrazed areas. Reduced root biomass and activity in grazed areas, unburned sites, and sites with a history of fire exclusion suggest that plants play a major role in determining soil JCO2 in this grassland. Soil temperature at 5 cm was related strongly to midday JCO2 in both annually burned sites (r2 = 0.58) and unburned sites (r2 = 0.71). In contrast, differences in soil moisture among sites, enhanced by comparing irrigated grassland to control areas, increased maximum monthly JCO2 by only 8\%. Thus, soil temperature was the primary abiotic determinant of soil JCO2 during this study. Maximum monthly estimates of soil JCO2 in tallgrass prairie ranged from 10.3 μmol CO2{\textperiodcentered}m-2{\textperiodcentered}s-1 in unburned sites to 15.1 μmol{\textperiodcentered}m-2{\textperiodcentered}s-1 in annually burned irrigated sites, whereas annual estimates varied from 4.7 to 7.8 kg CO2/m2. Over the 2-yr period, spring fire increased estimated annual soil JCO2 by 38{\textendash}51\% relative to unburned sites, while irrigation increased annual soil JCO2 by 13\%. These estimates for tallgrass prairie are much higher than those reported for most temperate ecosystems but are similar to estimates for tropical forests. Characteristics of undisturbed tallgrass prairie that may lead to high levels of soil JCO2 include: high above- and belowground productivity; a relatively high proportion of C stored belowground; levels of soil microbial biomass and activity that are among the highest in native ecosystems in the United States; and the lack of a single dominant factor such as temperature, moisture, or nutrient availability, that consistently limits biotic processes during the growing season. The sensitivity of soil JCO2 in tallgrass prairie to different land use practices (fire and grazing) suggests that it is critical to include these factors in the development of grassland C budgets, as well as in regional models that estimate biogeochemical responses to land use change.}, keywords = {LTER-KNZ, fire}, doi = {10.1890/1051-0761(1998)008[0760:DOSCFF]2.0.CO;2}, author = {Alan K. Knapp and Conard, S.L. and John M. Blair} } @inbook {KNZ00633, title = {Disturbance, diversity and species interactions in tallgrass prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {140 -156}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Scott. L. Collins and Steinauer, E.M.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00648, title = {Diversity of terrestrial macrofauna}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {101 -112}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {D.W. Kaufman and Fay, P.A. and Kaufman, G.A. and Zimmerman, J.L.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00632, title = {The dynamic tallgrass prairie: Synthesis and research opportunities}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {301 -315}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Scott. L. Collins and Alan K. Knapp and D.C. Hartnett and J. M. Briggs}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00639, title = {The flora of Konza Prairie: A historical review and contemporary patterns}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {69 -80}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Freeman, C.C.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00644, title = {Fluxes of CO2, water vapor, and energy from a prairie ecosystem during the seasonal transition from carbon sink to carbon source}, journal = {Agricultural and Forest Meteorology}, volume = {89}, year = {1998}, pages = {1 -14}, abstract = {In many temperate-zone ecosystems, seasonal changes in environmental and biological factors influence the dynamics and magnitude of surface-atmosphere exchange. Research was conducted to measure surface-layer fluxes of CO2, water vapor, and energy in a C4-dominated tallgrass prairie during the autumnal transition from carbon sink to carbon source. Data were collected between DOY 220 and 320, 1996 on the Konza Prairie Research Natural Area near Manhattan, KS, USA. Mass fluxes were measured with a tower-based conditional sampling (CS) system, and the surface energy balance was measured with Bowen ratio (BR) methods. Soil-surface CO2 fluxes were measured with a closed-chamber system. Carbon and energy fluxes decreased over the study period as the canopy senesced. When skies were clear, daily net CO2 exchange (NCE) varied from a maximum gain of 17.8 g CO2 m-2 day-1 on DOY 226 to a maximum loss of -10.3 g CO2 m-2 day-1 on DOY 290. Over the 100-day study period, the ecosystem had a net loss of -217 g CO2 m-2, with the change from sink to source occurring on about DOY 255. Soil-surface CO2 fluxes were -0.4 mg CO2 m-2 s-1 at the start of the study but declined to -0.04 mg CO2 m-2 s-1 on DOY 320. The Bowen ratio increased from 0.5 to 4 over the study period. The seasonal trend in NCE was governed by the senescence of the canopy and not abrupt changes in weather. Senescence also influenced canopy conductance, which caused a seasonal transformation in the surface energy balance. Data suggest that any climatic or management factors that affect the rate and timing of the autumnal sink-source transition can have a strong influence, on the carbon and water balance in the ecosystem.}, keywords = {LTER-KNZ, Carbon sink-source transition, Seasonal variation, Surface-atmosphere exchange, Tallgrass prairie ecosystem}, doi = {10.1016/S0168-1923(97)00062-2}, author = {J.M. Ham and Alan K. Knapp} } @inbook {KNZ00657, title = {Geomorphology of the Konza Prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {35 -47}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Oviatt, C.G.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @book {KNZ001010, title = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {364 -364}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00652, title = {Grasslands, Konza Prairie and long-term ecological Research}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {3 -15}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Alan K. Knapp and Seastedt, T.R.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00642, title = {Hydrology and aquatic chemistry}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {159 -176}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Gray, L.J. and G. L. Macpherson and Koelliker, J.K. and W. K. Dodds}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00626, title = {A landscape perspective of patterns and processes in tallgrass prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {265 -279}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {J. M. Briggs and Nellis, M.D. and Turner, C.L. and Henebry, G.M. and Su, H.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00656, title = {Is leaf-level photosynthesis related to plant success in a highly productive grassland?}, journal = {Oecologia}, volume = {117}, year = {1998}, pages = {40 -46}, abstract = {We addressed the question: {\textquotedblleft}Are short-term, leaf-level measurements of photosynthesis correlated with long-term patterns of plant success?{\textquotedblright} in a productive grassland where interspecific competitive interactions are important. To answer this question, seasonal patterns of leaf-level photosynthesis were measured in 27 tallgrass prairie species growing in sites that differed in species composition and productivity due to differences in fire history. Our specific goals were to assess the relationship between gas exchange under field conditions and success (defined as aerial plant cover) for a wide range of species, as well as for these species grouped as dominant and sub-dominant grasses, forbs, and woody plants. Because fire increases productivity and dominance by grasses in this system, we hypothesized that any relationship between photosynthesis and success would be strongest in annually burned sites. We also predicted that regardless of fire history, the dominant species (primarily C4 grasses) would have higher photosynthetic rates than the less successful species (primarily C3 grasses, forbs and woody plants). Because forbs and woody species are less abundant in annually burned sites, we expected that these species would have lower photosynthetic rates in annually burned than in infrequently burned sites. As expected, the dominant C4{\textvisiblespace}grasses had the highest cover on all sites, relative to{\textvisiblespace}other growth forms, and they had the highest maximum and seasonally averaged photosynthetic rates (17.6 {\textpm} 0.42 μmol m-2 s-1). Woody species had the lowest average cover as well as the lowest average photosynthetic rates, with subdominant grasses and forbs intermediate in both cover and photosynthesis. Also as predicted, the highest overall photosynthetic rates were found on the most productive annually burned site. Perhaps most importantly, a positive relationship was found between leaf-level photosynthesis and cover for a core group of species when data were combined across all sites. These data support the hypothesis that higher instantaneous rates of leaf-level photosynthesis are indicative of long-term plant success in this grassland. However, in contrast to our predictions, the subdominant grasses, forbs and woody species on the annually burned site had higher photosynthetic rates than in the less frequently burned sites, even though their average cover was lower on annually burned sites, and hence they were less successful. The direct negative effect of fire on plant cover and species-specific differences in the availability of resources may explain why photosynthesis was high but cover was low in some growth forms in annually burned sites.}, keywords = {LTER-KNZ, Competition, grassland, photosynthesis, Plant cover, tallgrass prairie}, doi = {10.1007/s004420050629}, author = {McAllister, C.A. and Alan K. Knapp and Maragni, L.A.} } @inbook {KNZ00650, title = {Long-term ecological consequences of varying fire frequency in a humid grassland}, booktitle = {Fire in Ecosystem Management: Shifting The Paradigm From Suppression to Prescription}, year = {1998}, pages = {173 -178}, publisher = {Tall Timbers Research Station}, organization = {Tall Timbers Research Station}, address = {Tallahassee, FL}, keywords = {LTER-KNZ, fire}, author = {Alan K. Knapp and John M. Blair and J. M. Briggs}, editor = {Pruden, T.L. and Brennan, L.A.} } @article {KNZ00631, title = {Modulation of diversity by grazing and mowing in native tallgrass prairie}, journal = {Science}, volume = {280}, year = {1998}, pages = {745 -747}, abstract = {Species diversity has declined in ecosystems worldwide as a result of habitat fragmentation, eutrophication, and land-use change. If such decline is to be halted ecological mechanisms that restore or maintain biodiversity are needed. Two long-term field experiments were performed in native grassland to assess the effects of fire, nitrogen addition, and grazing or mowing on plant species diversity. In one experiment, richness declined on burned and fertilized treatments, whereas mowing maintained diversity under these conditions. In the second experiment, loss of species diversity due to frequent burning was reversed by bison, a keystone herbivore in North American grasslands. Thus, mowing or the reestablishment of grazing in anthropogenically stressed grasslands enhanced biodiversity.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1126/science.280.5364.745}, author = {Scott. L. Collins and Alan K. Knapp and J. M. Briggs and John M. Blair and Steinauer, E.M.} } @inbook {KNZ00651, title = {Patterns and controls of aboveground net primary production in tallgrass prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {193 -221}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Alan K. Knapp and J. M. Briggs and John M. Blair and Turner, C.L.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00645, title = {Plant populations: Patterns and processes}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {81 -100}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {D.C. Hartnett and Fay, P.A.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00646, title = {Regional climate and the distribution of tallgrass prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {19 -34}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://www.colostate.edu/Depts/GDPE/Distinguished_Ecologists/2005/Hayden/grassland\%20dynamics\%20ch2.pdf}, author = {Hayden, B.P.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00621, title = {Responses of soil respiration to clipping and grazing in a tallgrass prairie}, journal = {Journal of Environmental Quality}, volume = {27}, year = {1998}, pages = {1539 -1548}, abstract = {Soil-surface CO2 flux (Fs) is an important component in prairie C budgets. Although grazing is common in grasslands, its effects on Fs have not been well documented. Three clipping treatments: (i) early-season clipping (EC); (ii) full-season clipping (FC); and (iii) no clipping (NC); which represented two grazing strategies and a control, were applied to plots in a tallgrass prairie in northeastern Kansas, USA. Measurements of Fs were made with a portable gas-exchange system at weekly to monthly intervals for 1 yr. Concurrent measurements of soil temperature and volumetric soil water content at 0.1 m were obtained with dual-probe heat-capacity sensors. Measurements of Fs also were obtained in grazed pastures. Fs ranged annually from 8.8 {\texttimes} 10-3 mg m-2 S-1 during the winter to 0.51 mg m-2 s-1 during the summer, following the patterns of soil temperature and canopy growth and phenology. Clipping typically reduced Fs 21 to 49\% by the second day after clipping despite higher soil temperatures in clipped plots. Cumulative annual Fs were 4.94, 4.04, and 4.11 kg m-2 yr-1 in NC, EC, and FC treatments, respectively; thus, dipping reduced annual Fs by 17.5\%. Differences in Fs between EC and FC were minimal, suggesting that different grazing strategies had little additional impact on annual Fs. Daily Fs in grazed pastures was 20 to 37\% less than Fs in ungrazed pastures. Results suggest that grazing moderates Fs during the growing season by reducing canopy photosynthesis and slowing translocation of carbon to the rhizosphere.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2134/jeq1998.00472425002700060034x}, author = {D. Bremer and J.M. Ham and Owensby, C.E. and Alan K. Knapp} } @article {KNZ00638, title = {Responses to short-term shade in soybean leaves: Effects of leaf position and drought stress}, journal = {International Journal of Plant Science}, volume = {159}, year = {1998}, pages = {805 -811}, abstract = {This study examined how leaf position in the canopy affected photosynthetic and stomatal responses to short-term, minutes-long shade periods during a drought cycle in soybean (Glycine max [L.] Merr.). All soybean leaves had similar basic responses to short-term shade, including rapidly decreased photosynthetic rates (Aco2 ), slower decreases in stomatal conductance (g,), and delayed stomatal reopening and photosynthetic recovery after leaves were reilluminated. Drought stress lowered overall Aco2 and restricted photosynthetic and stomatal responses to short-term shade with the negative effects of drought being stronger in lower than in upper leaves. Some of the negative effects of drought persisted after drought was relieved, causing reduced overall water use efficiency, especially in lower leaves. These results indicate that leaf position effects on stomatal responses to short-term shade events become important during and after stress.}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/2475151}, author = {Fay, P.A. and Alan K. Knapp} } @inbook {KNZ00660, title = {Soils and soil biota}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {48 -66}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Ransom, M.D. and C. W. Rice and Todd, T.C. and Wehmueller, W.A.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00643, title = {Structure and dynamics of aquatic communities}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {177 -189}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Gray, L.J. and W. K. Dodds}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @inbook {KNZ00620, title = {Terrestrial nutrient cycling in tallgrass prairie}, booktitle = {Grassland Dynamics: Long-term Ecological Research}, year = {1998}, pages = {222 -243}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, author = {John M. Blair and Seastedt, T.R. and C. W. Rice and Ramundo, R.A.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00653, title = {Variability in leaf optical properties among 26 species from a broad range of habitats}, journal = {American Journal of Botany}, volume = {85}, year = {1998}, pages = {940 -946}, abstract = {Leaves from 26 species with growth forms from annual herbs to trees were collected from open, intermediate, and shaded understory habitats in Mississippi and Kansas, USA. Leaf optical properties including reflectance, transmittance, and absorptance in visible and near infrared (NIR) wavelengths were measured along with leaf thickness and specific leaf mass (SLM). These leaf properties and internal light scattering have been reported to vary with light availability in studies that have focused on a limited number of species. Our objective was to determine whether these patterns in leaf optics and light availability were consistent when a greater number of species were evaluated. Leaf thickness and SLM varied by tenfold among species sampled, but within-habitat variance was high. Although there was a strong trend toward thicker leaves in open habitats, only SLM was significantly greater in open vs. understory habitats. In contrast, leaf optical properties were strikingly similar among habitats. Reflectance and reflectance/transmittance in the NIR were used to estimate internal light scattering and there were strong relationships (r1 \> 0.65) between these optical properties and leaf thickness. We concluded that leaf thickness, which did not vary consistently among habitats, was the best predictor of NIR reflectance and internal light scattering. However, because carbon allocation to leaves was lower in understory species (low SLM) yet gross optical properties were similar among all habitats, the energy investment by shade leaves required to achieve optical equivalence with sun leaves was lower. Differences in leaf longevity and growth form within a habitat may help explain the lack of consistent patterns in leaf optics as the number of species sampled increases.}, keywords = {LTER-KNZ}, url = {http://www.amjbot.org/content/85/7/940.short}, author = {Alan K. Knapp and Carter, G.A.} } @article {KNZ00590, title = {Photosynthetic gas exchange and water relations responses of three tallgrass prairie species to elevated carbon dioxide and moderate drought}, journal = {International Journal of Plant Science}, volume = {158}, year = {1997}, pages = {608 -616}, abstract = {Undisturbed tallgrass prairie was exposed to ambient and elevated (twice-ambient) levels of atmospheric CO2 and experimental dry periods. Seasonal and diurnal midday leaf water potential (Ψ leaf), net photosynthesis $(A_{\text{net}})$, and stomatal conductance (g s) responses of three tallgrass prairie growth forms{\textemdash}a C4 grass, Andropogon gerardii; a broad-leaved woody C3 shrub, Symphiocarpos orbiculatus; and a C3 perennial forb, Salvia pitcheri{\textemdash}were assessed. $\Psi _{\text{leaf}}$ in A. gerardii and S. orbiculatus was higher under elevated CO2, regardless of soil moisture, while $\Psi _{\text{leaf}}$ in S. pitcheri responded only to drought. Elevated CO2 always stimulated $A_{\text{net}}$ in the C3 species, while A. gerardii $A_{\text{net}}$ increased only under dry conditions. However, $A_{\text{net}}$ under elevated CO2 in the C3 species declined with drought but not in the C4 grass. Under wet conditions, g s reduced in elevated CO2 for all species. During dry periods, gs at elevated CO2 was sometimes higher than in ambient CO2. Our results support claims that elevated CO2 will stimulate tallgrass prairie productivity during dry periods and possibly reduce temporal and spatial variability in productivity in these grasslands.}, keywords = {LTER-KNZ, Water relations}, url = {http://www.jstor.org/stable/2474921}, author = {Hamerlynck, E.P. and McAllister, C.A. and Alan K. Knapp and J.M. Ham and Owensby, C.E.} } @inbook {KNZ00597, title = {Plant strategies for coping with variable light regimes}, booktitle = {Mechanisms of Environmental Stress Resistance in Plants}, year = {1997}, pages = {191 -212}, publisher = {Harwood Academic Press}, organization = {Harwood Academic Press}, address = {UK}, keywords = {LTER-KNZ}, url = {https://books.google.com/books?hl=en\&lr=\&id=cVUvZePMNfMC\&oi=fnd\&pg=PA191\&dq=\%22Plant\%2Bstrategies\%2Bfor\%2Bcoping\%2Bwith\%2Bvariable\%2Blight\%2Bregimes\%22+Knapp\&ots=4FA7O50id2\&sig=Iyq7j1EZaVkuv5Vkhqg25qu6b5c$\#$v=onepage\&q=\%22Plant\%2Bstrategies\%2Bfor\%2Bcoping\%2}, author = {Alan K. Knapp and Fay, P.A.}, editor = {Basra, A.S. and Basra, R.K.} } @article {KNZ00608, title = {Water vapor fluxes and their impact under elevated CO2 in a C4 tallgrass prairie}, journal = {Global Change Biology}, volume = {3}, year = {1997}, pages = {189 -195}, abstract = {We measured leaf-level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy-level water vapour fluxes in a C4-tallgrass prairie in Kansas exposed to ambient and elevated CO2. Stomatal conductance was reduced by as much as 50\% under elevated CO2 compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO2 compared to ambient. The result was an improved water status for plants exposed to elevated CO2 which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO2. Sap flow rates were 20 to 30\% lower for plants exposed to elevated CO2 than for those exposed to ambient CO2. At the canopy level, evapotranspiration was reduced by 22\% under elevated CO2. The reduced water use by the plant canopy under elevated CO2 extended the photosynthetically-active period when water became limiting in the ecosystem. The result was an increased above- and belowground biomass production in years when water stress was frequent.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1046/j.1365-2486.1997.00084.x}, author = {Owensby, C.E. and J.M. Ham and Alan K. Knapp and D. Bremer and Auen, L.M.} } @article {KNZ00573, title = {Biomass and density responses in tallgrass prairie legumes to annual fire and topographic position}, journal = {American Journal of Botany}, volume = {83}, year = {1996}, pages = {175 -179}, abstract = {Annually burned tallgrass prairie is purported to be a nitrogen-limited system, especially when compared to unburned prairie. To test the hypothesis that legumes, potential nitrogen-fixers, would increase in relative abundance in annually burned sites, we assessed their density and biomass for two seasons on upland and lowland soils in annually burned and unburned watersheds. Total legume density was significantly higher in burned (8.0 {\textpm} 1.0 [SE] stems/m2) than in unburned watersheds (3.0 {\textpm} 0.3 stems/m2). Species with higher (P < 0.05) densities in burned than in unburned prairie included Amorpha canescens, Dalea candida, Dalea purpurea, Lespedeza violacea, Psoralea tenuiflora, and Schrankia nuttallii. Desmodium illinoense was the only legume that responded negatively to annual fire. Total legume biomass did not differ between burned (11.3 {\textpm} 1.3 g/m2) and unburned prairie (10.5 {\textpm} 0.9 g/m2). Biomass productions of Dalea candida and Psoralea tenuiflora were higher (P < 0.05) in burned than in unburned sites, but biomasses of other legumes were similar between burn treatments. Average individual stem masses of Amorpha canescens and Baptisia bracteata were significantly greater in unburned than in burned prairie. Legumes were affected differentially by topographic location. Total legume density was higher (P < 0.05) on lowland soils (6.6 {\textpm} 1.0 stems/m2) than on upland soils (4.3 {\textpm} 0.5 stems/m2). However, total legume biomass was not different between lowland soils (12.0 {\textpm} 1.2 g/m2) and upland soils (9.9 {\textpm} 1.0 g/m2). Densities and biomasses of Amorpha canescens, Desmodium illinoense, and Lespedeza capitata were higher on lowland sites than on upland sites, whereas densities and biomasses of Baptisia bracteata and Dalea purpurea were higher on upland than on lowland soils. Most legume species are either fire tolerant or exhibit a positive response to fire and their persistence in annually burned prairie suggests that they may play an important role in the nitrogen budget of this ecosystem.}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://www.jstor.org/stable/2445935}, author = {Towne, E.G. and Alan K. Knapp} } @article {KNZ00570, title = {Detecting spatial and temporal patterns of aboveground production in a tallgrass prairie using remotely-sensed data}, year = {1996}, pages = {2361 -2365}, abstract = {Spatial and temporal patterns of aboveground production in a tallgrass prairie ecosystem constitute one of the important spatial components associated with ecological processes and biophysical resources (e.g., water and nutrients). This study addresses the effects of disturbance, topography, and climate on the spatial and temporal patterns of North American tallgrass prairie at a landscape level by using high resolution satellite data. Spatial heterogeneity (SH) derived from the satellite data was related to the impacts of the disturbance of fire and grazing, topographical gradient, and amount of precipitation during the growing season. The result suggests that ecological processes and biophysical resources can be quantified with high resolution satellite data for tallgrass prairie management}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1109/IGARSS.1996.516987}, author = {Su, H. and J. M. Briggs and Alan K. Knapp and John M. Blair and Krummel, J.R.} } @article {KNZ00545, title = {Early season cuticular conductance and gas exchange in two oaks near the western edge of their range}, journal = {Trees}, volume = {10}, year = {1996}, pages = {403 -409}, abstract = {Seasonal changes in minimum leaf conductance to water vapor (gmin), an estimate of cuticular conductance, and photosynthetic gas exchange in two co-occurring oak species in north-east Kansas (USA) were examined to determine if leaf gas exchange characteristics correlated with differences in tree distribution. Bur oak (Quercus macrocarpa Michx.) is more abundant in mesic gallery forest sites, whereas chinquapin oak (Quercus muehlenbergii Englm.) is more abundant in xeric sites. Early, during leaf expansion, gmin was significantly lower in chinquapin oak than in bur oak, though midday water potentials were similar. After leaves had fully expanded, gmin decreased to seasonal minimum values of 4.57 ({\textpm}0.274) mmol m-2 s-1 in bur oak, and 2.66 ({\textpm}0.156) mmol m-2 s-1 in chinquapin oak. Water potentials at these times were significantly higher in chinquapin oak. As leaves were expanding, photosynthesis (Anet) was significantly higher in chinquapin oak than in bur oak. Later in the growing season, Anet and gleaf increased dramatically in both species, and were significantly higher in bur oak relative to chinquapin oak. We concluded that bur and chinquapin oak have a number of leaf gas exchange characteristics that minimize seasonal water loss. These characteristics are distinct from trees from more mesic sites, and are consistent with the distribution patterns of these trees in tall-grass prairie gallery forests.}, keywords = {LTER-KNZ, Cuticular conductance, Gallery forests, Leaf development, photosynthesis, Quercus}, doi = {10.1007/BF02185644}, author = {Hamerlynck, E.P. and Alan K. Knapp} } @inbook {KNZ00564, title = {Ecosystem level responses of tallgrass prairie to elevated CO2}, booktitle = {Carbon Dioxide and Terrestrial Ecosystems}, year = {1996}, pages = {147 -162}, publisher = {Academic Press}, organization = {Academic Press}, address = {London}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Owensby, C.E. and J.M. Ham and Alan K. Knapp and C. W. Rice and Coyne, P.I. and Auen, L.M.}, editor = {Koch, G.W and Mooney, H.A.} } @article {KNZ00546, title = {Photosynthetic and stomatal responses to high temperature and light in two oaks at the western limit of their range}, journal = {Tree Physiology}, volume = {16}, year = {1996}, pages = {557 -565}, abstract = {Bur oak (Quercus macrocarpa Michx.) and chinquapin oak (Q. muehlenbergii Engl.) leaves were exposed to high temperatures at various photosynthetic photon flux densities under laboratory conditions to determine if species-specific responses to these factors were consistent with the distribution of these oaks in gallery forests in the tallgrass prairies of northeastern Kansas, USA. Measurements of the ratio of chlorophyll fluorescence decrease, Rfd, indicated that chinquapin oak maintained greater photosynthetic capacity than bur oak across all tested combinations of irradiance (100, 400, 700 and 1000 μmol m-2 s-1) and temperature (40, 42, 44, 46 and 48 {\textdegree}C). In both oak species, manipulation of leaf temperature to about 47 {\textdegree}C for 45 min in the field led to a 45\% decrease in carbon assimilation up to one week after the heat treatment, and to sharp reductions in stomatal conductance. Photosynthetic recovery patterns indicated that bur oak took longer to recover from heat stress than chinquapin oak, suggesting that heat stress may be important in determining distribution patterns of these oak species. Based on a comparison of the results with data from other forest species, we conclude that the photosynthetic temperature tolerances of bur oak and chinquapin oaks facilitate their dominance at the western limit of the eastern deciduous forest.}, keywords = {LTER-KNZ, Chlorophyll fluorescence, heat stress, Quercus macrocarpa, Quercus muehlenbergii}, doi = {10.1093/treephys/16.6.557}, author = {Hamerlynck, E.P. and Alan K. Knapp} } @article {KNZ00539, title = {Photosynthetic and stomatal responses to variable light in a cool-season and warm-season prairie forb}, journal = {International Journal of Plant Science}, volume = {157}, year = {1996}, pages = {303 -308}, abstract = {Species differences in photosynthetic and stomatal responses to steady-state and variable light were examined in two co-occurring tallgrass prairie forbs, the cool-season legume Baptisia bracteata var. glabrescens and the warm-season composite Helianthus annuus. Previous studies indicated that these species might have similar responses to short-term, minutes-long shade because of their similar growth forms. However, photosynthetic carbon gain, oxygen evolution, transpiration, and leaf xylem pressure potential measurements showed that Helianthus was far more responsive than Baptisia to changes in light availability. Helianthus had higher photosynthetic capacity, photosynthetic temperature optimum, stomatal conductance, and transpiration rates, rapid stomatal closure during shade, and delayed photosynthetic recovery when light levels increased, traits common to species exposed to high temperatures or periodic drought stress. Baptisia, active under cooler, wetter conditions than Helianthus, had lower photosynthetic capacity, photosynthetic temperature optimum, stomatal conductance, and transpiration, and no stomatal response to shade, responses typifying species that experience little water stress. We suggest that environmental and physiological factors may combine to reinforce greatly different photosynthetic and stomatal responses to short-term shade in species with similar growth form, especially in habitats with long, seasonally varying growing conditions.}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/2475266}, author = {Fay, P.A. and Alan K. Knapp} } @article {KNZ00540, title = {Plant tolerance of gall-insect attack and gall-insect performance}, journal = {Ecology}, volume = {77}, year = {1996}, pages = {521 -534}, abstract = {We examined plant tolerance of gall{\textemdash}insect attack and gall{\textemdash}insect performance in rosinweed (Silphium integrifolium, Asteraceae) and its apical meristem galler Antistrophus silphii (Hymenoptera: Cynipidae). Gall densities were varied in field rosinweed populations, while gall densities, water, and nutrients were varied for rosinweed in an experimental garden. Field plants grew under prevailing resource and competitive conditions, but garden plants grew free from competition, so gall{\textemdash}insect impacts, rosinweed regrowth, and gall{\textemdash}insect performance were observed under widely different growing conditions. Seasonal measures of rosinweed growth and leaf physiology, and end{\textemdash}of{\textemdash}season measures of biomass, reproduction, gall{\textemdash}wasp emergence, growth, sex ratios, and parasitism were made for both experiments. Rosinweed poorly tolerated Antistrophus gall damage in the field. Galls reduced plant height, leaf area, and inflorescence production. Rosinweed diverted biomass to stems, but produced no regrowth from axillary meristems. In the garden, rosinweed was much more tolerant of Antistrophus gall damage. Galls initially reduced plant height and leaf area, but axillary meristems grew profusely after gall formation, producing nearly all galled plant inflorescences and more than replacing leaf area initially lost to gall formation. Water{\textemdash} and nutrient{\textemdash}supplemented rosinweed were most tolerant of gall damage, experiencing little loss of total biomass or reproductive output. Field rosinweed failed to mount a tolerance{\textemdash}enhancing regrowth response because galls, resource availability, and competition combined to constrain axillary meristem growth. Gall{\textemdash}wasp performance was largely independent of rosinweed tolerance. Emergence, growth, sex ratios, and parasitism were comparable in field and garden, and only slightly affected by resource availability. Gall{\textemdash}insect performance may be buffered from environmental variation, disconnecting plant and herbivore population dynamics. Rosinweed{\textquoteright}s poor tolerance of gall damage may typify forb responses to herbivory in highly competitive grassland plant communities.}, keywords = {LTER-KNZ}, doi = {10.2307/2265627}, author = {Fay, P.A. and D.C. Hartnett and Alan K. Knapp} } @article {KNZ00556, title = {Responses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form}, journal = {Vegetatio}, volume = {125}, year = {1996}, pages = {31 -41}, abstract = {Responses in stomatal conductance (g st ) and leaf xylem pressure potential (ψ leaf ) to elevated CO2 (2x ambient) were compared among 12 tallgrass prairie species that differed in growth form and growth rate. Open-top chambers (OTCs, 4.5 m diameter, 4.0 m in height) were used to expose plants to ambient and elevated CO2 concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August, ψ leaf was usually higher in all species at elevated CO2 and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO2 resulted in decreased g st in 10 of the 12 species measured. Greatest decreases in g st (ca. 50\%) occurred in growth forms with the highest potential growth rates (C3 and C4 grasses, and C3 ruderals). In contrast, no significant decrease in g st was measured in the two C3 shrubs. During a dry period in September, reductions in g st at elevated CO2 were measured in only two species (a C3 ruderal and a C4 grass) whereas increased g st at elevated CO2 was measured in the shrubs and a C3 forb. These increases in g st were attributed to enhanced ψ leaf in the elevated CO2 plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in g st were measured in response to elevated CO2. Thus, there was significant interspecific variability in stomatal responses to CO2 that may be related to growth form or growth rate and plant water relations. The effect of growth in the OTCs, relative to field plants, was usually positive for g st and was greatest (>30\%) when water availability was low, but only 6{\textendash}12\% when ψ leaf was high. The results of this study confirm the importance of considering interactions between indirect effects of high CO2 of plant water relations and direct effects of elevated CO2 on g st , particularly in ecosystems such as grasslands where water availability often limits productivity. A product of this interaction is that the potential exists for either positive or negative responses in g st to be measured at elevated levels of CO2.}, keywords = {LTER-KNZ, elevated CO2, Growth form, Stomatal Conductance, tallgrass prairie, Water relations}, doi = {10.1007/BF00045202}, author = {Alan K. Knapp and Hamerlynck, E.P. and J.M. Ham and Owensby, C.E.} } @article {KNZ00574, title = {Responses of a C4 grass and three C3 forbs to variation in nitrogen and light in tallgrass prairie}, journal = {Ecology}, volume = {77}, year = {1996}, pages = {1738 -1749}, abstract = {In tallgrass prairie, high plant species diversity results not from a large number of grass species, but from a large number of forb (nongrass, herbaceous) species. Forbs exhibit morphological, life history, and ecophysiological characteristics that contrast sharply with those of the dominant C4 grasses. Success of the subdominant forbs varies strongly with topographic position and burning regime, and landscape scale patterns of abundance are well documented. But comparatively little is known about the mechanisms determining these patterns in persistent tallgrass prairie forbs. To elucidate these mechanisms, (1) leaf{\textemdash}level physiological characteristics of the dominant C4 grass, Andropogon gerardii, and four co{\textemdash}occurring C3 forbs were measured in response to natural and experimentally manipulated gradients of N availability, and (2) seasonal light environments of forbs in contrasting topographic positions and burning regimes and their morphological and physiological responses in these environments were compared to determine whether resource availability and utilization patterns contributed to patterns of forb distribution and abundance. The effects of burning regime and topographic position on maximum rate of photosynthesis (A) and stomatal conductance to water vapor (g) measured at the leaf level were not consistent with patterns of forb abundance. Nitrogen did not appear to limit forb physiological processes, even though increased N availability resulted in higher tissue N concentrations and greater biomass. There was no consistent increase in (A) or decrease in (g) in response to fertilization. However, (A) at low light levels was as much as 67\% higher in fertilized Vernonia baldwinii and A. gerardii compared to unfertilized plants. Greater light availability to forbs in the canopy was associated with lower grass biomass production in uplands compared to lowlands and in unburned compared to burned sites. Forbs did not appear to adjust morphologically (leaf area and plant height) to different light environments at different sites. As a result, as much as 90\% of forb leaf area in the burned lowland was displayed in low light, whereas as little as 30\% of forb leaf area was in low light in the uplands at midseason. Estimates of potential whole{\textemdash}plant carbon uptake, based on leaf area distribution relative to available light and (A) as a function of light availability, agreed well with patterns of forb abundance and production. Differences in light availability may account for much of the variability in forb abundance related to burning regime and topographic position by limiting carbon gain in forbs more in burned lowlands than in other sites.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/2265779}, author = {Turner, C.T. and Alan K. Knapp} } @proceedings {KNZ00550, title = {Spatio-temporal dynamics of soil moisture in tallgrass prairie using ERS-1 SAR image time series: first year results}, year = {1996}, pages = {166 -168}, publisher = {IEEE}, address = {Piscataway, NJ}, abstract = {Radar remote sensing of soil moisture has been demonstrated in croplands and rangelands. The authors are interested in synoptic estimates of soil moisture to aid long term ecological investigation of tallgrass prairie and have initiated a three year study to develop this technology. The authors{\textquoteright} study area, Konza Prairie Research Natural Area (KPRNA), exhibits a variety of factors to complicate soil moisture estimation via synthetic aperture radar (SAR) including significant terrain variation, heterogeneous soil types and depths, and watershed-level experimental treatments including prescribed burning, grazing by large ungulates, and water supplementation. Several indicators of the moisture status of soil-water-vegetation continuum were gathered during the 1995 growing season including soil moisture via neutron probe and TDR, xylem pressure,potential values, and aboveground biomass (wet and dry weights). Due to orbital track overlap, eleven ERS-1 SAR images of KPRNA were acquired from mid-April to early October. Spring through mid-summer were unusually cool and wet, thereby delaying phenological development and attenuating the water stress typical of the later growing season. Backscattering values show some consistent agreement with ground data. Watersheds burned in April showed markedly higher backscattering at the end of the growing season due to a lack of litter layer from previous seasons{\textquoteright} growth. Grazed units that were also burned showed the highest backscattering caused by no litter layer and more exposed soil}, keywords = {LTER-KNZ, tallgrass prairie}, url = {10.1109/IGARSS.1996.516279}, author = {Henebry, G.M. and Alan K. Knapp} } @article {KNZ00523, title = {Comparative gas exchange and nitrogen responses of the dominant C4 grass, Andropogon gerardii, and five C3 forbs to fire and topographic position in tallgrass prairie during a wet year}, journal = {International Journal of Plant Science}, volume = {156}, year = {1995}, pages = {216 -226}, abstract = {High plant species diversity in tallgrass prairie results not from a large number of grass species but, rather, from a large number of forb (nongrass, herbaceous) species. Although morphological characteristics of tallgrass prairie forbs differ strongly from those of the grasses and their patterns of abundance vary substantially with burning regime and topographic position, comparatively little is known about the ecology of persistent tallgrass prairie forbs. We measured leaf-level physiological characteristics of the dominant C4 grass, Andropogon gerardii, and five co-occurring C3 forb species in response to burning regime and topographic position to determine if there were either absolute or seasonal differences in resource utilization patterns that could contribute to the patterns of forb distribution and abundance in tallgrass prairie. In a wet year, neither fire nor topographic position affected leaf-level physiological characteristics in the grass and forbs studied, and thus responses were not consistent with extant patterns of forb abundance (greater forb abundance in unburned and upland sites). Water use efficiency in A. gerardii was 40\%-170\% greater and foliar N concentration was consistently lower in the grass than in the forbs, but there were no distinct or consistent differences between the grass and forbs in maximum rates of photosynthesis and stomatal conductance or xylem pressure potential. Moreover, seasonal patterns of gas exchange in forbs and the grass were not different, as might be expected when comparing C3 and C4 species. These results indicate that, when assessed at the leaf level, responses of these prairie plants to fire or topographic position during a wet year are not indicative of their success in these sites. However, seasonal water stress is a typical feature of the tallgrass prairie environment and water use patterns indicate that the prevalence of soil moisture limitations in most years may be a key factor influencing plant distribution and success in this ecosystem.}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://www.jstor.org/stable/2474960}, author = {Turner, C.T. and Kneisler, J.R. and Alan K. Knapp} } @proceedings {KNZ00490, title = {Environmental and physiological factors influencing the distribution of oaks near the edge of their range}, year = {1995}, pages = {17 -20}, keywords = {LTER-KNZ}, url = {http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC14/reference/econatres.napc14.ehamerlynck.pdf}, author = {Hamerlynck, E.P. and Alan K. Knapp}, editor = {D.C. Hartnett} } @article {KNZ00507, title = {Gas exchange characteristics of Typha latifolia L. from nine sites across North America}, journal = {Aquatic Biology}, volume = {49}, year = {1995}, pages = {203 -215}, abstract = {Gas exchange characteristics were measured in the field for nine populations of Typha latifolia L. from Florida to Minnesota in North America. These populations spanned a substantial gradient in growing season length and environmental conditions. The purpose of this study was to assess geographic variability in stomatal conductance (gst) in T. latifolia populations, as well as to identify key environmental and plant factors that may affect gst and, potentially, trace gas emissions through stomata. Midday rates of net photosynthesis (A) and gst were measured under full sunlight conditions at each site, and more intensive measurements of diurnal and seasonal variability were made at selected sites. In general, A varied less than gst (1.5 vs. 3 fold) among sites with maximum A (27.4 μmol m-2 s-1) and gst (1076.7 mmol m-2 s-1) measured at the southern sites. We found that A and gst in T. latifolia increased significantly with increasing temperature and light level, and varied diurnally and seasonally. Moreover, stomata closed completely at night, in contrast to some other wetland plants. Both A and gst also increased significantly along the length of leaves from the base of plants to the upper canopy. Finally, although gst was quite variable, A did not appear to be limited by gst under typical field conditions. It was concluded that generalizations that stomata in wetland plants are relatively unresponsive to environmental factors are not consistent with field responses measured in T. latifolia. As a result, because gst may influence trace gas flux in this species, spatial and temporal variations in gst need to be considered when emissions are estimated.}, keywords = {LTER-KNZ}, doi = {10.1016/0304-3770(94)00433-M}, author = {Alan K. Knapp and Yavitt, J.B.} } @article {KNZ00500, title = {The influence of fire on Spartina pectinata wetland communities in a northeastern Kansas tallgrass prairie}, journal = {Canadian Journal of Botany}, volume = {73}, year = {1995}, pages = {84 -90}, abstract = {Wetlands dominated by the C4 grass Spartina pectinata were investigated to quantify differences in plant species composition and diversity in response to fire frequency. The study site was a tallgrass prairie in northeastern Kansas that included Spartina wetlands subjected to spring fires at 1-, 2-, 4-, 10-, and 20-year intervals. Because C3 forbs in these wetlands responded strongly to different fire frequencies, the light environment and gas exchange responses of the ubiquitous forb Asclepias syriaca were also assessed. In general, species diversity was lower in annually burned wetlands because of lower forb diversity. Maximum H' in annually burned sites was 1.64 versus 2.77 in 10- and 20-year burned sites. However, individual forb responses varied. Asclepias increased and Solidago canadensis and Galium aparine decreased in importance with increasing fire frequency. Canopy sunlight interception was greater in annually burned wetlands than in wetlands with lower burn frequencies. Despite reduced light availability, midseason photosynthetic rates of Asclepias were higher in annually burned sites. The results suggest that frequent fire in tallgrass prairie wetlands results in less diverse plant communities, similar to responses of upland prairie, and that the timing of fire, relative to life history and phenology of the subordinate species, strongly influences responses of individual forbs. Key words: wetlands, tallgrass prairie, Spartina pectinata communities, fire.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1139/b95-011}, author = {Johnson, S.R. and Alan K. Knapp} } @article {KNZ00478, title = {Interannual variability in primary production in tallgrass prairie: climate, soil moisture, topographic position and fire as determinants of aboveground biomass}, journal = {American Journal of Botany}, volume = {82}, year = {1995}, pages = {1024 -1030}, abstract = {From 1975 to 1993, aboveground net primary production (NPP) at the Konza Prairie Research Natural Area in NE Kansas varied from 179 g/m2 to 756 g/m2. Across a variety of sites, NPP was significantly related to precipitation (r2 = 0.37), but much variability was unexplained. Thus, we evaluated the relationship between NPP with meteorological variables and soil moisture measurements in tallgrass prairie sites that varied in fire frequency and topographic position. Annually burned lowland sites had significantly higher NPP than either annually burned upland or unburned sites. NPP in burned sites was more strongly related to meteorological variables and soil moisture when compared to unburned sites. The lack of significant correlation between soil moisture with NPP on unburned sites suggests that factors other than water availability limit production in these sites. When NPP data were analyzed separately by life forms, interannual variability in forb NPP was not correlated with any meteorological variables, but was negatively correlated with grass NPP (r = -0.49). The inability of a single factor, such as precipitation to explain a large portion of the interannual variability in NPP is consistent with the concept that patterns of NPP in tallgrass prairie are a product of spatial and temporal variability in light, water, and nutrients, driven by a combination of topography, fire history, and climate.}, keywords = {LTER-KNZ, tallgrass prairie}, url = {~http://www.jstor.org/stable/2446232}, author = {J. M. Briggs and Alan K. Knapp} } @article {KNZ00525, title = {Methane emissions to the atmosphere through emergent cattail (Typha latifolia L.) plants}, journal = {Tellus}, volume = {47}, year = {1995}, pages = {521 -534}, keywords = {LTER-KNZ}, doi = {10.3402/tellusb.v47i5.16065}, author = {Yavitt, J.B. and Alan K. Knapp} } @proceedings {KNZ00502, title = {The role of fire in Spartina pectinata wetlands dominated tallgrass prairie wetlands}, year = {1995}, pages = {92 -101}, publisher = {Tall Timbers Research Station}, address = {Tallahassee, FL}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Johnson, S.R. and Alan K. Knapp}, editor = {Cerulean, S.I. and Engstroms, R.T.} } @article {KNZ00486, title = {Stomatal and photosynthetic responses to variable light in sorghum, soybeans and eastern gammagrass}, journal = {Physiologia Plantarum}, volume = {94}, year = {1995}, pages = {613 -620}, abstract = {We studied photosynthetic and stomatal responses of grain sorghum (Sorghum bicolor [L.] Moench cv. Pioneer 8500), soybean (Glycine max L. cv. Flyer) and eastern gamagrass (Tripsacum dactyloides L.) during experimental sun and shade periods simulating summer cloud cover. Leaf gas exchange measurements of field plants showed that short-term (5 min) shading of leaves to 300{\textendash}400 μmol m-2 s-1 photosynthetic photon flux density reduced photosynthesis, leaf temperature, stomatal conductance, transpiration and water use efficiency and increased intercellular CO2 partial pressure. In all species, photosynthetic recovery was delayed when leaves were reilluminated, apparently by stomatal closure. The strongest stomatal response was in soybean. Photosynthetic recovery was studied further with soybeans grown indoors (maximum photosynthetic photon flux density 1 200 μmol m-2 s-1). Plants grown indoors had responses to shade similar to those of field plants, except for brief nonstomatal limitation immediately after reillumination. These responses indicated the importance of the light environment during leaf development on assimilation responses to variable light, and suggested different limitations on carbon assimilation in different parts of the soybean canopy. Photosynthetic oxygen evolution recovered immediately upon reillumination, indicating that the light reactions did not limit soybean photosynthetic recovery. While shade periods caused stomatal closure and reduced carbon gain and water loss in all species, the consequences for carbon gain/water loss were greatest in soybean. The occurrence of stomatal closure in all three species may arise from their shared phenologies and herbaceous growth forms.}, keywords = {LTER-KNZ, A:Ci curves, eastern gamagrass, Glycine max, induction, light, oxygen evolution, photosynthesis, sorghum, Sorghum bicolor, soybean, Stomatal Conductance, Tripsacum dactyloides, Water use efficiency}, doi = {10.1111/j.1399-3054.1995.tb00975.x}, author = {Fay, P.A. and Alan K. Knapp} } @proceedings {KNZ00524, title = {Wetlands may change tallgrass prairie from a sink to a source for atmospheric methane}, year = {1995}, pages = {137 -141}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC14/reference/econatres.napc14.jyavitt.pdf}, author = {Yavitt, J.B. and Alan K. Knapp}, editor = {D.C. Hartnett} } @proceedings {KNZ00436, title = {Aboveground biomass in tallgrass prairie: effect of time since last fire}, year = {1994}, pages = {165 -170}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC13/reference/econatres.napc13.jbriggs.pdf}, author = {J. M. Briggs and Fahnestock, J.T. and Ward, L. and Alan K. Knapp}, editor = {Wickett, R.G. and Lewis, P.D. and Woodliffe, A. and Pratt, P.} } @article {KNZ00455, title = {Effect of elevated C02 on stomatal density and distribution in a C4 grass and a C3 forb under field conditions}, journal = {Annals of Botany}, volume = {74}, year = {1994}, pages = {595 -599}, abstract = {Two common tallgrass prairie species, Andropogon gerardii, the dominant C4 grass in this North American grassland, and Salvia pitcheri, a C3 forb, were exposed to ambient and elevated (twice ambient) CO2 within open-top chambers throughout the 1993 growing season. After full canopy development, stomatal density on abaxial and adaxial surfaces, guard cell length and specific leaf mass (SLM; mg cm-2) were determined for plants in the chambers as well as in adjacent unchambered plots. Record high rainfall amounts during the 1993 growing season minimized water stress in these plants (leaf xylem pressure potential was usually > -1{\textperiodcentered}5 MPa in A. gerardii) and also minimized differences in water status among treatments. In A. gerardii, stomatal density was significantly higher (190 {\textpm} 7 mm-2; mean {\textpm} s.e.) in plants grown outside of the chambers compared to plants that developed inside the ambient CO2 chambers (161 {\textpm} 5 mm-2). Thus, there was a significant {\textquoteright}chamber effect{\textquoteright} on stomatal density. At elevated levels of CO2, stomatal density was even lower (P < 0{\textperiodcentered}05; 121 {\textpm} 5 mm-2). Most stomata were on abaxial leaf surfaces in this grass, but the ratio of adaxial to abaxial stomatal density was greater at elevated levels of CO2. In S. pitcheri, stomatal density was also significantly lower when plants were grown in the open-top chambers (235 {\textpm} 10 mm-2 outside vs. 140 {\textpm} 6 mm-2 in the ambient CO2 chamber). However, stomatal density was greater at elevated CO2 (218 {\textpm} 12 mm-2) compared to plants from the ambient CO2 chamber. The ratio of stomata on adaxial vs. abaxial surfaces did not vary significantly in this herb. Guard cell lengths were not significantly affected by growth in the chambers or by elevated CO2 for either species. Growth within the chambers resulted in lower SLM in S. pitcheri, but CO2 concentration had no effect. In A. gerardii, SLM was lower at elevated CO2. These results indicate that stomatal and leaf responses to elevated CO2 are species specific, and reinforce the need to assess chamber effects along with treatment effects (CO2) when using open-top chambers.}, keywords = {LTER-KNZ, Andropogon gerardii, elevated CO2, Salvia pitcheri, stomatal density, tallgrass prairie}, doi = {10.1006/anbo.1994.1159}, author = {Alan K. Knapp and Cocke, M. and Hamerlynck, E.P. and Owensby, C.E.} } @inbook {KNZ00469, title = {Effects of topography and fire on spatial and temporal distribution of soil moisture in a tallgrass prairie}, booktitle = {Time Domain Reflectometry in Environmental, Infrastructure and Mining Applications}, year = {1994}, pages = {154 -162}, publisher = {United States Department of Interior Bureau of Mines}, organization = {United States Department of Interior Bureau of Mines}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Su, H. and Alan K. Knapp and J. M. Briggs} } @article {KNZ00454, title = {Elevated CO2 alters dynamic stomatal responses to sunlight in a C4 grass}, journal = {Plant Cell and Environment}, volume = {17}, year = {1994}, pages = {189 -195}, abstract = {Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO2 levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C4 grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m-2 s-1 versus 350 μmol m-2 s-1 shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO2 when compared to plants at ambient CO2. This was due primarily to the 50\% reduction in stomatal conductance at elevated CO2, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29{\textendash}33\%) at elevated CO2. As a result, water loss was decreased by as much as 57\% (6.5\% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO2 enhanced plant water status. CO2-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO2 on plant water relations in all ecosystems.}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-3040.1994.tb00282.x}, author = {Alan K. Knapp and Fahnestock, J.T. and Owensby, C.E.} } @article {KNZ00443, title = {Leaf-level responses to light and temperature in two co-occurring Quercus (Fagaceae ) species: implications for tree distribution patterns}, journal = {Forest Ecology and Management}, volume = {68}, year = {1994}, pages = {149 -159}, abstract = {Leaf-level responses to light environment (sun vs. shade) and high temperature were compared in two gallery forest oaks, bur oak (Quercus macrocarpa) and chinquapin oak (Quercus muehlenbergii). Our goal was to determine if species-specific differences in response to light or temperature could explain the distribution of these oaks in tallgrass prairie gallery forests. On the Konza Prairie Research Natural Area in NE Kansas (USA), bur oak is more abundant than chinquapin oak along lower reaches of streams where the forests are most productive and canopy closure is greatest. We hypothesized that bur oak is better able to acclimate physiologically to reduced light availability than chinquapin oak. Leaf level acclimation to low light in both oaks occurred through reduced photosynthetic light compensation points (PLC) (bur oak, 60.77 {\textpm} 3.02 μmol photons m-2 s-1 in sun leaves vs. 29.25 {\textpm} 0.29 photons m-2 s-1 in shade leaves; chinquapin oak, 49.47 {\textpm} 2.82 photons m-2 s-1 in sun leaves vs. 26.48 {\textpm} 1.74 photons m-2s-1 in shade leaves), resulting from nearly 50\% reductions in dark respiration and specific leaf mass in both species. Apparent quantum requirement (Qreq; mol photons mol-1 O2) did not differ between sun and shade leaves in either species, and only bur oak showed differences in sun and shade leaf shape. We also hypothesized that chinquapin oak, which is more abundant in exposed, upper stream reaches, has greater photosynthetic tolerance to the high temperatures characteristics of this region. The maximum temperature tolerances (Tmax) of these oaks were compared by measuring increases in chlorophyll fluorescence. For most of the growing season, Tmax in chinquapin oak was 2.0{\textdegree}C higher than in bur oak. Field measurements of leaf vs. air temperatures in seedlings and adult trees indicated that Tmax was more likely to be exceeded in bur than in chinquapin oak, particularly in seedlings exposed to high solar radiation loads. We concluded that differences in thermal tolerance, in addition to previously documented differences in water relations are important determinants of distributional patterns of these oaks within gallery forests.}, keywords = {LTER-KNZ, gallery forest, Light acclimation, photosynthesis, Quercus, tallgrass prairie, Temperature tolerance}, doi = {10.1016/0378-1127(94)90042-6}, author = {Hamerlynck, E.P. and Alan K. Knapp} } @article {KNZ00438, title = {Mathematical simulation of C4 grass photosynthesis in ambient and elevated C02}, journal = {Ecological Modelling}, volume = {73}, year = {1994}, pages = {63 -80}, abstract = {A mechanistic leaf photosynthesis model was developed for C4 grasses based on a general simplified scheme of C4 plant carbon metabolism. In the model, the PEPcase-dependent C4-cycle was described in terms of CO2 concentration in the mesophyll space using Michaelis-Menten kinetics, and the activity of PEPcase was related to the incident PAR to take account of the influence of light on the activity of C4-cycle processes. The CO2 refixation by Rubisco in the bundle sheath was described using a widely accepted C3 photosynthesis model. The model assumes a steady state balance among CO2 diffusion from surrounding atmosphere into the mesophyll space, CO2 transport into the bundle sheath by the C4-cycle, CO2 refixation by the C3-cycle in the bundle sheath, and CO2 leakage from the bundle sheath. The response to temperature of photosynthesis was incorporated via the temperature dependence of model parameters. The photosynthesis model was coupled with a stomatal conductance model in order to predict leaf photosynthesis rates at different atmospheric conditions. The empirical model of Ball et al. (1987) was adopted and slightly modified to describe responses in stomatal conductance. The coupled model was parameterized for the C4 grass Andropogon gerardii grown in both ambient (350 ppm) and elevated (700 ppm) CO2 atmospheres. The key parameters of the model were estimated by fitting the model to the measured data using non-linear regression. The model was validated by comparison the predicted photosynthetic response to PAR in both CO2-pretreatments with the measured data from an independent gas exchange experiment. The predicted photosynthesis and stomatal conductance matched the measured data quite well for both atmospheric CO2-pretreatments. At 25{\textdegree}C, the estimated maximum carboxylation rate of Rubisco Vcm,25, potential electron transport rate Jm,25 and quantum efficiency α were increased by CO2 enrichment. The maximum PEPcase activity Vpm,25 was lower in elevated CO2. The model predicted that the light-saturated leaf photosynthesis will increase by about 10\% with the rising of atmospheric CO2 from 350 to 700 ppm at 30{\textdegree}C, and that the optimal temperature of photosynthesis will shift from 37 to 38.5{\textdegree}C. The estimated slope of the stomatal conductance model was increased by atmospheric CO2 enrichment. Stomatal conductance was significantly reduced by increasing atmospheric CO2 concentration.}, keywords = {LTER-KNZ, Andropogon gerardii, C4 grass, CO2 enrichment, photosynthesis, Stomatal Conductance}, doi = {10.1016/0304-3800(94)90098-1}, author = {Chen, D.X. and Coughenour, M.B. and Alan K. Knapp and Owensby, C.E.} } @article {KNZ00439, title = {Responses of forbs and grasses to selective grazing by bison: interactions between herbivory and water stress}, journal = {Vegetatio}, volume = {115}, year = {1994}, pages = {123 -131}, abstract = {Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53\% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10 ?C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40\% greater and reproductive biomass and head number of V. baldwinii was 45\% and 40\% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.}, keywords = {LTER-KNZ, bison, Light reduction, Selective herbivory, tallgrass prairie, water stress}, doi = {10.1007/BF00044867}, url = {http://link.springer.com/article/10.1007/BF00044867}, author = {Fahnestock, J.T. and Alan K. Knapp} } @article {KNZ00444, title = {Stomatal responses to variable sunlight in Bur Oak (Quercus macrocarpa Michx.) leaves with different photosynthetic capacities}, journal = {International Journal of Plant Science}, volume = {155}, year = {1994}, pages = {583 -587}, abstract = {Interspecific comparisons of stomatal response to variable light have generally supported the hypothesis that stomatal adjustments are more rapid in plants with high photosynthetic capacity. We compared stomatal responses to variable light in bur oak (Quercus macrocarpa) leaves that had low photosynthetic capacity and high apparent anthocyanins levels ("red leaves") with responses in green leaves. When incident sunlight was experimentally varied, stomatal conductance (gs) in red leaves was no less responsive than in green leaves. In both leaf types, gs declined at the same rate ($18.9\ {\rm mmol}\ {\rm m}^{-2}\ {\rm s}^{-1}\ {\rm min}^{-1}$) upon shading and increased at the same rate ($13.4\ {\rm mmol}\ {\rm m}^{-2}\ {\rm s}^{-1}\ {\rm min}^{-1}$) when reilluminated. The only difference in stomatal response in red leaves was an overall reduction in gs compared with green leaves. In shade, net photosynthetic rates (${\rm A}_{\text{net}}$) in red leaves were very low (${\rm ca}.\ 2.3\ \mu {\rm mol}\ {\rm m}^{-2}\ {\rm s}^{-1}$) but still nearly 70\% of full sun rates ($3.2\ \mu {\rm mol}\ {\rm m}^{-2}\ {\rm s}^{-1}$). In contrast, ${\rm A}_{\text{net}}$ in green leaves reduced by nearly 50\% in shade ($7.2\ \mu {\rm mol}\ {\rm m}^{-2}\ {\rm s}^{-1}$) relative to full sun rates ($14.1\ \mu {\rm mol}\ {\rm m}^{-2}\ {\rm s}^{-1}$). Internal CO2 concentrations ( C i) were always ${\rm ca}.\ 50\ \mu {\rm L}\ {\rm L}^{-1}$ higher in red than in green leaves. Upon reillumination, ${\rm A}_{\text{net}}$ in red leaves immediately returned to preshade levels, while green leaves recovered more slowly, perhaps because of stomatal limitation, as indicated by reduced C i. We concluded that, although rapid stomatal responses are often correlated with photosynthetic capacity among species, within Quercus macrocarpa, stomatal responses to rapidly varying light are not related to leaf photosynthetic capacity.}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/2475031}, author = {Hamerlynck, E.P. and Alan K. Knapp} } @proceedings {KNZ00456, title = {Water relations and biomass responses to irrigation across a topographic gradient in tallgrass prairie}, year = {1994}, pages = {215 -220}, keywords = {LTER-KNZ, Water relations}, url = {http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC13/reference/econatres.napc13.aknapp.pdf}, author = {Alan K. Knapp and Koelliker, J.K. and Fahnestock, J.T. and J. M. Briggs}, editor = {Wickett, R.G. and Lewis, P.D. and Woodliffe, A. and Pratt, P.} } @article {KNZ00417, title = {Biomass production in a tallgrass prairie ecosystem exposed to ambient and elevated CO2}, journal = {Ecological Applications}, volume = {3}, year = {1993}, pages = {644 -653}, keywords = {LTER-KNZ, Aboveground biomass, Andropogon gerardii, Carbon dioxide, elevated CO2, kansas, Poa pratensis, Root biomass, tallgrass prairie, water-use efficiency, xylem pressure potential}, doi = {10.2307/1942097}, author = {Owensby, C.E. and Coyne, P.I. and Ham, J.M. and Auen, L.M. and Alan K. Knapp} } @article {KNZ00391, title = {Comparative water relations of seedling and adult Quercus species during gallery forest expansion in tallgrass prairie}, journal = {Forest Ecology and Management}, volume = {56}, year = {1993}, pages = {29 -41}, abstract = {Seasonal patterns in xylem pressure potential (ψ) and stomatal conductance to water vapor (g) were compared between seedlings (1{\textendash}5 years old) invading tallgrass prairie and adult trees of Quercus macrocarpa (Bur oak) and Quercus muehlenbergii (Chinquapin oak) in adjacent gallery forest. Water is often limiting to productivity in the Flint Hills of NE Kansas, thus, we hypothesized that seedlings invading a grassland would have lower ψ and g compared with adult trees, reflecting competitive interactions for water between seedlings and the dominant grasses. We found that ψ was usually lower in adult trees than in older seedlings, but in the very smallest seedlings ψ was significantly reduced compared with mature (greater than 50 years) individuals. Adult and 1- to 2-year-old seedlings had similar g during the season with highest g occurring in older (4- to 5-year-old) seedlings. In general, Q. muehlenbergii had higher ψ and lower g than Q. macrocarpa consistent with reports that Q. muehlenbergii is more drought tolerant. Rapid and extensive root growth characteristic of Quercus seedlings reduced the period of time when seedlings invading prairie must compete with the grasses for water, although water stress may still be an important factor affecting first-year seedlings in this ecosystem. In contrast, strong intraspecific competition for water within mature gallery forests may have reduced ψ and g in forest trees. Thus, the potential for water stress to impact Quercus survival and growth in this drought-prone ecosystem is greatest during establishment and then after forest development is complete. Older seedlings (greater than 4{\textendash}5 years old), saplings and isolated trees within the prairie may experience the least water stress due to a reduction in both inter- and intraspecific competition for water. These data suggest that in the absence of severe fire or drought occurring during the growing season, gallery forests will continye to expand into tallgrass prairie.}, keywords = {LTER-KNZ, Water relations}, doi = {10.1016/0378-1127(93)90101-R}, author = {Bragg, W.K. and Alan K. Knapp and J. M. Briggs} } @article {KNZ00427, title = {Consequences of non-equilibrium resource availability across multiple time scales: the transient maxima hypothesis}, journal = {American Naturalist}, volume = {141}, year = {1993}, pages = {621 -633}, abstract = {Nonequilibrium biotic responses to changes in resource limitation dominate the behavior of tallgrass prairie ecosystems. Rates of leaf photosynthesis on a time scale of minutes, amounts of annual plant productivity, patterns in the productivity of certain consumer groups, and amounts of soil organic matter accumulation over millennia all reflect biotic responses to frequent and recurring shifts in limiting resources. Productivity is higher during a transition period when the relative importance of an essential resource is changing than during an equilibrium interval generated by single resource limitation. These "transient maxima" are both characteristic and easily measurable in the tallgrass prairie because of the unpredictable climate and ecological constraints such as grazing and recurrent fires that modify water, nitrogen, and light availability. Such diverse phenomena as overcompensation for herbivory, the intermediate disturbance hypothesis, maximum levels of productivity observed in successional ecosystems, and widespread nitrogen limitation in terrestrial and aquatic ecosystems can be explained by biotic response to shifts in limiting resources.}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/2462753}, author = {Seastedt, T.R. and Alan K. Knapp} } @article {KNZ00410, title = {The effect of fire on gas exchange and aboveground production in Spartina pectinata wetlands}, journal = {Wetlands}, volume = {13}, year = {1993}, pages = {299 -303}, abstract = {Photosynthetic and growth responses ofSpartina pectinata were compared in annually and biennially burned wetlands in a northeastern Kansas tallgrass prairie. Photosynthetic CO2 uptake was consistently higher inSpartina pectinata in annually burned wetlands, and there was a seasonal difference in maximum photosynthetic rates. Plants in annually burned wetlands reached a maximum photosynthetic rate of 38.2 μmol m-2 s-1 in late spring, while maximum photosynthesis in plants of biennially burned wetlands was 28.6 μmol m-2 s-1 in late summer. Culm density in these wetlands was not affected by fire; however, aboveground biomass production, inflorescence density and plant height at anthesis were significantly greater in annually burned wetlands. These results indicate thatSpartina pectinata may be a fire-dependent species, with post-fire responses similar to the dominant grasses in tallgrass prairie as well as otherSpartina congeners.}, keywords = {LTER-KNZ, fire, gas exchange, prairie wetlands, production, Spartina pectinata, tallgrass prairie}, doi = {10.1007/BF03161296}, author = {Johnson, S.R. and Alan K. Knapp} } @article {KNZ00412, title = {Gas exchange dynamics in C3 and C4 grasses: consequences of differences in stomatal conductance}, journal = {Ecology}, volume = {74}, year = {1993}, pages = {113 -123}, abstract = {In full sunlight (photosynthetic photon flux density, 0.4{\textemdash}0.7. μm, PPFD), stomatal conductance to water vapor (g) and transpiration rates (E) are usually higher, and water use efficiency lower, in C3 than C4 species. In the C3 grass Bromus inermis, the difference in conductance between full sun (photosynthetic photon flux [PPFD] > 1500 μmol{\textdegree}m{\textemdash}2{\textdegree}S{\textemdash}1) and shade (PPFD {\textdegree} 400 μmol{\textdegree}m{\textemdash}2{\textdegree}s{\textemdash}1) was twice as large as in the C4 species Sorghastrum nutans (226.7 vs. 97.8 mmol.m{\textemdash}2.S{\textemdash}1, respectively). Similar differences were measured in four additional C3 and C4 grasses. Variability in PPFD incident on leaves is very common in grasslands, and stomata are known to respond relatively rapidly to fluctuations in this resource. This study evaluated the consequences of different responses in stomatal conductance between C3 and C4 grasses during periods of sunlight variability. Photosynthesis (A, measured as CO2 uptake) and g varied substantially in both B. inermis and S. nutans as PPFD fluctuated between sun and shade levels (4{\textemdash}10 min intervals). Time constants derived form first{\textemdash}order equations describing responses in g after abrupt changes in PPFD indicated that g approached steady{\textemdash}state levels more rapidly in S. nutans than in B. inermis. However, because of the greater magnitude of change in g between sun and shade PPFD in B. inermis, the absolute rate of change in g in this C3 grass was twice as rapid compared to S. nutans (25.7 vs. 12.4 mmol{\textdegree}m{\textemdash}2{\textdegree}s{\textemdash}1{\textdegree}min{\textemdash}1 for reductions in g, respectively). New steady{\textemdash}state levels in g were achieved in 12{\textemdash}13 min after a change in PPFD in S. nutans and 15{\textemdash}19 min in B. inermis. A statistically based two{\textemdash}state (sun/shade) simulation model was used to quantify costs in A and E of the nonsteady{\textemdash}state responses measured for both species as PPFD varied. Costs were calculated as the difference between model output of A or E based on field responses and model output based on hypothetical gas exchange responses in which A and g changed instantaneously. Results suggested that even though the absolute rate of change in g changed was greater in the C3 grass, costs in E during periods of short{\textemdash}term (2{\textemdash}6 min) variations in PPFD were as much as sixfold higher than in the C4 species. Lower cost in E in C4 species during periods of variable PPFD (due to inherently smaller alterations in g) is a previously unrecognized consequence of the C4 photosynthetic pathway. C4 species, and grasses in particular, often are most successful in high{\textemdash}PPFD, water{\textemdash}limited environments in which leaf{\textemdash}level variability in PPFD occurs daily from cloud cover and within{\textemdash}canopy shading. In these environments, stomatal responses that conserve water under nonsteady{\textemdash}state conditions may contribute significantly to the success of C4 species.}, keywords = {LTER-KNZ}, doi = {10.2307/1939506}, author = {Alan K. Knapp} } @article {KNZ00399, title = {Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps}, journal = {Oecologia}, volume = {93}, year = {1993}, pages = {114 -120}, abstract = {Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (Ψ). Preliminary data showed that in drought-stressed plants galled shoots had 36\% greater A, and 10\% greater stem Ψ than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher Ψ, g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and Ψ, but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO2 concentration relationships were consistent with the explanation that increased Ψ allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low-Ψ limitations on non-stomatal components of A. Our working hypothesis is that galls increase Ψ and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased Ψ and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.}, keywords = {LTER-KNZ, Gall insect, photosynthesis, Plant-insect interactions, Silphium integrifolium, Water potential}, doi = {10.1007/BF00321200}, author = {Fay, P.A. and D.C. Hartnett and Alan K. Knapp} } @article {KNZ00413, title = {Landscape patterns in soil-water relations and primary production in tallgrass prairie}, journal = {Ecology}, volume = {74}, year = {1993}, pages = {549 -560}, abstract = {Landscape variation in soil water relations, leaf xylem pressure potential ({\textdegree}) and leaf{\textemdash}level net photosynthesis (A) in Andropogon gerardii, and net primary production (NPP) were evaluated during the 1989 and 1990 growing seasons in a northeast Kansas (USA) tallgrass prairie. Landscape patterns were assessed along transects that spanned upland and lowland topographic positions in an annually burned and a long{\textemdash}term unburned watershed. Landscape variability in volumetric soil water content ({\textdegree}) was significantly greater in the unburned watershed (coefficient of variation [CV] = 0.425 and 0.479 for 0{\textemdash}15 and 0{\textemdash}30 cm soil depths in unburned prairie vs. 0.285 and 0.330 for similar depths in the burned watershed). In both watersheds, significantly higher {\textdegree} and total soil water content (0{\textemdash}30 cm) were measured in lowlands compared to uplands. Topographic anomalies, such as a lowland ridge, resulted in local, small{\textemdash}scale variation in soil moisture that equaled watershed variation. Variation across landscapes in predawn {\textdegree}, which was expected to reflect soil water content, was similar in both watersheds (CV = 0.312). Variation in midday @j was significantly greater across the burned than the unburned watershed in 1990 (maximum range in @j from uplands to lowlands was 0.708 MPa at predawn and 0.662 MPa at midday in the burned watershed). In both watersheds, variation in midday @j was much lower relative to {\textdegree}. Landscape patterns in leaf{\textemdash}level A in A. gerardii, the dominant species in this tallgrass prairie, were inconsistent when upland and lowland sites were compared. During an extended period of drought, A was significantly higher in plants in the unburned watershed. In both watersheds, NPP was strongly correlated with {\textdegree}. However, variability in NPP across topographic gradients in the unburned watershed was much less pronounced (CV = 0.224{\textemdash}0.245) than in the annually burned watershed (CV = 0.364{\textemdash}0.430). Moreover, the slope of the relationship between NPP and {\textdegree} was significantly greater in the annually burned watershed. We propose that relatively uniform energy limitations across topographic gradients in unburned tallgass prairie, caused by detrital accumulation that absorbs/reflects sunlight, reduced topographic variability in NPP in unburned watersheds. This pattern occurred in unburned watersheds despite greater landscape variation in {\textdegree} relative to burned watersheds. Analysis of long{\textemdash}term records of NPP from several watersheds supported the hypothesis that variability in NPP associated with topographic position is lower in unburned vs. burned watersheds. Variability in @j across watersheds and between years was muted by negative feedback of canopy leaf area (transpiring surface) on plant{\textemdash}soil water relations. We concluded that patterns in landscape variability in A and @j which may vary significantly over short time intervals, were not good predictors of seasonal carbon dioxide exchange or productivity in this tallgrass prairie. Nonetheless, interactions between A and @j, when combined with nitrogen and energy limitations to A. provide the mechanisms for integrated responses measured across these landscapes.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/1939315}, author = {Alan K. Knapp and Fahnestock, J.T. and Hamburg, S.P. and Statland, L.J. and Seastedt, T.R. and Schimel, D.S.} } @article {KNZ00397, title = {Photosynthetic and stomatal responses of Avena sativa (Poaceae) to a variable light environment}, journal = {American Journal of Botany}, volume = {80}, year = {1993}, pages = {1369 -1373}, abstract = {The net photosynthetic (A), stomatal conductance to water vapor (g), water use efficiency (WUE = A/transpiration), and leaf water potential (Ψ) responses of cultivated oats (Avena sativa) were determined under an experimental regime of alternating full sun (photosynthetic photon flux density, PPFD > 1,700 μmol{\textperiodcentered}m-2{\textperiodcentered}sec-1) and shade (300-400 μmol{\textperiodcentered}m-2 sec-1 PPFD). Less extensive measurements were made on winter wheat (Triticum aestivum) to test the generality of crop species{\textquoteright} responses. The rates of stomatal opening/closing after changes in PPFD in Avena and Triticum were compared with previously determined rates for native grasses and forbs to assess how domestication might have altered stomatal dynamics. Characteristics of Avena under alternating sun and shade were 1) rapid fluctuations in A between full sun (\~{}27 μmol{\textperiodcentered}m-2 sec-1) and shade rates (\~{}12-13 μmol m-2{\textperiodcentered}sec-1); 2) slower changes in g, causing progressive stomatal closure during the measurement sequence and possible stomatal limitation of A; 3) no change in leaf Ψ; and 4) a net reduction in WUE. Triticum had similar sun/shade A, g, and WUE dynamics, except that Triticum stomata returned to full-sun g between shade periods. The rates of change of g in Avena and Triticum were lower than for some desert and subalpine native species, but were similar to rates for species in adjoining native habitat. The basic stomatal dynamics of Avena and Triticum may typify many cultivated C3 species, and these data indicate that crop stomatal behavior has not diverged significantly from that of native species.}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/2445664}, author = {Fay, P.A. and Alan K. Knapp} } @article {KNZ00396, title = {Water relations and growth of tallgrass prairie forbs in response to selective herbivory by bison}, journal = {International Journal of Plant Science}, volume = {154}, year = {1993}, pages = {432 -440}, abstract = {The indirect effects of selective grass herbivory by bison (Bison bison) on the xylem pressure potentials (ψ) and primary production of ungrazed tallgrass prairie forbs were assessed during the 1990 growing season on the Konza Prairie Research Natural Area (KPRNA) in northeast Kansas. Responses in three forb species, Vernonia baldwinii, Ambrosia psilostachya, and Aster ericoides, were measured in grazed and adjacent ungrazed patches. Seasonal predawn and midday ψ responses in these forbs were also compared with responses in Andropogon gerardii, the dominant grass in tallgrass prairie. Reductions in grass leaf area by herbivory may result in greater availability of soil water to the remaining forbs, if the forbs and grasses utilize similar soil water resources. Overall, selective herbivory of the grasses had an inconsistent effect on leaf ψ in the remaining forbs. When ψ were relatively high during the growing season, no significant differences in water status were detected between A. gerardii and the forbs. However, when ψ were low, predawn and midday ψ were higher in V. baldwinii (as much as 1.0 MPa), and midday ψ were higher in A. psilostachya (0.5 MPa) compared with A. gerardii. In contrast, ψ were not significantly different between A. ericoides and A. gerardii. Grazing by bison substantially increased light availability to ungrazed forbs (> 40\%) in grazed relative to ungrazed areas. Compared with ungrazed areas, end-of-season aboveground biomass in grazed patches was 40\% greater for individuals of A. psilostachya and 33\% and 26\% greater for reproductive biomass and floret numbers, respectively, in V. baldwinii. We conclude that the potential benefits of selective grass herbivory on forb water relations may be reduced or offset by other factors such as significant alterations in the microclimate of grazed patches. Nonetheless, increased productivity in grazed patches in two of the three forbs studied indicates that selective consumption of grasses by bison may benefit some forb species.}, keywords = {LTER-KNZ, Water relations}, url = {http://www.jstor.org/stable/2995662}, author = {Fahnestock, J.T. and Alan K. Knapp} } @article {KNZ00387, title = {Water relations of Juniperus virginiana and Andropogon gerardii in an unburned tallgrass prairie watershed}, journal = {Southwestern Naturalist}, volume = {38}, year = {1993}, pages = {325 -330}, abstract = {We assessed several factors influencing the success of Juniperus virginiana in unburned tallgrass prairie by mapping the distribution of individuals in a 21-ha watershed on the Konza Prairie Research Natural Area in NE Kansas. We also compared the water relations of this tree with those of the dominant grass Andropogon gerardii. No general relationships were detected between the distribution of J. virginiana and plant water status, plant size, soil type, slope, aspect or elevation. Leaf xylem pressure potential (ψ) in upland trees was significantly lower (0.44 MPa) than in lowland trees, but only during the driest portion of the growing season. In contrast, topographic position influenced ψ more strongly in A. gerardii with ψ in upland plants as much as 1.0 MPa lower than in lowland plants. During wetter periods, photosynthetic rates in A. gerardii were higher than in J. virginiana in uplands, but the reverse occurred during the driest period of the growing season. The ability of J. virginiana to maintain higher ψ and higher photosynthetic rates relative to A. gerardii during periods of water limitation may contribute to its success in tallgrass prairie protected from fire.}, keywords = {LTER-KNZ, Water relations}, doi = {10.2307/3671610}, author = {Axmann, B.D. and Alan K. Knapp} } @article {KNZ00368, title = {Evaluation of the closed-chamber method for estimating methane emissions from aquatic plants}, journal = {Tellus}, volume = {44B}, year = {1992}, pages = {63 -71}, abstract = {

Closed flow chambers are commonly used to measure CH4 emissions from aquatic plants. We monitored environmental and plant physiological responses to closed chambers during measurements of CH4 emission from Typha latifolia (cattails) to evaluate potential errors in emission estimates. In 1990, two wetlands were studied: one in NE Kansas and one in central New York. Leaves and stems of Typha enclosed in gas-impermable, collapsible bags were exposed to rapid increases in chamber air temperature and, thus, tissue temperatures (up to 9oC above ambient) as well as rapid reductions in both CO2 partial pressure due to photosynthesis and the leaf-to-air water vapor pressure deficit due to transpiration. These rapid (\< 5 min) environmental changes within chambers are known to affect stomatal aperture through which CH4 exists Typha leaves. Indeed, stomatal conductance (g) increased up to 42\% compared to control leaves after 19 min of enclosure in chambers, and chamber effects on interactions between irradiance and g were detected. The duration of enclosure also critically affected the CH4 gradient within chambers. Estimated CH4 emission from leaves in chambers sampled after 19 min (3 nmol m-2s-1) was 10-fold lower than estimates based on 30-s sampling periods (30 nmol m-2s-1) due to reduction in the leaf-to-air concentration gradient for CH4. Since closed chambers 1)induce alterations in g, 2)may result in rapid environmental changes, and 3)lead to almost instantaneous reductions in leaf-to-air concentration gradient for CH4 diffusion, we suggest that only very short enclosure times should be used when measuring CH4 emission from plants

}, keywords = {LTER-KNZ}, doi = {10.1034/j.1600-0889.1992.00006.x}, author = {Alan K. Knapp and Yavitt, J.B.} } @article {KNZ00367, title = {Leaf gas exchange in Quercus macrocarpa (Fagaceae): rapid stomatal responses to variability in sunlight in a tree growth form}, journal = {American Journal of Botany}, volume = {79}, year = {1992}, pages = {599 -604}, keywords = {LTER-KNZ}, doi = {http://www.jstor.org/stable/2444875}, author = {Alan K. Knapp} } @article {KNZ00307, title = {Estimating aboveground biomass in tallgrass prairie with the harvest method: determining proper sample size using jackknifing and Monte Carlo simulations}, journal = {The Southwestern Naturalist}, volume = {36}, year = {1991}, pages = {1 -6}, abstract = {

Sample adequacy for estimating aboveground biomass in the Flint Hills (Kansas) tallgrass prairie was determined by using a combination of jackknifing and Monte Carlo simulations. Based on this analysis, we concluded that 14 and 16 0.1 -m2 (20 by 50 cm) quadrats should be harvested in burned and unburned sites, respectively, when estimating total, live graminoid and previous years dead biomass. However, 20 quadrats should be harvested to detect at least a 20\% difference in production between sites due to treatments such as fire. Due to high variability in forb biomass, other measurements (cover or density) may be more appropriate for detecting fire effects on this group of plants

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/3672110}, author = {J. M. Briggs and Alan K. Knapp} } @article {KNZ00335, title = {Physiological interactions along resource gradients in a tallgrass prairie}, journal = {Ecology}, volume = {72}, year = {1991}, pages = {672 -684}, abstract = {

Spatial variability in availability of resources that limit photosynthesis (water and N) leads to variation in rates of atmosphere- biosphere exchange. N content and allocation are canopy properties that link ecosystem, physiological, and biophysical processes and that vary in space at scales relevant to atmosphere-biosphere interaction. We studied landscape-scale variation in these and related canopy properties in Kansas Tallgrass Prairie (USA). The tallgrass ecosystem was suited to this investigation because primary production in the prairie is constrained by N availability. This work was designed to aid in interpertation and spatial extrapolation of gas exchange measurements made using aerodynamic techniques as part of FIFE (First ISLSCP Field Experiment), a NASA- supported study. We collected data on spatial disrtibution of biomass, leaf area index (LAI), canopy N mass. N concentration ([N]), and gas exchange along topographic and management gradients. We also measured height distribution of N, light interception, and gas exchange within canopies as a function of position in the landscape. Substantial variation in biomass, LAI, N accumulation, and N allocation occurred over time, with topography, and as a result of previous burning. The verticle gradient of [N] and photosynthetic capacity within canopies were correlated, in space and time, with biomass and canopy light interception. The gradients were steper in high biomass sites than in low biomass sites. In addition, proportional N allocation to the upper layer increased with time (12\% in June, 32\% in August) as biomass increased. As nutrient uptake increased within the tallgrass landscape, biomass increased and light limitation in the lower canopy was induced. As this light limitation increased with increasing biomass, or with accumulation of dead vegetation, allocation of N to the upper canopy increased. Height distribution of photosyntheticd capicity paralleled within-canopy N allocation and light interception. As resource ratios (light, water and nitrogen) varied in the landscape, so did rates of gas exchange. This work suggests that in interactions between light extinction, N allocation, and photosynthesis that have been proposed for monospecific stands apply to the multispecies, but structurally simple, canopy of the tallgrass prairie. Models of plant performance based on evolutionary arguments may provide a powerful basis for spatial extarapolation of atmosphere-ecosystem exchange rates from sites to landscape and larger regions. Key words: atmosphere-ecosystem exchange, FIFE; Konza Prairie Long-Term Ecological Research site; leaf area index; light interception; nitrogen allocation; photosynthetic capacity; photosynthetically active radiation; remote sensing; toposequences

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/2937207}, author = {Schimel, D.S. and Kittel, T.G.F. and Alan K. Knapp and Seastedt, T.R. and Parton, W.J. and Brown, V.B.} } @article {KNZ00290, title = {Influence of plant size on the carbon and water relations of Cucurbita foetidissima HBK}, journal = {Functional Ecology}, volume = {4}, year = {1990}, pages = {789 -797}, abstract = {

Plant size varies by two orders of maginitude within populations of Cucurbita foetidissima HBK. a perennial, prostrate vine of deserts and grasslands in the western USA. Differences in plant size in in this species growing in Kansas (USA) brome grassland were most evident in leaf size (10 times greater in larger vs 13 g in small plants). and taproot mass (c 7000g in large vs 13g in small plants). This study examined the influence of plant size on net photosynthetic CO2 uptake (A), stomatal conductance to water vapour (g), transpiration rate (E), xylem pressure potential and leaf temperature (Tleaf). During a period of drought in which co-occurring grasses and forbs experienced significant water stress (xylem pressure potential\< -2*5 MPa), xylem pressure potential in large- and intermediate-sized plants of C. foetidissima was always\> -0*6 MPa. In small plants, minimum xylem pressur potential was c-1*3 MPa. No substantial diurnal reductions in A or g were measured in the two groups of larger plants, but CO2 uptake was reduced to zero after midday in small plants, presumably due to water stress. Increased mass of succulent taproots and greater rooting depth may have contributed to greater xylem pressure potential in larger vs small individuals of C. foetidissima. Water use efficiency (WUE) was greater in larger vs small individuals of C. foetidissima. Water use efficiency (WUE) was greater in leaves from large- and intermediate-sized plants relative to small plants under all environmental conditions. During brief periods of low irradiance due to clouds or artificial shading, 5-10oC reduction in Tleaf in plants with large leaves resulted in low E, enhancing WUE. Much smaller reductions in Tleaf and E occurred during shade in the small leaves of small individuals. These results indicate the greater size in C. foetidissima, while not increasing interception of irradiance in this grassland, resulted in greater independence from resource limitations as well as water conservation during periods of variable sunlight. Key words: Cucurbita, photosynthesis, plant size, water stress

}, keywords = {LTER-KNZ, Water relations}, doi = {~10.2307/2389445}, author = {Alan K. Knapp and Fahnestock, J.T.} } @article {KNZ00274, title = {Response of Andropogon gerardii to simulated acid rain}, journal = {Transactions of the Kansas Academy of Science}, volume = {93}, year = {1990}, pages = {85 -90}, abstract = {

In this study the authors examined the effects that moderate exposure to acidic precipation had on growth and physiological responses in Andropogon gerardii, the dominant grass in tallgrass prairie. Naturally occurring monocultures of A. gerardii were treated with simulated acid rain (pH 4.0 or 3.0) or precipitation with a pH similar to natural rainfall (pH 5.0) throughout a growth season. No visual effects of the acid rain treatments were observed and there were no statistically significant effects on net photosynthetic CO2 uptake, stomatal conductance to water vapor, or tiller mass in A. gerardii. However, tiller density was significantly reduced as treatment pH decreased. These data suggest that a moderate increase in the number of rainfall events with low pH will have little discernible short-term impact on tallgrass prairie dominated by A. gerardii, but longer-term effects need to be evaluated

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/3628150}, author = {Fahnestock, J.T. and Alan K. Knapp} } @article {KNZ00257, title = {Death Camas (Zigadenus nuttallii) in Kansas: pollen collectors and a florivore}, journal = {Journal of the Kansas Entomological Society}, volume = {62}, year = {1989}, pages = {411 -412}, abstract = {

All parts of plants of the genus Zigadenus (Liliaceae) contain numerous alkaloids through both vegetative and reproductive periods. The purpose of this note is to report the findings of incidental observations and insect collections made during a sutdy of the ecophysiologiy of Z. nuttallii

}, keywords = {LTER-KNZ, insect}, author = {Tepedino, V.J. and Alan K. Knapp and Eickwort, G.C. and Ferguson, D.C.} } @article {KNZ00142, title = {Canopy rainfall interception and throughfall in burned and unburned tallgrass prairie}, journal = {The Southwestern Naturalist}, volume = {32}, year = {1987}, pages = {267 -271}, abstract = {

Interception of precipitation by grass canopies in annually burned and unburned tallgrass prairie was measured for individual precipitation events from 1983 to 1984. Seasonal patterns of interception closely reflected differences in grass canopy development throughout the growing season in burned but not unburned prairie. Mean interception for the study period was 38 and 19\% for unburned and burned prairie, respectively, indicating tht througfall volume in burned prairie was approximately 1.3 times that of unburned prairie on an annual basis. Thus, water availability for plant uptake may be initally higher in burned prairie, especially early in the growing season

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/3671570}, author = {Gilliam, F.S. and Seastedt, T.R. and Alan K. Knapp} } @article {KNZ00121, title = {Detritus accumulation limits productivity of tallgrass prairie}, journal = {BioScience}, volume = {36}, year = {1986}, pages = {662 -668}, abstract = {

A number of ecosystems in North American are maintained by fire, and other systems such as the Serengetti grasslands and taiga forest decline in productivity when fire or grazing is excluded. We believe, however, that the tallgrass prairie is unique among the North American biomes because of the strong negative impacts of plant ligtter production on subsequent ecosystem function. These deleterious effects on plant litter include: a reduction in available light energy to the system; an alteration of the microclimate and physiology of emerging shoots such that carbon dioxide uptake is reduced; a conversion of immediately usable inorganic notrogen in rainwater to less readily available organic nitrogen in microbial biomass; an inhibition of nitrogen fixation by free living microbes and blue-green algae as a result of phosphorus and/or light limitation; and a reduction in soil temperatures, which diminishes root productivity, invertebrate activities, and probably microbial activities as well. Fire and grazing therefore become necessary, integral ecosystem processes that maintain productivity of tallgrass prairie by the removal of standing and fallen litter

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {http://www.jstor.org/stable/1310387}, author = {Alan K. Knapp and Seastedt, T.R.} } @article {KNZ00119, title = {Ecophysiology of Zigadenus nuttallii , a toxic spring ephemeral in a warm season grassland: effect of defoliation and fire}, journal = {Oecologia}, volume = {71}, year = {1986}, pages = {69 -74}, abstract = {

Zigadenus nuttallii, a highly toxic spring ephemeral in tallgrass prairie, was studies in 1985 to ascertain: 1) several ecophysiological characteristics of the species, 2) seasonal patterns of biomass accumulation, and 3) its response to defoilation and fire. The maximum photosynthetic rate of Z. nuttallii measured in unburned prairie was 13.2 micromoles CO2 m-2s-1 which occurred at 24-28o C and an incident quantum flux of 0.8-1.0 mmoles m-2s-1. Maximum stomatal conductance measured was 5.4 mm s-1. Early in the season, belowground storage organs (bulbs) decreased in mass and supplied much of the energy for growth of leaves, even though CO2 uptake was possible. Bulb mass did not increase until about 6 weeks after shoot emergence implying that, at this time, leaves had become a source rather than a sink for carbohydrates. The result of a single, severe defoliation event was a decrease in biomass of bulbs, leaves and reproductive structures in Z. nuttallii. Intrinsic compensatory mechanisms were not detected. In contrast, fire, which also defoliated plants, did not result in any biomass decrease at the end of the season. Improved post-fire microclimate and increased nutrient supply (extrinsic factors) may have contributed to higher photosynthetic rates and led to biomass compensation in burned prairie. These data support arguments that intrinsic compensatory mechanisms have evolved in response to chronic herbivory. Key words: Photosynthesis, carbon storage, fire, herbivory, Zigadenus

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1007/BF00377323}, author = {Alan K. Knapp} } @article {KNZ00118, title = {Postfire water relations, production, and biomass allocation in the shrub, Rhus glabra, in tallgrass prairie}, journal = {Botanical Gazette}, volume = {147}, year = {1986}, pages = {90 -97}, abstract = {

Several populations of Rhus glabra were studies to assess the effect of fire on productivity, biomass allocation, N and chlorophyll content, and seasonal water relations in a North American tallgrass prairie. Compared with shoots in unburned prairie, shoots from burned sites had reduced biomass and fewer leaves; however, a significant postfire increase in shoot density led to similar midseason leaf area indices for both populations. Productivity was greater in the unburned population principally because of the biomass of sexual reproductive structures, which were absent in the resprouting shoots. However, burned populations allocated a greater proportion of biomass to leaves and wood, utilized for vertical growth, than unburned populations. Leaves in unburned prairie had greater stomatal density, specific leaf mass, and chlorophyll and N content than leaves from the burned population. For much of the growing season, stomatal conductances were greater in unburned prairie. Despite lower soil water potentials at the burned site, differences in midday leaf water potentials were small between populations; hence, differences in N content probably contributed more to the responses. The postfire reduction in productivity and sexual reproduction in R. glabra indicates that fire reduces its fitness

}, keywords = {LTER-KNZ, Water relations}, doi = {http://www.jstor.org/stable/2474813}, author = {Alan K. Knapp} } @article {KNZ00120, title = {Production, density and height of flower stalks of three grasses in annually burned and unburned eastern Kansas tallgrass prairie: a four year record}, journal = {The Southwestern Naturalist}, volume = {31}, year = {1986}, pages = {235 -241}, abstract = {

The production, density and height of flower stalks of Andropogon gerardii, Schizachrium scoparium and Sorghastrum nutans were surveyed over a four-year period (1981-1984) on burned vs unburned and shallow vs deep soil sites in native, undisturbed tallgrass prairie. In general, flower stalk height was greater on deep than the shallow soils but the response of height to fire was variable. In contrast, density and production of flower stalks were usually greater on the annualy burned sites, whereas soil effects were inconsistent. Production of flower stalks for all three species combined ranged from 0 to 670 g/m2 during the 4-year period with maximum production occurring in a mesic year which followed a drought year

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/3670564}, author = {Alan K. Knapp and Hulbert, L.C.} } @article {KNZ00107, title = {Seasonal water relations of three gallery forest hardwood species in northeast Kansas}, journal = {Forest Science}, volume = {32}, year = {1986}, pages = {687 -696}, abstract = {

Field and laboratory studies were used to evaluate several water relations characteristics for saplings of three species: hackberry (Celtis occidentalis), a mesic, late successional tree, and bur and chinquapin oak (Quercus macrocarpa and Q. muehlenbergii), more xeric, early successional trees growing in a northeast Kansas gallery forest understory during a dry summer. For all species, maximum stomatal conductance (gwv) (8.9-9.5 mm*s-1) was recorded early in the season when soil moisture was greatest. Seasonally, sunlit leaves generally had significantly higher gwv than shaded leaves. Both gwv and leaf water potential decreased during the season in each species, with minimum leaf water potential values ranging from - 2.13 to -2.48 MPa. Seasonally, leaf water potential remained consistently higher than the osmotic potential at zero turgor for each species (minimum values of zero turgor ranged from -2.61 to -3.02 MPa). Hackberry, the most mesic species, experienced the largest seasonal decrease in gwv in both shaded and sunlit leaves (69\% and 78\% reduction, respectively). Moreover, in contrast to the two oaks, a significant seasonal decline in 600 h leaf water potential was shown for hackberry. By season\&$\#$39;s end, hackberry had the lowest gwv and leaf water potential suggesting that this species was better able to utilize late season precipitation. Additional key words: Quercus, Celtis, ecophysiology, prairie, drought

}, keywords = {LTER-KNZ, Water relations}, author = {Abrams, M.D. and Alan K. Knapp} } @article {KNZ00106, title = {A ten-year record of aboveground biomass in a Kansas tallgrass prairie: effects of fire and topographic position}, journal = {American Journal of Botany}, volume = {73}, year = {1986}, pages = {1509 -1515}, abstract = {

Measurements of mid-season live and dead aboveground biomass are reported for a 10 year period (1975- 1984) in a northeast Kansas tallgrass prairie. Study sites included shallow, rocky upland and deep, nonrocky lowland soils in annually burned (April) and unburned watersheds. Lowland sites had significantly greater live biomass than upland sites for both burned and unburned prairie for the 10 year period. Moreover, live biomass was greater on burned than unburned lowland sites, but was not significantly increased by fire on the upland sites. Averaged across upland and lowland sites, mid-season live biomass was 422 g m-2 on annually burned and 364 g m-2 on unburned sites for the 10 year period. Each site had its lowest live biomass value during the severe drought year of 1980 (range=185-299 g m-2). During the study period, live biomass was most strongly correlated with seasonal pan water evaporation (r=-0.45 to -0.82), whereas dead biomass was correlated with the previous years precipitation (r=0.62 and 0.90 for upland and lowland sites, respectively). When aboveground biomass was sampled throughout the 1984 season and separated into several components, biomass of the graminoids was 40\% lower, whereas that of forbs and woody plants was 200-300\% greater in the unburned than in the annually burned site

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {~http://www.jstor.org/stable/2443856}, author = {Abrams, M.D. and Alan K. Knapp and Hulbert, L.C.} } @article {KNZ0089, title = {Early season production and microclimate associated with topography in a C4 dominated grassland}, journal = {Oecologia Plant}, volume = {6}, year = {1985}, pages = {337 -346}, abstract = {

Greater early season primary production in lowland vs. upland sites has been documented in a C4 dominated grassland in eastern Kansas (USA). Since lower production in uplands is probably not due to differences in water stress between sites at this time of year, an alternative hypothesis tested was that low air temperatures limit early season production in grassland and reduced windspeeds at lowland sites may allow leaf temperatures to be nearer their optimum for growth compared to leaves at exposed upland sites. Numerous microclimatic and physiological measurements were made early in the growing season at a lowland and at two upland sites (one shielded form the wind) in an eastern Kansas tallgrass prairie to test this hypothesis. During May, leaf temperatures of Andropogon gerardii, the dominant C4 grass in this grassland, were as much as 6 degrees Celcius greater (and windspeeds approximately 50\% less) in the lowland and shielded plots compared to the upland plot. These higher leaf temperatures were attributed to reduced convective cooling of the grasses in the low wind plots. No substantial differences in water stress were detected between sites but greater stomatal conductance to water vapor diffusion was measured in lowland plants. The photosynthetic temperature response of A. gerardii indicated strong temperature limitation to carbon uptake at typically early season air temperatures. The net result of shielding an upland plot from wind was a significant increase in total aboveground primary production and shoot mass of A. gerardii compared to the unshielded upland plot. Furthermore, production in the shielded plot was similar to that in the lowland for the early season. Thus, greater early season production in lowlands in this grassland which cannot be attributed to increased moisture availability (important later in the season) may be due to a more favorable early season microclimate in lowlands for growth of these warm season grasses. Key words: Leaf temperature, Microclimate, Net photosynthesis, Primary production, Tallgrass prairie, Water relations, Wind

}, keywords = {LTER-KNZ, Wind}, author = {Alan K. Knapp} } @article {KNZ0088, title = {Effect of fire and drought on the ecophysiology of Andropogon gerardii and Panicum virgatum in a tallgrass prairie}, journal = {Ecology}, volume = {66}, year = {1985}, pages = {1309 -1320}, abstract = {The effect of fire and drought on net photosynthesis, stomatal conductance to water vapor diffusion, leaf water potential, leaf osmotic potential, leaf nitrogen content, and growth were evaluated for the dominant tallgrass prairie species. Andropogon gerardii, and a less common (more mesic) grass, Panicum virgatum, at the Konza Prairie Research Natural Area in northeast Kansas. Early in the growing season in a burned plot, leaves of A. gerardii exhibited greater photosynthesis (maximum CO2 uptake=41.6 micromol*m-2*s-1), leaf conductance, leaf nitrogen content, nitrogen use efficiency, leaf thickness, and shoot biomass compared to shoots from an unburned plot. Postburn differences in P. virgatum were less pronounced, and CO2 uptake rates (maximum=46.0 micromol*m-2*s-1) were not significantly different between burned and unburned sites for this grass. Both grasses had broad temperature optima for photosynthesis, and neither was light-saturated at 2.2 mmol*m-2*s-1 quantum flux. The response of A. gerardii to burning reflects the documented postfire vigor of this grass, and is indicative of a substantial shift in the water, energy, and nutrient dynamics of tallgrass prairie following fire. Due to a midseason drought in 1983, leaf water potentials at midday decreased to \< -6.6 and -5.74 MPa in A. gerardii, and -4.53 and -3.51 MPa in P. virgatum in burned and unburned plots, respectively. Lower leaf water potential in the burned site was a result of greater leaf area and transpiration per unit ground area in burned than in unburned prairie. As water stress increased, the osmotic potential at full turgor and at zero turgor decreased more rapidly in the burned plot for both grasses, but minimum values of each parameter (-2.04 and -2.68 MPa in A. gerardii; -2.04 and -2.56 MPa in P. virgatum) were recorded at the end of the season in the unburned plot. When water stress was most severe, photosynthesis increased from 28 to 48\% of the early season rates. Attributes of A. gerardii that may contribute to its dominance in tallgrass prairie were the ability of this grass to maintain high rates of carbon gain over a greater range of leaf temperatures and at lower leaf water potential than P. virgatum.}, keywords = {LTER-KNZ, Andropogon gerardii, fire, net photosynthesis, nitrogen use dfficiency, osmotic potential, Panicum virgatum, tallgrass prairie, water stress}, doi = {10.2307/1939184}, author = {Alan K. Knapp} } @article {KNZ0091, title = {An evaluation of beta attenuation for estimating aboveground biomass in a tallgrass prairie}, journal = {Journal of Range Management}, volume = {38}, year = {1985}, pages = {556 -558}, abstract = {

The attenuation of beta particles by vegetation was evaluated as a nondestructive method for estimating aboveground biomass in tallgrass prairie in northeast Kansas. Regression equations using the sum of beta attenuation measurements for each of 5 height classes within the vegetation and mean midday leaf water potential as the independent variables were used to predict live and total biomass. Live and total biomass were better predicted on burned (r2=.91 and .88, respectively) and unburned sites (r2=.71 and .70, respectively). Greater variability in the relationship between beta attentuation and biomass in unburned prairie was a result of the large and variable amount of dead biomass on unburned sites. Dead biomass was poorly predicted by beta attentuation (r2=.24- .49). Beta attentuation predicted biomass in burned tallgrass prairie within +/- 5\% of harvest values until late season vegetative senescence. In unburned prairie, predictions were poorer, but the technique could still be useful if the required accuracy need be only +/- 25\%

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/3899752}, author = {Alan K. Knapp and Abrams, M.D. and Hulbert, L.C.} } @article {KNZ0090, title = {Response of Andropogon gerardii (poaceae ) to fire-induced high vs.low irradiance environments in tallgrass prairie: leaf structure and photosynthetic pigments}, journal = {American Journal of Botany}, volume = {72}, year = {1985}, pages = {1668 -1671}, abstract = {

Photosynthetic pigments and several structural characteristics were measured in leaves of Andropogon gerardii from tallgrass prairie populations in an unburned, low-irradiance site and a burned, high-irradiance site to determine if these species displayed sun/shade differences similar to those documented in forest species. Early in the growing season, leaves of A. gerardii in the low-irradiance, unburned site had significantly lower stomatal density, pore length, and conductance, as well as specific leaf mass and thickness than leaves from the high-irradiance, burned site. Moreover, the chlorophnyll a:b ratio, carotenoid content, and bundle sheath-vascular complex area were significantly lower, but chlorophyll content (mass/mass) was greater in leaves in unburned vs burned sites. These differences are consistent with sun/shade adaptations reported for forest understory plants and may contribute to the low productivity of A. gerardii in unburned tallgrass prairie

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {http://www.jstor.org/stable/2443720}, author = {Alan K. Knapp and Gilliam, F.S.} } @article {KNZ0067, title = {Effect of fire in tallgrass prairie on seed production of Vernonia baldwinii Torr. (Compositae)}, journal = {The Southwestern Naturalist}, volume = {29}, year = {1984}, pages = {242 -243}, abstract = {

Fire, an important natural component of tallgrass prairie ecosystems, may be detrimental to the invasion and survival of many nongraminoid species. In the absence of fire, perennial forbs are reported to increase significantly in tallgrass prairie. Most studies involving fire and tallgrass prairie have concentrated on the dominant tallgrass species, but have failed to document the reproductive response of prairie forbs to fire. This study compares seed production of the common tallgrass prairie forb, Vernonia baldwinii Torr., among five native prairie sites which differ in time since the most recent burn

}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Alan K. Knapp} } @article {KNZ0065, title = {Postburn differences in solar radiation, leaf temperature, and water stress influencing production in a lowland tallgrass prairie}, journal = {American Journal of Botany}, volume = {71}, year = {1984}, pages = {220 -227}, abstract = {

The influence of standing dead biomass on available solar radiation, leaf temperature and leaf water potential of Andropogon gerardii in unburned tallgrass prairie was compared to burned prairie in eastern Kansas. The standing dead reduced photosynthetically active radiation incident on emerging shoots by 58.8\% in unburned compared to burned prairie during the initial 30 days of the growing season. Aboveground production in unburned prairie was similarly reduced during this period (55.4\%) compared to burned prairie. Leaf temperatures in A. gerardii were greater in unburned prairie than in burned early in the season, but were nearly equal by the end of the growing season. The maximum elevation of leaf temperature in unburned prairie above burned was 9.5 C. The maximum unburned leaf temperature measured was 41.5 C compared to 39.4 C in burned prairie. Lower windspeed adjacent to leaves in unburned prairie resulting in reduced convective cooling may have caused higher leaf temperature in unburned prairie. Leaf water potential was significantly lower in unburned prairie than in burned prairie early in the season but was higher in unburned prairie by late season. The seasonal minimum leaf water potential in burned prairie was -1.60 MPa compared to -1.45 MPa in unburned prairie. The combined effect of these post-burn differences in solar radiation, leaf temperature and leaf water potential may be significant in contributing to the lower production in unburned compared to burned tallgrass prairie

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {http://www.jstor.org/stable/2443749}, author = {Alan K. Knapp} } @article {KNZ0066, title = {Water relations and growth of three grasses during wet and drought years in a tallgrass prairie}, journal = {Oecologia}, volume = {65}, year = {1984}, pages = {35 -43}, abstract = {

The water relations and growth of three tallgrass prairie species Panicum virgatum, Andropogon gerardii and A. scoparius were examined in irrigated and unwatered prairie in easter Kansas (USA). Measurement of the osmotic potential at full turgor, at zero turgor, and growth of vegetative and reproductive tillers were made in a year with above-normal precipitation and a drought year to evaluate: 1) the ability of these grasses to osmotically adjust in response to water stress and 2) the effect of drought or supplemental water on growth of these species. Although these grasses adjusted osmotically even in the wet year, the degree of adjustment of full turgor and zero turgor in the drought year was relatively large (0.60-0.78 MPa and 0.88-1.34 MPa, respectively) compared to reports for other species. Seasonal minimum values of full turgor and zero turgor for these grasses in the drought year were lower than in most mesic species and seasonal fluctuations in full turgor and zero turgor were greater than reported for most mesic or xeric species. The relatively frequent occurrence of drought in sub-humid tallgrass prairies may partially explain the greater than expected magnitude of osmotic adjustment in these grasses. Irrigation in the wet year increased reproductive biomass in the mesic grass P. virgatum, but had no effect on A. gerardii or the more xeric grass A. scoparius. However, irrigation in the drought year increased maximum shoot biomass in all three grasses significantly with the largest increase in P. virgatum. Reproduction in P. virgatum was also increased more by irrigation in the drought year compared to the other grasses. Irrigation did not increase season\&$\#$39;s end production of A. gerardii in the wet year, but in the drought year production was 28\% greater in irrigated than unwatered prairie. The combination of these water relations and growth responses of the three grasses to wetter than normal and drought years supports their reported distribution along a moisture gradient in tallgrass prairies

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.1007/BF00384460}, author = {Alan K. Knapp} }