@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 {KNZ001742, title = {Analysis and estimation of tallgrass prairie evapotranspiration in the central United States}, journal = {Agricultural and Forest Meteorology}, volume = {232}, year = {2017}, pages = {35-47}, abstract = {

Understanding the factors controlling evapotranspiration (ET) of spatially distributed tallgrass prairie is crucial to accurately upscale ET and to predict the response of tallgrass prairie ecosystems to current and future climate. The Moderate Resolution Imaging Spectroradiometer (MODIS)-derived enhanced vegetation index (EVI) and ground-based climate variables were integrated with eddy covariance tower-based ET (ETEC) at six AmeriFlux tallgrass prairie sites in the central United States to determine major climatic factors that control ET over multiple timescales and to develop a simple and robust statistical model for predicting ET. Variability in ET was nearly identical across sites over a range of timescales, and it was most strongly driven by photosynthetically active radiation (PAR) at hourly-to-weekly timescales, by vapor pressure deficit (VPD) at weekly-to-monthly timescales, and by temperature at seasonal-to-interannual timescales at all sites. Thus, the climatic drivers of ET change over multiple timescales. The EVI tracked the seasonal variation of ETEC well at both individual sites (R2\ \>\ 0.70) and across six sites (R2\ =\ 0.76). The inclusion of PAR further improved the ET-EVI relationship (R2\ =\ 0.86). Based on this result, we used ETEC, EVI, and PAR (MJ\ m\−2 d\−1) data from four sites (15 site-years) to develop a statistical model (ET\ =\ 0.11 PAR\ +\ 5.49 EVI \− 1.43, adj. R2\ =\ 0.86, P\ \<\ 0.0001) for predicting daily ET at 8-day intervals. This predictive model was evaluated against additional two years of ETEC data from one of the four model development sites and two independent sites. The predicted ET (ETEVI+PAR) captured the seasonal patterns and magnitudes of ETEC, and correlated well with ETEC, with R2 of 0.87-0.96 and RMSE of 0.35-0.49\ mm\ d\−1, and it was significantly improved compared to the standard MODIS ET product. This study demonstrated that tallgrass prairie ET can be accurately predicted using a multiple regression model that uses EVI and PAR which can be readily derived from remote sensing data.

}, keywords = {LTER-KNZ, Artificial neural network, Eddy covariance, Empirical model, ET modeling, remote sensing, Wavelet cross-correlation analysis}, doi = {10.1016/j.agrformet.2016.08.005}, url = {https://www.sciencedirect.com/science/article/pii/S0168192316303604?via\%3Dihub}, author = {Wagle, P. and Xiao, X. and Gowda, P. and Basara, J. and N. Brunsell and Steiner, J. and Anup, K.C.} } @article {KNZ001820, title = {Assessing the roles of fire frequency and precipitation in determining woody plant expansion in central U.S. grasslands}, journal = {Journal of Geophysical Research - Biogeosciences}, volume = {122}, year = {2017}, pages = {2683{\textendash}2698}, abstract = {

Woody plant expansion into grasslands and savannas is occurring and accelerating worldwide and often impacts ecosystem processes. Understanding and predicting the environmental and ecological impacts of encroachment has led to a variety of methodologies for assessing its onset, transition, and stability, generally relying on dynamical systems approaches. Here we continue this general line of investigation to facilitate the understanding of the roles of precipitation frequency and intensity and fire frequency on the conversion of grasslands to woody-dominated systems focusing on the central United States. A low-dimensional model with stochastic precipitation and fire disturbance is introduced to examine the complex interactions between precipitation and fire as mechanisms that may suppress or facilitate increases in woody cover. By using Lyapunov exponents, we are able to ascertain the relative control exerted on woody encroachment through these mechanisms. Our results indicate that precipitation frequency is a more important control on woody encroachment than the intensity of individual precipitation events. Fire, however, exerts a much more dominant impact on the limitation of encroachment over the range of precipitation variability considered here. These results indicate that fire management may be an effective strategy to slow the onset of woody species into grasslands. While climate change might predict a reduced potential for woody encroachment in the near future, these results indicate a reduction in woody fraction may be unlikely when considering anthropogenic fire suppression.

}, keywords = {LTER-KNZ}, doi = {10.1002/2017JG004046}, url = {https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017JG004046}, author = {N. Brunsell and van Vleck, E.S. and Nosshi, M. and Z. Ratajczak and Jesse B. Nippert} } @article {KNZ001794, title = {Changes in spatial variance during a grassland to shrubland state transition}, journal = {Journal Ecology}, volume = {105}, year = {2017}, pages = {750-760}, abstract = {
  1. State transitions are changes in ecosystem structure and self-reinforcing feedbacks that are initiated when an exogenous driver variable crosses a threshold. Reversing state transitions is difficult and costly. While some state transitions are relatively rapid, many take years to decades. Outside of theoretical models, very little is known about slower state transitions and how they unfold in time and space.
  2. We quantified changes in spatial variance as a mesic grassland ecosystem shifts to a shrub-dominated state, using long-term experiments and simulations that maintain grasslands with annual fires or initiate a state transition to shrub dominance by decreasing fire frequency.
  3. In the experiments, the susceptibility to state transitions varied substantially in space. In the less frequent fire treatment, some plots became shrub-dominated around year 20 and grass extirpations began in year 25, but a third of the plots were still grass-dominated in year 37. Variable rates of state transition resulted in increasing spatial variance of grass cover over time, whereas shrub cover variance decreased. In the annually burned treatment, grasses remained dominant and the spatial variance of grass cover declined.
  4. In a separate experiment, less frequent fires were maintained for 23\ years and then switched to annual fires. The switch to annual fires occurred shortly after grass variance started to increase and a majority of these plots quickly returned to a grass dominated state.
  5. In simulations, spatial variance remained low and average grass cover was high under frequent fires. If fire frequency decreased below a threshold, the ecosystem transitioned to shrubland, with a transient increase in the spatial variance of grass cover during the transition between states.
  6. Synthesis. Spatial variability in the rate and susceptibility to state transitions is indicative of a system with a patchy spatial structure, high spatial heterogeneity and low connectivity between patches. Increases in spatial variance can serve as an indication that some patches have begun a state transition and that management interventions are needed to avoid widespread transitions. This is one of the first empirical examples where altering management after an increase in spatial variance prevented state transitions.

}, keywords = {LTER-KNZ}, doi = {10.1111/1365-2745.12696}, url = {https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2745.12696}, author = {Z. Ratajczak and D{\textquoteright}Odorico, P.D. and Jesse B. Nippert and Scott. L. Collins and N. Brunsell and Ravi, S.} } @article {KNZ001739, title = {Productivity of North American grasslands is increased under future climate scenarios despite rising aridity}, journal = {Nature Climate Change}, volume = {6}, year = {2016}, pages = {710-714}, abstract = {

Grassland productivity is regulated by both temperature and the amount and timing of precipitation1, 2. Future climate change is therefore expected to influence grassland phenology and growth, with consequences for ecosystems and economies. However, the interacting effects of major shifts in temperature and precipitation on grasslands remain poorly understood and existing modelling approaches, although typically complex, do not extrapolate or generalize well and tend to disagree under future scenarios3, 4. Here we explore the potential responses of North American grasslands to climate change using a new, data-informed vegetation\–hydrological model, a network of high-frequency ground observations across a wide range of grassland ecosystems and CMIP5 climate projections. Our results suggest widespread and consistent increases in vegetation fractional cover for the current range of grassland ecosystems throughout most of North America, despite the increase in aridity projected across most of our study area. Our analysis indicates a likely future shift of vegetation growth towards both earlier spring emergence and delayed autumn senescence, which would compensate for drought-induced reductions in summer fractional cover and productivity. However, because our model does not include the effects of rising atmospheric CO2 on photosynthesis and water use efficiency5, 6, climate change impacts on grassland productivity may be even larger than our results suggest. Increases in the productivity of North American grasslands over this coming century have implications for agriculture, carbon cycling and vegetation feedbacks to the atmosphere.

}, keywords = {LTER-KNZ}, issn = {1758-678X}, doi = {10.1038/nclimate2942}, url = {https://www.nature.com/articles/nclimate2942}, author = {Hufkens, K. and Keenan, T.F. and Flanagan, L.B. and Scott, R.L. and Bernacchi, C.J. and Joo, E. and N. Brunsell and Verfaillie, J. and Richardson, A.D.} } @article {KNZ001709, title = {The sensitivity of carbon exchanges in Great Plains grasslands to precipitation variability}, journal = {Journal of Geophysical Research: Biogeosciences}, volume = {121}, year = {2016}, pages = {280-294}, abstract = {

In the Great Plains, grassland carbon dynamics differ across broad gradients of precipitation and temperature, yet finer-scale variation in these variables may also affect grassland processes. Despite the importance of grasslands, there is little information on how fine-scale relationships compare between them regionally. We compared grassland C exchanges, energy partitioning and precipitation variability in eight sites in the eastern and western Great Plains using eddy covariance and meteorological data. During our study, both eastern and western grasslands varied between an average net carbon sink and a net source. Eastern grasslands had a moderate vapor pressure deficit (VPD = 0.95 kPa) and high growing season gross primary productivity (GPP = 1010 \± 218 g C m\−2 m\−2 y\−1). Western grasslands had a growing season with higher VPD (1.43 kPa) and lower GPP (360 \± 127 g C m\−2 m\−2 y\−1). Western grasslands were sensitive to precipitation at daily timescales, whereas eastern grasslands were sensitive at monthly and seasonal timescales. Our results support the expectation that C exchanges in these grasslands differ as a result of varying precipitation regimes. Because eastern grasslands are less influenced by short-term variability in rainfall than western grasslands, the effects of precipitation change are likely to be more predictable in eastern grasslands because the timescales of variability that must be resolved are relatively longer. We postulate increasing regional heterogeneity in grassland C exchanges in the Great Plains in coming decades.

}, keywords = {LTER-KNZ}, doi = {10.1002/2015JG003205}, url = {https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JG003205}, author = {Petrie, M.D. and N. Brunsell and Vargas, R. and Scott. L. Collins and Flanagan, L.B. and Hanan, N.P. and Litvak, M.E. and Suyker, A.E.} } @article {KNZ001738, title = {A thermodynamic approach for assessing agroecosystem sustainability}, journal = {Ecological Indicators}, volume = {67}, year = {2016}, pages = {204-214}, abstract = {

By revisiting theoretical concepts in biogeography and the importance of thermodynamic laws in biosphere-atmosphere interactions, ecological sustainability in agricultural systems may be better defined. In this case study, we employed a multidisciplinary methodology for exploring agroecosystem sustainability by using eddy covariance (EC) data to compute thermodynamic entropy production () and relate it to water, energy and carbon cycling in croplands and grasslands of the Central US. From 2002 to 2012, the biophysical metric of  was compared across AmeriFlux sites, each with site-specific land management practices of irrigation, crop rotation, and tillage. Results show that  is most correlated with net ecosystem exchange (NEE) of carbon, and when cropland and grassland sites are close to being carbon neutral,  values range from 0.51\–1.0 W K\−1 m\−2 for grasslands, 0.81\–1.0 W K\−1 m\−2 for rainfed croplands, and 0.81\–1.1 W K\−1 m\−2 for irrigated croplands. Irrigated maize stressed by hydrologic and high temperature anomalies associated with the 2012 drought exhibit the greatest increase in , indicating the possibility of decreased sustainability compared to rainfed croplands and grasslands. These results suggest that maximizing carbon uptake with irrigation and fertilizer use tends to move agroecosystems further away from thermodynamic equilibrium, which has implications for ecological sustainability and greenhouse gas (GHG) mitigation in climate-smart agriculture. The underlying theoretical concepts, multidisciplinary methodology, and use of eddy covariance data for biophysical indicators in this study contribute to a unique understanding of ecological sustainability in agricultural systems.

}, keywords = {LTER-KNZ}, doi = {10.1016/j.ecolind.2016.01.045}, url = {https://kundoc.com/pdf-a-thermodynamic-approach-for-assessing-agroecosystem-sustainability-.html}, author = {Cochran, F. V. and N. Brunsell and Sukyer, A.} } @article {KNZ001729, title = {Warm spring reduced carbon cycle impact of the 2012 US summer drought}, journal = {Proceedings of the National Academy of Sciences}, year = {2016}, pages = {201519620}, abstract = {

The global terrestrial carbon sink offsets one-third of the world\’s fossil fuel emissions, but the strength of this sink is highly sensitive to large-scale extreme events. In 2012, the contiguous United States experienced exceptionally warm temperatures and the most severe drought since the Dust Bowl era of the 1930s, resulting in substantial economic damage. It is crucial to understand the dynamics of such events because warmer temperatures and a higher prevalence of drought are projected in a changing climate. Here, we combine an extensive network of direct ecosystem flux measurements with satellite remote sensing and atmospheric inverse modeling to quantify the impact of the warmer spring and summer drought on biosphere-atmosphere carbon and water exchange in 2012. We consistently find that earlier vegetation activity increased spring carbon uptake and compensated for the reduced uptake during the summer drought, which mitigated the impact on net annual carbon uptake. The early phenological development in the Eastern Temperate Forests played a major role for the continental-scale carbon balance in 2012. The warm spring also depleted soil water resources earlier, and thus exacerbated water limitations during summer. Our results show that the detrimental effects of severe summer drought on ecosystem carbon storage can be mitigated by warming-induced increases in spring carbon uptake. However, the results also suggest that the positive carbon cycle effect of warm spring enhances water limitations and can increase summer heating through biosphere\–atmosphere feedbacks.

}, keywords = {LTER-KNZ, biosphere{\textendash}atmosphere feedbacks, carbon uptake, ecosystem fluxes, Eddy covariance, seasonal climate anomalies}, issn = {0027-8424}, doi = {10.1073/pnas.1519620113}, url = {https://www.pnas.org/content/113/21/5880}, author = {Wolf, S. and Keenan, T.F. and Fisher, J.B. and Baldocchi, D.D. and Desai, A.R. and Richardson, A.D. and Scott, R.L. and Law, B.E. and Litvak, M.E. and N. Brunsell and Peters, W. and van der Laan-Luijkx, I.T.} } @article {KNZ001711, title = {Biophysical controls on carbon and water vapor fluxes across a grassland climatic gradient in the United States}, journal = {Agricultural and Forest Meteorology}, volume = {214-215}, year = {2015}, month = {Jan-12-2015}, pages = {293 - 305}, abstract = {

Understanding of the underlying causes of spatial variation in exchange of carbon and water vapor fluxes between grasslands and the atmosphere is crucial for accurate estimates of regional and global carbon and water budgets, and for predicting the impact of climate change on biosphere\–atmosphere feedbacks of grasslands. We used ground-based eddy flux and meteorological data, and the Moderate Resolution Imaging Spectroradiometer (MODIS) enhanced vegetation index (EVI) from 12 grasslands across the United States to examine the spatial variability in carbon and water vapor fluxes and to evaluate the biophysical controls on the spatial patterns of fluxes. Precipitation was strongly associated with spatial and temporal variability in carbon and water vapor fluxes and vegetation productivity. Grasslands with annual average precipitation \<600 mm generally had neutral annual carbon balance or emitted small amount of carbon to the atmosphere. Despite strong coupling between gross primary production (GPP) and evapotranspiration (ET) across study sites, GPP showed larger spatial variation than ET, and EVI had a greater effect on GPP than on ET. Consequently, large spatial variation in ecosystem water use efficiency (EWUE = annual GPP/ET; varying from 0.67 \± 0.55 to 2.52 \± 0.52 g C mm\−1 ET) was observed. Greater reduction in GPP than ET at high air temperature and vapor pressure deficit caused a reduction in EWUE in dry years, indicating a response which is opposite than what has been reported for forests. Our results show that spatial and temporal variations in ecosystem carbon uptake, ET, and water use efficiency of grasslands were strongly associated with canopy greenness and coverage, as indicated by EVI.

}, keywords = {LTER-KNZ, Ecosystem water use efficiency, Eddy covariance, Enhanced vegetation index, Evapotranspiration, Grasslands, Gross primary production}, issn = {01681923}, doi = {10.1016/j.agrformet.2015.08.265}, url = {http://linkinghub.elsevier.com/retrieve/pii/S0168192315007005}, author = {Wagle, Pradeep and Xiao, Xiangming and Scott, Russell L. and Kolb, Thomas E. and Cook, David R. and N. Brunsell and Baldocchi, Dennis D. and Basara, Jeffrey and Matamala, Roser and Zhou, Yuting and Bajgain, Rajen} } @article {KNZ001710, title = {Influence of drought on growing season carbon and water cycling with changing land cover}, journal = {Agricultural and Forest Meteorology}, volume = {213}, year = {2015}, month = {Jan-11-2015}, pages = {217 - 225}, abstract = {

Grasslands around the world are experiencing increased woody encroachment while drought frequency and severity is expected to increase with climate change. These processes will result in changes in the local carbon, water, and energy balances. Two closely located eddy covariance towers on native prairie sites in Kansas with differing burn regimes provided a unique opportunity to investigate those changes with minimal confounding climate factors. Daytime turbulent fluxes were analyzed throughout the growing season during two wet years (2008, 2009) and one drought year (2011). The site experiencing more woody encroachment (K4B) was a greater carbon sink every year, especially in 2011, when the annually burned site (KON) became a negligible sink and potential source of carbon. K4B also had greater seasonal water use efficiency (WUE) at all times except during drought and when burned. Evidence of water loss from deeper, stored sources was shown at K4B through cumulative evaporative loss that exceeded precipitation during drought conditions. Changes in the nature of the turbulent fluxes were also investigated through deviations from similarity theory, quadrant analysis, and wavelet decomposition. Wavelet analysis determined the size of eddies primarily involved in net exchange, showing differences between sites across small to medium scales (average diameter of 4.0\–108 m). Increased surface heterogeneity and roughness with woody encroachment was seen through smaller eddy size of maximum transport and slightly weakened applicability of similarity theory at K4B. Eddy size decreased over the course of the growing season, in a strikingly inverse relation to grass leaf area index, likely corresponding to greater canopy height and roughness. Variance of the correlation between carbon and water, indicative of more variable plant productivity, is much greater across all scales in 2011 at KON only. Overall, results indicate woody encroachment increases resilience to drought. Though carbon sequestration remains stronger at the woody encroachment site, it comes at the cost of far increased water loss, including from deeper groundwater resources, which could prove detrimental in combination with predicted climate change.

}, keywords = {LTER-KNZ, Drought, Eddy covariance, Turbulence, Water use efficiency}, issn = {01681923}, doi = {10.1016/j.agrformet.2015.07.002}, url = {https://www.sciencedirect.com/science/article/pii/S0168192315002099?via\%3Dihub}, author = {Logan, K.E. and N. Brunsell} } @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 {KNZ001608, title = {Impacts of seasonality and surface heterogeneity on water-use efficiency in mesic grasslands}, journal = {Ecohydrology}, volume = {7}, year = {2014}, pages = {1223 -1233}, abstract = {

Woody encroachment is occurring in grasslands worldwide, with largely unknown effects on local carbon and water fluxes and the energy balance. Water-use efficiency (λ) is a measure of carbon assimilation per evapotranspiration. Here, a was compared among three different grassland ecosystems in eastern KS, USA, by using the eddy covariance technique. Variation in λ was examined at multiple timescales and across different burning regimes. Site-specific variations in λ were more readily observed at seasonal and inter-annual timescales rather than daily and monthly averages. Annually burned grassland with homogeneous C4 grass cover had less negative values of λ [lower water-use efficiency (WUE)] than infrequently burned grassland that is presently undergoing woody encroachment and a transition to a shrub-dominated ecosystem. The most likely explanation for differences in λ are differences in rooting depth and source-water acquisition between encroaching woody plants and the native grass community. Reliance on a deeper water source by the woody community may buffer the negative consequences of forecasted climate variability and drought, resulting in greater landscape WUE and reduced susceptibility to water stress when compared with the coexisting grass species. Copyright \© 2013 John Wiley \& Sons, Ltd.

}, keywords = {LTER-KNZ}, doi = {10.1002/eco.1455}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/eco.1455}, author = {N. Brunsell and Jesse B. Nippert and Buck, T.L.} } @article {KNZ001531, title = {Comparing surface and mid-tropospheric CO2 concentrations from central U.S. grasslands}, journal = {entropy}, volume = {15}, year = {2013}, pages = {606 -623}, abstract = {

Comparisons of eddy covariance (EC) tower measurements of CO2 concentration with mid-tropospheric observations from the Atmospheric Infrared Sounder (AIRS) allow for evaluation of the rising global signal of this greenhouse gas in relation to surface carbon dynamics. Using an information theory approach combining relative entropy and wavelet multi-resolution analysis, this study has explored correlations and divergences between mid-tropospheric and surface CO2 concentrations in grasslands of northeastern Kansas. Results show that surface CO2 measurements at the Kansas Field Station (KFS) and the Konza Prairie Biological Stations 1B (KZU) and 4B (K4B) with different land-cover types correlate well with mid-tropospheric CO2 in this region at the 512-day timescale between 2007 and 2010. Relative entropy further reveals that AIRS observations are indicative of surface CO2 concentrations for all land-cover types on monthly (32-day) and longer timescales. AIRS observations are also similar to CO2 concentrations at shorter timescales at sites KFS and K4B experiencing woody encroachment, though these results require further investigation. Differences in species composition and microclimate add to the variability of surface concentrations compared with mid-tropospheric observations.

}, keywords = {LTER-KNZ, Atmospheric Infrared Sounder, Eddy covariance, information theory, Konza Prairie, relative entropy, wavelets}, doi = {10.3390/e15020606}, url = {https://www.mdpi.com/1099-4300/15/2/606}, author = {Cochran, F.V. and N. Brunsell and Mechem, D.B.} } @article {KNZ001740, title = {EO-1 Hyperion reflectance time series at calibration and validation sites: stability and sensitivity to seasonal dynamics}, journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing}, volume = {6}, year = {2013}, month = {Jan-04-2013}, pages = {276 - 290}, abstract = {

This study evaluated Earth Observing 1 (EO-1) Hyperion reflectance time series at established calibration sites to assess the instrument stability and suitability for monitoring vegetation functional parameters. Our analysis using three pseudo-invariant calibration sites in North America indicated that the reflectance time series are devoid of apparent spectral trends and their stability consistently is within 2.5-5 percent throughout most of the spectral range spanning the 12+ year data record. Using three vegetated sites instrumented with eddy covariance towers, the Hyperion reflectance time series were evaluated for their ability to determine important variables of ecosystem function. A number of narrowband and derivative vegetation indices (VI) closely described the seasonal profiles in vegetation function and ecosystem carbon exchange (e.g., net and gross ecosystem productivity) in three very different ecosystems, including a hardwood forest and tallgrass prairie in North America, and a Miombo woodland in Africa. Our results demonstrate the potential for scaling the carbon flux tower measurements to local and regional landscape levels. The VIs with stronger relationships to the CO2 parameters were derived using continuous reflectance spectra and included wavelengths associated with chlorophyll content and/or chlorophyll fluorescence. Since these indices cannot be calculated from broadband multispectral instrument data, the opportunity to exploit these spectrometer-based VIs in the future will depend on the launch of satellites such as EnMAP and HyspIRI. This study highlights the practical utility of space-borne spectrometers for characterization of the spectral stability and uniformity of the calibration sites in support of sensor cross-comparisons, and demonstrates the potential of narrowband VIs to track and spatially extend ecosystem functional status as well as carbon processes measured at flux towers.

}, keywords = {LTER-KNZ, calibration, geophysical techniques, remote sensing, vegetation}, issn = {1939-1404}, doi = {10.1109/JSTARS.2013.2246139}, url = {https://ieeexplore.ieee.org/document/6507569}, author = {Campbell, P.P.K. and Middleton, E.M. and Thome, K.J. and Kokaly, R.F. and Huemmrich, K.F. and Lagomasino, D. and Novick, K.A. and N. Brunsell} } @article {KNZ001440, title = {Climate change alters growing season flux dynamics in mesic grasslands}, journal = {Theoretical and Applied Climatology}, volume = {107}, year = {2012}, pages = {427 -440}, abstract = {

Changing climate could affect the functioning of grassland ecosystems through variation in climate forcings and by altering the interactions of forcings with ecological processes. Both the short and long-term effects of changing forcings and ecosystem interactions are a critical part of future impacts to ecosystem ecology and hydrology. To explore these interactions and identify possible characteristics of climate change impacts to mesic grasslands, we employ a low-dimensional modeling framework to assess the IPCC A1B scenario projections for the Central Plains of the United States; forcings include increased precipitation variability, increased potential evaporation, and earlier growing season onset. These forcings are also evaluated by simulations of vegetation photosynthetic capacity to explore the seasonal characteristics of the vegetation carbon assimilation response for species at the Konza Prairie in North Central Kansas, USA. The climate change simulations show decreases in mean annual soil moisture and and carbon assimilation and increased variation in water and carbon fluxes during the growing season. Simulations of the vegetation response show increased variation at the species-level instead of at a larger class scale, with important heterogeneity in how individual species respond to climate forcings. Understanding the drivers and relationships behind these ecosystem responses is important for understanding the likely scale of climate change impacts and for exploring the mechanisms shaping growing season dynamics in grassland ecosystems.

}, keywords = {LTER-KNZ, ecohydrology, Konza Prairie, Low-dimensional modeling, Nonlinear interactions, Soil moisture feedback}, doi = {10.1007/s00704-011-0484-y}, url = {https://link.springer.com/article/10.1007\%2Fs00704-011-0484-y}, author = {Petrie, M.D. and N. Brunsell and Jesse B. Nippert} } @article {KNZ001396, title = {The role of precipitation variability on the ecohydrology of grasslands}, journal = {Ecohydrology}, volume = {5}, year = {2012}, pages = {337 -345}, abstract = {

Precipitation event timing and magnitude are important drivers of ecosystem processes and are instrumental in creating landscape heterogeneity. Ecosystems respond to precipitation and other driving variables at different spatial and temporal scales, which complicates understanding of the relationships that govern ecosystem conditions. To better characterize the ecosystem response, we present a low-dimensional framework for simulating the influence of precipitation event timing and magnitude on grassland ecosystems, with particular focus on characterizing the temporal sensitivity of water and carbon fluxes to climate forcings and the feedback of water and carbon on soil moisture availability. Results show variation in daily through seasonal sensitivity of ecosystem water and carbon fluxes and identifies the way these sensitivities change at daily to annual timescales to shape long-term ecosystem states. This provides for a better understanding of the nonlinearities inherent to ecosystem interactions during the growing season and provides assessment of the extent that precipitation variance has on grassland functioning and heterogeneity. Copyright \© 2011 John Wiley \& Sons, Ltd.

}, keywords = {LTER-KNZ}, doi = {10.1002/eco.224}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/eco.224}, author = {Petrie, M.D. and N. Brunsell} } @article {KNZ001741, title = {Temporal scales of tropospheric CO2, precipitation, and ecosystem responses in the central Great Plains}, journal = {Remote Sensing of Environment}, volume = {127}, year = {2012}, pages = {316 - 328}, abstract = {

Natural and anthropogenic sources of CO2 around the globe contribute to mid-tropospheric concentrations, yet it remains unknown how measurements of mid-tropospheric CO2 relate to regional ecosystem dynamics. NASA Atmospheric Infrared Sounder (AIRS) measurements of CO2 concentrations in the mid-troposphere from 2002 to 2010 were examined in relation to precipitation and vegetation phenology across the US Great Plains. Wavelet multi-resolution analysis and the information theory metric of relative entropy were applied to assess regional relationships between mid-tropospheric CO2, Normalized Difference Vegetation Index (NDVI), and precipitation (PPT). Results show that AIRS observations of mid-tropospheric CO2 exchange greater amounts of information with regional PPT and NDVI at seasonal, annual, and longer time scales compared to shorter time scales. PPT and NDVI contribute to mid-tropospheric CO2 at the 18-month time scale, while spatial patterns seen at this time scale for PPT and mid-tropospheric CO2 are reflective of the influence of PPT on NDVI at the annual scale. Identification of these dominant temporal scales may facilitate utilization of AIRS CO2 for monitoring regional source/sink dynamics related to climate and land-use/cover change.

}, keywords = {LTER-KNZ, Atmospheric Infrared Sounder, Land{\textendash}atmosphere interactions, relative entropy, wavelets}, issn = {00344257}, doi = {10.1016/j.rse.2012.09.012}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0034425712003665?via\%3Dihub}, author = {Cochran, F.V. and N. Brunsell} } @article {KNZ001464, title = {The timing of climate variability and grassland productivity}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {109}, year = {2012}, pages = {3401 -3405}, abstract = {

Changes in precipitation amount and variability have the potential to alter the structure and function of grasslands, but we know little about how changes in the timing of precipitation might affect grasslands. Here, we analyze long-term records from a tallgrass prairie to show that shifts in the timing of precipitation during the growing season have little effect on primary productivity or grass reproduction, but can greatly affect grazer performance. While greater late-season precipitation increases the weight gain of adult and young bison, greater mid-season precipitation decreases their weight gain. In addition, calving rates are lower after years with greater mid-season precipitation and higher after years with greater late-season precipitation. As well-timed drought can actually increase grazer weight gain and reproduction, it will be necessary to generate predictions of within-season distribution of precipitation to successfully forecast future grazer performance.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1600-0706.2012.20400.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1600-0706.2012.20400.x}, author = {Craine, J.M. and Jesse B. Nippert and Elmore, A.J. and Skibbe, A.M. and Hutchinson, S.L. and N. Brunsell} } @article {KNZ001387, title = {Linking plant growth responses across topographic gradients in tallgrass prairie}, journal = {Oecologia}, volume = {166}, year = {2011}, pages = {1131 -1142}, abstract = {

Aboveground biomass in grasslands varies according to landscape gradients in resource availability and seasonal patterns of growth. Using a transect spanning a topographic gradient in annually burned ungrazed tallgrass prairie, we measured changes in the height of four abundant C4 grass species, LAI, biomass, and cumulative carbon flux using two closely located eddy flux towers. We hypothesized that seasonal patterns of plant growth would be similar across the gradient, but the magnitude of growth and biomass accumulation would vary by topographic position, reflecting spatial differences in microclimate, slope, elevation, and soil depth. Thus, identifying and measuring local growth responses according to topographic variability should significantly improve landscape predictions of aboveground biomass. For most of the growth variables measured, classifying topography into four positions best captured the inherent spatial variability. Biomass produced, seasonal LAI and species height increased from the upland and break positions to the slope and lowland. Similarly, cumulative carbon flux in 2008 was greater in lowland versus upland tower locations (difference of 64 g m\−2 by DOY 272). Differences in growth by topographic position reflected increased production of flowering culms by Andropogon gerardii and Sorghastrum nutans in lowland. Varying growth responses by these species may be a significant driver of biomass and carbon flux differences by topographic position, at least for wet years. Using a digital elevation model to classify the watershed into topographic positions, we performed a geographically weighted regression to predict landscape biomass. The minimum and maximum predictions of aboveground biomass for this watershed had a large range (86\–393 t per 40.4 ha), illustrating the drastic spatial variability in growth within this annually-burned grassland.

}, keywords = {LTER-KNZ, ANPP, Eddy covariance, Flux footprint, LAI, Mesic grassland, topography}, doi = {10.1007/s00442-011-1948-6}, url = {https://link.springer.com/article/10.1007\%2Fs00442-011-1948-6}, author = {Jesse B. Nippert and Ocheltree, T.W. and Skibbe, A.M. and Kangas, L.C. and J.M. Ham and Shonkwiler-Arnold, K.B. and N. Brunsell} } @article {KNZ001348, title = {Validating remotely sensed land surface fluxes in heterogeneous terrain with large aperture scintillometry}, journal = {International Journal of Remote Sensing}, volume = {32}, year = {2011}, pages = {6295 -6314}, abstract = {

The Large Aperture Scintillometer (LAS) has emerged as one of the best tools for quantifying areal averaged fluxes over heterogeneous land surfaces. This is particularly useful as a validation of surface energy fluxes derived from satellite sources. We examine how changes in surface source area contributing to the scintillometer and eddy covariance measurements relate to satellite derived estimates of sensible heat flux. Field data were collected on the Konza Prairie in Northeastern Kansas, included data from two eddy covariance towers: one located on an upland, relatively flat homogeneous area, and the second located in a lowland area with generally higher biomass and moisture conditions. The large aperture scintillometer spanned both the upland and lowland areas and operated with a path length of approximately 1 km specifically to compare to Moderate Resolution Imaging Spectroradiometer (MODIS) derived estimates of surface fluxes. The upland station compares well with the LAS (correlation of 0.96), with the lowland station being slightly worse (correlation of 0.84). Data from the MODIS sensor was used to compute surface fluxes using the \‘triangle\’ method which combines the remotely sensed data with a soil-vegetation-atmosphere-transfer scheme and a fully developed atmospheric boundary layer model. The relative contribution to the surface observations is estimated using a simple footprint model. As wind direction varies, the relative contribution of upland and lowland sources contributing to the LAS measurements varies while the MODIS pixel contribution remains relatively constant. With the footprint model, we were able to evaluate the relationship between the LAS observations and the remotely sensed estimates of the surface energy balance. The MODIS derived sensible heat flux values correspond better to the LAS measurements (percentage error: 0.04) when there was a larger footprint compared to a time with a smaller footprint (percentage error:\ \−0.13). Results indicate that the larger the footprint, the better the agreement between satellite and surface observations.

}, keywords = {LTER-KNZ}, doi = {10.1080/01431161.2010.508058}, url = {https://doi.org/10.1080/01431161.2010.508058}, author = {N. Brunsell and J.M. Ham and Arnold, K.A.} } @mastersthesis {KNZ001398, title = {Climate forcings and the nonlinear dynamics of grassland ecosystems}, volume = {MS Thesis}, year = {2010}, school = {University of Kansas}, type = {M.S. Thesis}, address = {Lawrence, KS}, abstract = {

The nonlinear interaction of climate forcings and ecosystem variables is instrumental in creating the temporal and spatial heterogeneity of grasslands. Ecosystem processes are a product of these interactions and vary in sensitivity to them across time. How forcings aggregate and shape ecosystem responses is an important aspect of grassland states and defines how they respond to changes in environmental conditions. Characterizing the relationship between climate drivers and ecosystem variables helps sharpen analysis of ecosystem flux dynamics during the growing season and identifies likely deviations from mean functioning. To address the question of how climate forcings and ecosystem variables interact to shape seasonal water and carbon dynamics in grasslands, this thesis is split into two analysis chapters. The first (Chapter 3) is a characterization of water and carbon flux responses to variable precipitation timing and magnitude. Particular focus is placed on temporal sensitivity to inputs, seasonality in water flux dynamics, and the linkage between precipitation, soil moisture, evapotranspiration, and potential evaporation. Chapter 4 extends International Panel on Climate Change (IPCC A1B) regional climate scenario projections for the Central Plains of the United States to assess mesic grassland responses. The specific focus is assessing the ecosystem response to increased precipitation variability, increased potential evaporation, and earlier growing season onset. Effects of these forcings are shaped by simulations of constant and seasonally-varying water-use efficiency to assess the role of vegetation on grassland carbon assimilation, and also to explore species-specific responses at the Konza Prairie in North Central Kansas, USA. Results from both chapters show variation in seasonal sensitivity of fluxes to precipitation, with varying relationships between drivers, variable conditions, and fluxes. This research provides for a better understanding of ecosystem processes and provides assessment of the magnitude and extent that forcing variation has on grassland function. Results from the second chapter show increased seasonal water and carbon flux variability and increased frequency of water stress conditions. Vegetation responses suggest climate change will impact species and habitat compositions through changing environmental conditions and partitioning of carbon assimilation periods. This illustrates potential effects to grassland functioning and growing season dynamics.

}, keywords = {LTER-KNZ}, url = {http://hdl.handle.net/1808/6633}, author = {Petrie, M.} } @mastersthesis {KNZ001397, title = {The impact of land cover change on water and carbon cycling in the US central plains grasslands}, volume = {MS Thesis}, year = {2010}, school = {University of Kansas}, type = {M.S. Thesis}, address = {Lawrence, KS}, keywords = {LTER-KNZ}, url = {https://kuscholarworks.ku.edu/handle/1808/6992}, author = {Buck, T.} } @article {KNZ001211, title = {Assessing the multi-resolution information content of remotely sensed variables and elevation for evapotranspiration in a tall-grass prairie environment}, journal = {Remote Sensing of Environment}, volume = {112}, year = {2008}, pages = {2977 -2987}, abstract = {

Understanding the spatial scaling behavior of evapotranspiration and its relation to controlling factors on the land surface is necessary to accurately estimate regional water cycling. We propose a method for ascertaining this scaling behavior via a combination of wavelet multi-resolution analysis and information theory metrics. Using a physically-based modeling framework, we are able to compute spatially distributed latent heat fluxes over the tall-grass prairie in North-central Kansas for August 8, 2005. Comparison with three eddy-covariance stations and a large aperture scintillometer demonstrates good agreement, and thus give confidence in the modeled fluxes. Results indicate that the spatial variability in radiometric temperature (a proxy for soil moisture) most closely controls the spatial variability in evapotranspiration. Small scale variability in the water flux can be ascribed to the small scale spatial variance in the fractional vegetation. In addition, correlation analysis indicates general scale invariance and that low spatial resolution data may be adequate for accurately determining water cycling in prairie ecosystems.

}, keywords = {LTER-KNZ, Entropy, information theory, Konza Prairie, Latent heat, MODIS, Spatial heterogeneity, SVAT model, wavelets}, doi = {10.1016/j.rse.2008.02.002}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0034425708000655?via\%3Dihub}, author = {N. Brunsell and J.M. Ham and Owensby, C.E.} }