@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.} } @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.} } @phdthesis {KNZ001209, title = {Sensitivity of grassland ecosystemsacross the Great Plains to present and future variability}, volume = {PhD Dissertation}, year = {2008}, pages = {1 -161}, school = {Colorado State University}, type = {Ph.D. Thesis}, address = {Fort Collins, CO}, abstract = {

Patterns and controls of aboveground net primary productivity (ANPP) have been of long-standing interest to ecologists because ANPP integrates key aspects of ecosystem structure and function through time. In many terrestrial biomes, water availability is a primary constraint to ANPP, and it is an ecosystem driver that will be affected by future climate change. To understand the sensitivity of temperate grasslands to inter- and intra-annual variability in precipitation, I analyzed long-term ANPP data, conducted a multi-site experimental manipulation in which the number of growing season rainfall events was varied, and simulated the effects of altered rainfall regimes using a terrestrial ecosystem model (DAYCENT). I conducted this research within the Great Plains of North America\—a region characterized by a strong west-east precipitation-productivity gradient and three distinct grassland types\—the semi-arid shortgrass, the mixed-grass prairie, and the mesic tallgrass prairie. My results demonstrate that temperate grasslands are indeed sensitive to both inter- and intra-variability in precipitation, but the ANPP response is contingent upon ecosystem structure and typical soil water levels. Additionally, both management strategies and topographic location may interact with precipitation to enhance or diminish coherence in the ANPP response. At the dry end of the gradient (semi-arid steppe), fewer, but larger rain events led to increased periods of above-average soil water content, reduced plant water stress and increased ANPP. The opposite response was observed at the mesic end of the gradient (tallgrass prairie), where longer dry intervals between large events led to extended periods of below-average soil water content, increased plant water stress, and reduced ANPP. Mixed grass prairie was intermediate along the gradient, characterized by the greatest plant species richness, and the most sensitive to within-season variability in rainfall. Comparison of these experimental data to model simulations revealed key differences in soil water dynamics and ANPP patterns, suggesting that more experimental data is needed to parameterize biological and physical processes that drive model simulations. In conclusion, these results highlight the difficulties in extending inference from single site experiments to whole ecosystems or biomes and demonstrate the complexity inherent in predicting how terrestrial ecosystems will respond to novel climate conditions.

}, keywords = {LTER-KNZ}, url = {https://search.proquest.com/docview/304623353/?pq-origsite=primo}, author = {Heisler, J.L.} } @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} } @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 {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 {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.} } @mastersthesis {KNZ00886, title = {An ecosystem in transition: Patterns and mechanisms of a shift in growth form dominance in mesic grasslands of central North America}, volume = {MS Thesis}, year = {2003}, pages = {1 -80}, school = {Arizona State University}, type = {M.S. Thesis}, address = {Tempe, AZ}, keywords = {LTER-KNZ}, author = {Heisler, J.L.} } @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} }