@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 {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 {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.} } @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 {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 {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 {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} }