@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 {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 {KNZ001701, title = {Invasibility of a mesic grassland depends on the time-scale of fluctuating resources}, journal = {Journal of Ecology}, volume = {103}, year = {2015}, pages = {1538 - 1546}, abstract = {

1. Global change is increasing the frequency and magnitude of resource fluctuations (pulses) at multiple time-scales. According to the fluctuating resource availability hypothesis (FRAH), susceptibility of an ecosystem to invasion (i.e. invasibility) is expected to increase whenever resource supply exceeds that which is utilized by native communities. Thus, global change has the potential to increase invasibility around the world. 2. Here, we test the FRAH by adding seeds of a target invader grass species to a long-term climate change experiment manipulating precipitation pulse size in tallgrass prairie in Kansas, USA. 3. Our experimental work yielded three important findings. First, contrary to predictions of the FRAH, invasibility was reduced with short time-scale resource pulses (intra-annual time-scale). Secondly, we found evidence to suggest that at inter-annual time-scales, the FRAH is supported. Wet years resulted in an increase in the number of established seedlings as well as the number of seedlings that persisted to the end of the season. Finally, we found that invasibility was positively related to native community richness and the density of individuals in the community suggesting that native communities facilitate establishment of invader species. Perhaps more importantly, results from this 10-year invasion study also show that resource availability drives invasion and that the biotic filters of plant community structure and diversity are secondary. 4. Synthesis. Our findings suggest that intensification of precipitation regimes may enhance resistance to invasion at intra-annual time-scales, but will have opposing effects if precipitation regimes include more wet years.

}, keywords = {LTER-KNZ}, doi = {10.1111/1365-2745.12479}, url = {https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12479}, author = {Koerner, S.E. and M.L. Avolio and Chang, C.C. and Grey, J. and D.L. Hoover and M.D. Smith} } @article {KNZ001690, title = {Contrasting sensitivities of two dominant C4 grasses to heat waves and drought}, journal = {Plant Ecology}, volume = {215}, year = {2014}, pages = {721 -731}, abstract = {

Heat waves and droughts are predicted to increase in frequency and intensity with climate change. However, we lack a mechanistic understanding of the independent and interactive effects of severe heat and water stress for most ecosystems. In a mesic tallgrass prairie ecosystem, we used a factorial experimental approach to assess ecophysiological and productivity responses of two dominant C4 grasses, Andropogon gerardii and Sorghastrum nutans, to a season-long drought and a mid-summer heat wave at four intensities. We hypothesized that drought would have greater impacts than heat waves, that combined effects would be greater than either factor alone, and that the dominant grasses would differ in their responses to heat and water heat stress. We detected significant reductions in photosynthesis, leaf water potential, and productivity with drought but few direct responses to the heat waves. Surprisingly, there was no additive effect of heat and water stress on any plant response. However, S. nutans was more sensitive than A. gerardii to drought. In this grassland, water stress will likely dominate photosynthetic and productivity responses caused by discrete drought and heat wave events, rather than direct or additive effects of heat stress, with differential sensitivity in these grasses altering future ecosystem structure and function.

}, keywords = {LTER-KNZ, Climate change, Climate extremes, Mesic grassland, photosynthesis, productivity}, doi = {10.1007/s11258-014-0345-8}, url = {https://link.springer.com/article/10.1007\%2Fs11258-014-0345-8}, author = {D.L. Hoover 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 {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.} }