02163nas a2200301 4500008004100000245007600041210006900117300001200186490000800198520129700206100001601503700001801519700002001537700001701557700001701574700001701591700001701608700001701625700001301642700001801655700001701673700002601690700001701716700001701733700001101750700002001761856008001781 2020 eng d00aMass ratio effects underlie ecosystem responses to environmental change0 aMass ratio effects underlie ecosystem responses to environmental a855-8640 v1083 a

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

1 aSmith, M.D.1 aKoerner, S.E.1 aKnapp, Alan, K.1 aAvolio, M.L.1 aChaves, F.A.1 aDenton, E.M.1 aDietrich, J.1 aGibson, D.J.1 aGray, J.1 aHoffman, A.M.1 aHoover, D.L.1 aKomatsu, Kimberly, J.1 aSilletti, A.1 aWilcox, K.R.1 aYu, Q.1 aBlair, John, M. uhttps://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2745.1333002051nas a2200205 4500008004100000245011100041210006900152300001200221490000800233520138900241653002301630653002101653653001201674653001801686653002201704100001701726700002001743700001601763856006601779 2017 eng d00aPhotosynthetic responses of a dominant C4 grass to an experimental heat wave are mediated by soil moisture0 aPhotosynthetic responses of a dominant C4 grass to an experiment a303-3130 v1833 a

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

10aCanopy temperature10aClimate extremes10aDrought10aEcophysiology10atallgrass prairie1 aHoover, D.L.1 aKnapp, Alan, K.1 aSmith, M.D. uhttps://link.springer.com/article/10.1007%2Fs00442-016-3755-603239nas a2200313 4500008004100000245011900041210006900160300001400229490000700243520231400250100002002564700001702584700001402601700002002615700002402635700001602659700001702675700002402692700001702716700001602733700001502749700001902764700001502783700001502798700001702813700001702830700001602847856006202863 2017 eng d00aPushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years0 aPushing precipitation to the extremes in distributed experiments a1774-17820 v233 a

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

1 aKnapp, Alan, K.1 aAvolio, M.L.1 aBeier, C.1 aCarroll, C.J.W.1 aCollins, Scott., L.1 aDukes, J.S.1 aFraser, L.H.1 aGriffin-Nolan, R.J.1 aHoover, D.L.1 aJentsch, A.1 aLoik, M.E.1 aPhillips, R.P.1 aPost, A.K.1 aSala, O.E.1 aSlette, I.J.1 aYahdjian, L.1 aSmith, M.D. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.1350402183nas a2200145 4500008004100000245010100041210006900142300001600211490000800227520167500235100001701910700002001927700001601947856007401963 2016 eng d00aThe immediate and prolonged effects of climate extremes on soil respiration in a mesic grassland0 aimmediate and prolonged effects of climate extremes on soil resp a1034 - 10440 v1213 a

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

1 aHoover, D.L.1 aKnapp, Alan, K.1 aSmith, M.D. uhttps://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JG00325602102nas a2200361 4500008004100000245011900041210006900160300001400229490000700243520104200250653004101292653000901333653001401342653001201356653002001368100001601388700002001404700002401424700002001448700001801468700001801486700001901504700002001523700001601543700001201559700002001571700001701591700001701608700001601625700001701641700001701658856006501675 2016 eng d00aShared drivers but divergent ecological responses: Insights from long-term experiments in mesic savanna grasslands0 aShared drivers but divergent ecological responses Insights from  a666 - 6820 v663 a

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.

10aAboveground net primary productivity10afire10agrassland10aGrazing10aplant community1 aSmith, M.D.1 aKnapp, Alan, K.1 aCollins, Scott., L.1 aBurkepile, D.E.1 aKirkman, K.P.1 aKoerner, S.E.1 aThompson, D.I.1 aBlair, John, M.1 aBurns, C.E.1 aEby, S.1 aForrestel, E.J.1 aFynn, R.W.S.1 aGovender, N.1 aHagenah, N.1 aHoover, D.L.1 aWilcox, K.R. uhttps://academic.oup.com/bioscience/article/66/8/666/246414104652nas a2200133 4500008004100000245006600041210006600107260004800173490002200221520420000243100001704443700002004460856003804480 2013 eng d00aEcological responses to climate extremes in a mesic grassland0 aEcological responses to climate extremes in a mesic grassland aFort Collins, CObColorado State University0 vPhD. Dissertation3 a

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

1 aHoover, D.L.1 aKnapp, Alan, K. uhttp://hdl.handle.net/10217/8247401884nas a2200193 4500008004100000245009100041210006900132260004100201300001000242490001400252520122500266653002401491653001501515653002901530653003301559653002701592100001701619856005401636 2008 eng d00aAltered rainfall due to climate change: Modeling the ecological effects on grasslands0 aAltered rainfall due to climate change Modeling the ecological e aStorrs,CTbUniversity of Connecticut a1 -630 vMS Thesis3 a

The acceleration of the hydrological cycle due to global climate change is predicted to cause more extreme precipitation, characterized by fewer but more intense storms. Such altered rainfall patterns will have significant impacts on water limited ecosystems such as grasslands. This project used a land surface model, the Community Land Model 3.0 coupled with a dynamic vegetation model, to examine how changing the size and frequency of rain events impacts plants, surface hydrology and surface energy budgets in the tallgrass prairie ecosystem. We applied a similar rainfall scheme as the Rainfall Manipulation Plots at Konza Prairie Biological Station in Kansas, which consisted of an altered rainfall treatment, with fewer but more intense events, and an ambient treatment, which replicated the natural pattern of rain. Both the model and field experiments show a reduction in grass productivity from the ambient to the altered treatment and a negative correlation between soil moisture variability and productivity. In addition, based on the modeling results, the altered rainfall treatment increases runoff, decreases evapotranspiration, and increases sensible heat flux at the expense of latent heat flux.

10aClimate variability10aEcosystems10aImpacts of global change10aLand/atmosphere interactions10astructure and dynamics1 aHoover, D.L. uhttp://adsabs.harvard.edu/abs/2008AGUFM.B21C0377H