Extreme climate conditions such as drought can prompt rapid and sustained shifts in ecosystem function with important economic ramifications. This is especially true for grasslands where there is high demand in supporting livestock despite increased drought around the world. There are a variety of mechanisms for reducing the impact of climate extremes in ecosystems, including high plant species richness, or a diverse range of traits. These plant community characteristics are hypothesized to serve as buffers against reduced grassland resilience under climate extremes, however these metrics do not consider species level interactions. Species interactions, including competition and facilitation, ultimately determine the outcome of ecosystem responses to environmental changes, however the potential for species interactions to provide stabilizing or amplifying feedbacks to climate extremes is difficult to examine in field settings. Our Pastures in Climate Extremes (PACE) experimental facility in eastern Australia uses a factorial manipulation of elevated air temperature (+3 °C) and winter + spring extreme drought (60% reduction) to examine pasture resilience and sensitivity under future climate scenarios. The experiment includes three mixed species pastures along with monoculture pastures of the composite species to examine the importance of competition and nutrient facilitation in influencing pasture sensitivity to drought and warming.
Results/Conclusions
We found that grass-legume mixed pastures had lower sensitivity to drought conditions on aboveground biomass than the comparable sensitivity of individual grass or legume pastures during our 2018 drought. However, during our 2019 drought this benefit was removed for one species pair (legume/C4 grass) but maintained for the second (legume/C3 grass). As expected, warmed conditions increased the sensitivity of mixed pastures to drought for both legume/grass combinations, tied to a reduction in biologically fixed nitrogen (N) by associated rhizobia under these conditions. In contrast, the mixed C3/C4 pasture had a greater reduction in biomass from drought than either grass alone suggesting competition for limited water resources. Importantly, warming conditions did not increase sensitivity to drought for the mixed C4/C3 pasture, although during the warmer summer months following the end of the drought period made this effect more negative. Our findings support the hypothesis that nutrient facilitation between grass and legumes provides stabilizing feedbacks on ecosystem response (aboveground biomass) to extreme drought provided that the legume component maintains a threshold level of N fixation. Increased species richness and trait diversity in our C3/C4 mixed pasture did not increase resilience to drought but did provide a benefit under warming.