PS 43-155 - Extreme drought consistently causes species loss and shifts in plant community composition

Abstract: Drought is predicted to increase in frequency and duration with climate change, which may impact the provisioning of ecosystem functions and services through changing plant community composition. Generalities of drought impacts on plant community composition, however, are few. The International Drought Experiment (IDE) was designed to study the impacts of severe drought on ecosystems globally. Here, we use data from 77 IDE sites on grasslands and shrublands spanning six continents to assess how three years of consecutive drought impacts plant community composition and dynamics. With this data, we addressed two questions. First, does drought affect plant community features such as changes in richness, evenness, dominance, and processes such as re-ordering, species gains and species losses as well as overall composition based on Bray-Curtis dissimilarity? Second, how do drought severity, duration and site characteristics (e.g. mean annual precipitation and percent of the community that are annual species) affect the magnitude of community change? To answer these questions, we calculated and compared changes in plant community structure and composition over time for all control and droughted plots. Our goal is to seek generality regarding the impacts of drought on plant community composition and dynamics. Drought altered plant communities; statistically significant differences were detected for many of our measures of changes in community structure and composition. Compared with control plots, we found that droughted plots had greater changes in evenness (p = 0.007) and overall community composition (p < 0.001) and had fewer species gains (p = 0.001) and more species losses (p < 0.001). Droughted plots, however, did not undergo greater changes in richness, dominance or re-ordering compared with control plots. Interestingly, the degree of change was not impacted by length or severity of drought —communities typically changed in the first year and then maintained those changes regardless of drought severity in subsequent years . We also found that drought led to greater changes in species richness at sites with more annual species (p = 0.03). Overall, our findings highlight some generalities about the impacts of drought on plant communities, which included losses of plant species and significant changes to community evenness and overall composition, especially at sites dominated by annuals.