Drought conditions reorder plant communities through expansion of spatially sparse and temporally intermittent species

Abstract

The frequency and severity of droughts are increasing in grasslands globally. Forecasting changes in community composition is challenging due to the direct and indirect effects of drought as well as varied species ecologies. Rare species are predicted to be more susceptible to perturbations, but extreme drought may also disproportionately reduce common species that are well adapted to modal environmental conditions. Resolving these contrasting effects is key to predicting species reordering in response to extreme drought. We explored how species drought responses varied by their spatial and temporal rarity, defined by their local abundances relative to co-occurring species, at six sites across a gradient of mean annual precipitation (MAP) in the Central USA. We found that in general, both spatially sparse and temporally intermittent species increased during and after drought, while common and persistent species declined. Responses were similar at four of the six sites across a gradient of MAP, but did not align with the precipitation gradient as expected. Instead, patterns depended on the characteristics of common species, where rarity was not related to drought responses at sites with many common, early-season, C₃ annual grasses that likely avoided the effects of the summer drought. Finally, the increase in rare species persisted for 4 years after the experimental drought was lifted, suggesting that this extreme drought resulted in lasting shifts to community composition. These findings highlight that rare species can be favored under non-modal environmental conditions like extreme drought and underscore the importance of assessing species recovery to periodic environmental disturbances.