Global diversity of drought tolerance and grassland climate-change resilience

Drought reduces plant productivity, induces widespread plant mortality and limits the geographic distribution of plant species1–7. As climates warm and precipitation patterns shift in the future8,9, understanding the distribution of the diversity of plant drought tolerance is central to predicting future ecosystem function and resilience to climate change10–12. These questions are especially pressing for the world’s 11,000 grass species13, which dominate a large fraction of the terrestrial biosphere14, yet are poorly characterized with respect to responses to drought. Here, we show that physiological drought tolerance, which varied tenfold among 426 grass species, is well distributed both climatically and phylogenetically, suggesting most native grasslands are likely to contain a high diversity of drought tolerance. Consequently, local species may help maintain ecosystem functioning in response to changing drought regimes without requiring long-distance migrations of grass species. Furthermore, physiologically drought-tolerant species had higher rates of water and carbon dioxide exchange than intolerant species, indicating that severe droughts may generate legacies for ecosystem functioning. In all, our findings suggest that diverse grasslands throughout the globe have the potential to be resilient to drought in the face of climate change through the local expansion of drought-tolerant species. Despite widespread agreement on the importance of drought in grasslands, predictions of grassland functioning under future climates is limited by our understanding of how grasses respond to drought and the geographic distribution of grasses with different functional traits. For example, plant productivity in Kansas and Nebraska grasslands was maintained during drought in the 1930s not by the immigration of drought-tolerant species, but by local expansion of these species after less-tolerant species perished15,16. Yet, it is unknown how diversity of drought tolerance among grassland species varies along climate gradients globally. Further research on the effects of functional diversity on resilience is still necessary, but grasslands containing a large range of drought tolerance among species should have greater resilience to drought compared with grasslands with less functional diversity. Grasslands without drought-tolerant species present are more likely to experience major declines with drought in grassland ecosystem function such as carbon uptake, productivity, soil retention and provision of forage to grazers following drought. Droughted grasslands may require long-distance migrations to restore such functions17,18, which may not be able to keep pace with the velocity of climate change19. Additionally, if drought-tolerant species have unique sets of functional traits beyond drought tolerance20–22, droughts might alter both the functional composition of grasslands and important grassland processes after cessation of drought.