OOS 33-1 - Multiyear drought effects on North American warm deserts

Background/Question/Methods
Over the past century, mean precipitation has decreased while precipitation variability has increased globally and in the southwestern U.S. The combination of reduced annual precipitation and increased variability implies increasing frequency of severe multiyear drought. Recent studies suggest that ecosystem sensitivity to precipitation change is greatest in drylands. Since drylands account for 40% of the Earth’s terrestrial surface, it is crucial that we understand the mechanisms by which drought affects dryland ecosystems. We used a drought experiment distributed across the three warm deserts of North America - Mojave, Sonoran, Chihuahuan - to assess the hierarchical effects of drought on desert plant communities. We compared the impacts of the drought treatment on community structure and species turnover in communities dominated by bet-hedging annuals and communities dominated by perennial grasses. We address (1) differences in drought response between annual-dominated and perennial-dominated communities, (2) relationships between species reordering and turnover with sensitivity of vegetative cover, and (3) whether drivers of cover responses change over time.

Results/Conclusions
Cover changes were attributable more to changes in grass cover than forb cover across years. Grass cover significantly decreased in the first year at three of six sites and in the second year at four of six sites. Responses of grass cover to the drought treatment were common across years, but forbs were generally unresponsive. Species reordering was correlated with cover responses in the first two treatment years (year one slope = -0.57, p= 0.0079; year two slope = -0.567, p = 0.001), but correlations with turnover responses were limited. With this distributed experiment, we demonstrate that drought impacts on desert ecosystem processes are immediate and concentrated in grasses. Community reordering plays an important role in the impacts of drought on desert ecosystem processes through heightened sensitivity of dominant species to abiotic change.