Understanding how plant communities will respond to altered precipitation regimes is critical for predicting future biodiversity patterns and ecosystem functioning. This is especially challenging in ecosystems with seasonal precipitation regimes, where species cope with a wide range of variability and predictability in precipitation. At the cross-roads of the Mediterranean and Sonoran ecosystems, annual plant communities receive both winter and summer rain resulting in distinct plant communities in each season. However, anthropogenic climate change has increased summer precipitation in this system. To assess the impact of altered precipitation on the annual plant community, we used a rainfall exclusion, collection, and distribution system to alter seasonal (winter and summer) precipitation regimes. We sampled biomass and species composition seasonally over two years in six subplots nested within each of 24 5x8m plots to: 1) assess the contribution of precipitation in each season (a: summer only rainfall treatment; b: winter only rainfall treatment); 2) assess the influence of increased precipitation in each season (c: ambient summer and 2x winter rainfall treatment; d: ambient winter and 2x summer rainfall treatment); and 3) to compare these changes in precipitation regime to ambient conditions (e: ambient summer and winter rainfall control plots).
Results/Conclusions
Under ambient rainfall, we found five times greater biomass, twice as many species, and higher diversity in winter as compared to summer. Summer only rainfall contributed little to summer biomass or composition, and plots that did not receive summer rain did not differ from control plots. Increasing summer precipitation resulted in less variation in biomass production and species richness across seasons, and increased biomass twofold in drier years. Winter only rainfall contributed strongly to winter biomass and composition, and plots that did not receive winter rain had 20 times lower biomass and 8 fewer species than control plots. Increasing winter precipitation had little impact on biomass production or species richness in winter but is beginning to show carryover effects where biomass and richness are increasing in summer communities that receive extra winter rain. Our results suggest that increasing warm season precipitation will likely have strong immediate effects on biodiversity and ecosystem function, while increasing cool season precipitation may impact biodiversity and ecosystem function via carryover effects during the next warm season. Taken together, these patterns suggest that altered seasonal precipitation may have larger positive impacts on biodiversity and ecosystem functioning in warmer than cooler seasons.