Global precipitation patterns are expected to shift in the coming decades as a consequence of climate change. Southern California is a region predicted to experience persistent drought, punctuated by extreme high rainfall years. Traditionally dominated by woody shrubs, these ecosystems are increasingly invaded by exotic herbaceous species. Here, we asked whether invasion alters the community-level response shifting rainfall regimes. To accomplish this, we experimentally manipulated rainfall amounts in adjacent communities initially dominated by native versus exotic species. Three experimental rainfall treatment (50%, 100%, and 150% of ambient rainfall) were imposed on 3 x 3 m plots of intact vegetation. Plot-level community composition data was monitored in a permanent central 1 x 1m area through three growing seasons, and PERMANOA analysis was used to analyze species composition using initial community composition, rainfall treatment, and growing season year as factors. The study site was located in a coastal sage scrub ecosystem in the Santa Margarita Ecological Reserve, located in San Diego County.
Results/Conclusions
We found that initial species composition (invaded versus native-dominated plots) mediated species composition responses to altered rainfall (invasion x rainfall interaction, p=0.003). Plots dominated by native vegetation were not significantly affected by rainfall treatment, maintaining consistently high cover from three native shrubs (Artemesia californica, Salvia apiana, S. mellifera). Invaded plots receiving high rainfall had greater community dominance of exotic annual grasses and forbs (notably Bromus hordeaceus, B. madritensis, and Centaurea melitensis) while those receiving 50% of ambient rainfall showed greater abundance of native herbaceous species (Stipa pulchra, Helianthus gracilentus). Differences between species composition treatment also varied between years (p=0.001), with exotic dominated plots having non-overlapping 95% confidence intervals for all rainfall treatments in the 2013 and 2015 growing seasons, but only showing differences between the 150% and 50% treatments in the 2014 growing season. Additionally, there were overall effects by all three factors (p=0.001 for all). These results give evidence that native-shrub dominated communities are more resilient to changing precipitation regimes than adjacent invaded communities. Within already invaded areas, reduced precipitation may lower the competitive dominance of exotic grasses and forbs, allowing for greater abundance and long-term persistence of native herbaceous grasses and forbs.