Climate change models predict that in mid-latitude regions rainfall event sizes will increase and frequency will decrease. These temporal changes in rainfall distribution may influence the success of invasive species, particularly when invaders and residents differ in their relative responses to soil moisture. California’s coastal sage scrub (CSS) shrublands have been invaded by exotic annual grasses. We hypothesized that competitive hierarchies between resident and invader species would shift in response to rainfall distribution. We predicted that decreased rainfall frequency would favor species that can maintain higher growth rates during the prolonged dry periods (CSS). We also predicted that larger rainfall events would allow water to infiltrate to deeper soil layers, giving deeply-rooted plants (CSS) an advantage when surface soils dry.  In contrast, more frequent, smaller events would allow plants with higher shallow root biomass and higher resource uptake rates (exotic grasses) to preempt soil water from deeper rooted species. In a rainfall manipulation experiment we established exotic grass plots, CSS seedling plots, and competition plots in an additive design. Each plot type was exposed to three precipitation treatments: frequent small events, infrequent large events (with same total water inputs as the previous treatment), and infrequent small events. 
Results/Conclusions

Monoculture plots verified differences in exotic grass and CSS shrub root distribution, with grasses showing higher uptake rates of shallow water than shrubs. Grasses had strong competitive effects on shrub seedlings in all treatments, with the strongest effect in the frequent small events rainfall treatment. These results suggest that climate change predictions of less frequent, larger rainfall events and more variable soil moisture conditions will increase CSS seedling success, and in the long term reduce post-disturbance invasion of CSS by exotic grasses. More generally, our results suggest that predictions of more variable and less frequent rainfall events in mid-latitude regions will increase the relative success of deeply rooted and drought tolerant species.