Climate change models predict that, in mid-latitude regions, rainfall event size will increase and storm frequency will decrease. These changes may strongly influence whether exotic species can invade native habitats, particularly in systems that are simultaneously impacted by disturbance. In semi-arid coastal sage scrub (CSS), exotic annual grasses threaten the highly diverse native community. In addition, fire disturbance is increasing in frequency. The proposed research will test the prediction that larger, less frequent, rainfall events increase the relative competitive ability of the native shrubs after fire, slowing the conversion of CSS to annual exotic grassland. These larger, less frequent events should favor species like the shrubs with deeper roots and greater tolerance to dry periods. To test these predictions, rainfall distribution will be manipulated (increasing event size and decreasing frequency) in burned and unburned CSS. Subplots planted with CSS, grasses, or a competitive mixture will be used to evaluate species' relative performance under the rainfall manipulations. Greenhouse measurements of species' soil moisture requirements will supplement field experiments to provide a strong test of ecological theory and a base for predictions of community response to multiple global change factors. The results will be used to inform conservation and restoration decisions in CSS and other systems subjected to increased invasion pressure, altered disturbance regimes, and changed precipitation patterns. In addition to influencing natural area management, the project will train undergraduate students though independent research and promote community awareness through meetings, presentations, and publications.
