While climate change predictions indicate that in mid-latitude regions rainfall event sizes will increase and frequency will decrease, relatively few climate change experiments have manipulated rainfall patterns. This increased variability in rainfall may impact competitive dynamics in plant communities, depending on how species respond to high soil moisture and tolerate dry periods between storms. Coastal sage scrub (CSS) communities in southern California have declined rapidly over the past century and have been replaced by exotic annual grasslands. The decline in CSS is associated with fire, and competition with exotic grass species after a fire may limit CSS reestablishment. We hypothesized that increased rainfall event sizes and decreased event frequency will decrease the competitive impact of exotic grasses on CSS shrubs, increasing CSS reestablishment after fire. Deep rooted CSS shrubs are expected to tolerate dry periods between storms better than exotic grasses. Using rainout shelters, we manipulated rainfall patterns for two years following a fire in a CSS community, keeping total rainfall constant. We established annual grass plots, CSS plots containing resprouting shrubs and seedlings, and competition plots to evaluate CSS and annual grass responses to rainfall manipulations, impacts on resource availability, and competitive success.
We found that exotic annual grasses had a strong competitive impact on CSS shrub survival and biomass, promoting grass invasion success after fire. However, contrary to predictions, we found that increasing rainfall event sizes and decreasing event frequency did not influence the outcome of competition. Although the number of rainfall events was decreased by 60% throughout the season, reduction in the number of these small storms had little effect on annual grass growth or their competitive effect on CSS. We suggest that under the more variable rainfall conditions, annual grass success was due to the opportunistic fast growth and use of pulses in resources from large early season storms. Our results suggest that projected changes to larger less frequent storms may not reduce invasion by opportunistic annual species that are strong competitors, and emphasize the vulnerability of many native communities to invasion after disturbances.