Coastal sage scrub (CSS) in Southern California harbors exceptional native plant diversity and is critical habitat for a number of threatened or endangered animal species. These communities are increasingly impacted by environmental changes including habitat loss, invasive species, and accelerating fire regimes. Re-establishment of native species in CSS following fire may be hindered by invasive annual grasses that benefit disproportionately from disturbance, evidenced by differing rates of germination, establishment, and survival following fire. Thus, frequent fire and invasion together could convert shrub-dominated native CSS to predominantly exotic herbaceous communities. Here, we evaluated the hypothesis that these represent alternate community states, as defined by Multiple Stable Equilibrium (MSE) theory. First, we evaluated the “mutual invasibility criterion” in a recently burned CSS site for which pre-fire vegetation data was available. We collected and planted seeds of native and exotic focal species in paired burned and unburned locations and monitored rates of germination, growth and survival. Second, to place these results in a longer-term context we surveyed vegetation in five CSS sites in San Diego County with varying fire histories to evaluate the trajectory of exotic and native species abundances following fire, building on a USGS long-term vegetation monitoring effort.
Results/Conclusions
In our experiment exotic grasses (Avena fatua, Bromus hordeaceus,Vulpia myuros) had significantly higher germination, growth and survivorship than native shrub species (Mimulus aurantiacus, Eriogonum fasciculatum, Salvia apiana, Encelia californica). Exotic species performance was enhanced disproportionately under burned conditions as compared with native species, evidenced by significant origin by burn interactions. For instance, exotic grasses had high germination rates on both burned (66%) and unburned (62%) soils. In contrast native shrubs had low germination overall, with higher germination on burned (7%) than unburned (4%) soils. Exotic grasses had lower mortality rates in burned (12%) than in unburned (36%) areas, and while native shrubs had a similar pattern (36% mortality in burned, 53% in unburned), they benefited less on burned sites than the exotic grasses. Similar responses were observed for growth of surviving individuals. In our vegetation surveys, exotic grass abundance increased following fire at most sites, and native shrub cover increased over time, although recovery rates differed among sites. In summary, our data suggest that dynamics of invaded post-fire communities do not strictly fit within MSE theory, but rather there is a variable (site specific) time period following burning in which conditions favor invasion by exotic species over native shrub re-establishment.