Invasive Species and Fire in California Ecosystems

nvasive plant species occur throughout all floristic regions of California, but their spatial extent, diversity, and impacts within these regions vary considerably. Alterations of natural disturbance regimes have made communities more susceptible to these invasions (Brooks et al. 2004). Fire is a natural and chronic disturbance in many California plant communities and has been observed to promote and be promoted by invasive species in several of the communities. Fire regimes—the type, frequency, intensity and timing of fire—have played an important role in the evolution of California plant communities, but human influences have changed fire regimes, sometimes in ways that shift the relative dominance of native and non-native species. Invasive plants may be directly responsible for changes in fire regimes through increased biomass, changes in the distribution of flammable biomass, increased flammability, and altered timing of fuel drying, while others may be “fire followers” whose abundances increase as a result of shortening of fire return intervals. California’s shrublands, woodlands, grasslands, wetlands, and forests occupy different elevation and moisture zones, creating unique fire regimes that have benefitted particular invasive species. Plant associations that differ in physiognomy, fire regimes, and fuel types may vary in their resistance and resilience to fire. Fire regimes have been best studied in conifer forests and shrublands, but are poorly understood in California’s grasslands. The role of fire in riparian or other wetland systems is particularly poorly known, presumably because these habitats have Invasive grasses and forbs have invaded this fuel break in the Santa Ynez Mountains above Santa Barbara. Photograph by C. D’Antonio.