INVASIVE SPECIES AND FIRE IN CALIFORNIA ECOSYSTEMS

nvasive plant species occur through-out all floristic regions of Cali-fornia, but their spatial extent, di-versity, and impacts within these regions vary considerably. Alter-ations of natural disturbance regimes have made communities more sus-ceptible to these invasions (Brooks et al. 2004). Fire is a natural and chronic disturbance in many Cali-fornia plant communities and has been observed to promote and be promoted by invasive species in sev-eral of the communities. Fire regimes—the type, fre-quency, intensity and timing of fire—have played an important role in the evolution of California plant communities, but human influences have changed fire regimes, some-times in ways that shift the relative dominance of native and non-native species. Invasive plants may be di-rectly responsible for changes in fire regimes through increased biomass, changes in the distribution of flam-mable biomass, increased flamma-bility, and altered timing of fuel dry-ing, while others may be “fire fol-lowers ” whose abundances increase as a result of shortening of fire re-turn intervals. California’s shrublands, wood-lands, grasslands, wetlands, and for-ests occupy different elevation and moisture zones, creating unique fire regimes that have benefitted particu-lar invasive species. Plant associa-tions 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 Cali-fornia’s grasslands. The role of fire in riparian or other wetland systems is particularly poorly known, pre-sumably 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.