UC Riverside

Exotic annual plant species have invaded large regions of southern California deserts. Certain areas have been especially impacted, such as the western edge of the Colorado Desert adjacent to Mt. San Gorgonio, Banning Pass, and Mt. San Jacinto. This landscape is highly invaded due to relatively high winter rainfall compared to interior desert locales and elevated anthropogenic nitrogen deposition from urban areas to the west. Invasive annual grasses, in particular, are abundant here and have fueled disastrous wildfires in creosote bush scrub (CBS) since the 1970s. Invasive annual plants and fire pose major threats to the sustainability of CBS. The purpose of this dissertation was to document the impact of fire on CBS perennial and annual plant components. The ability of various restoration treatments to remove invasive annual plants and to promote native species in both burned and unburned contexts was also tested. Fire disturbance was shown to dramatically reduce shrub species richness and diversity. Fire also altered the vertical and horizontal vegetation structure of CBS. Total shrub density returned to unburned levels within a decade or two after fire due to recruitment by Encelia farinosa, a relatively small, short-lived shrub. Fire also increased the abundance of invasive annual plants. These changes may result in a vegetation type that is more fire prone than unburned CBS. Post-fire increases in invasive annual plants were also shown to decrease native annual plant cover and species richness. However, if invasive annuals were removed after fire, then native annual plant abundance and richness increased greatly, resembling unburned, pristine stands. Positive responses to invasive plant removal were also documented by native annual plant assemblages in unburned CBS. Clearly, the combination of invasive annual plants and fire has a large negative impact on native plant components of CBS. Fortunately, there is great potential to reestablish native annuals if invasives can be controlled.