In terms of competition, invasive annual grasses and forbs hierarchically rank above native annuals in winter-rainfall, annual plant assemblages of the western United States. Therefore, removal of both invasive grasses and forbs results in maximum competitive release of native annual plants. Because invasive plants are typically more abundant in areas with relatively higher resources, the greatest positive response of natives to invasive plant removal should occur where resources are relatively high. Interspecific competition is thought to be greater in areas with higher resources and consequently, more productivity. In addition, the degree of competition can vary as resources are spatially patchy within a habitat and/or characterized by pulsed temporal availability. The purpose of this study was to evaluate the response of native annual plants to the simultaneous removal of invasive annual grasses and forbs in two contrasting microhabitats, resource poor shrub interspaces and relatively resource rich fertile island understories in creosote bush scrub. Four sites were used in this study that varied in their abundances of invasive annual grasses, invasive annual forbs, and native annuals. Invasive species removal was performed at these sites in 2005 or 2008.
We found that invasive plants were more abundant in understory than interspace microhabitats, and that native species richness was greater in interspace microhabitats but had the greatest relative increase in response to invasive plant removal in the understory. Based on cover of native annual plants, competition importance did not differ between microhabitats, and competition intensity was greater in understory microhabitat, but only at one of the four sites. These results suggest that areas with greater resources in desert environments are more susceptible to invasion and competitive exclusion, but may also be more resilient once invasives are removed.