Interactive effects of disturbance and exotic species on the structure and dynamics of an endemic sandhills plant community.

摘要

I examined the independent and interactive effects of disturbance and exotic plants on the structure and species composition of an endemic plant assemblage in the Santa Cruz sandhills. Restricted to outcroppings of sandy soils found only in Santa Cruz County (central coastal California), the sandhills support a diverse assemblage of unique native species that includes two federally endangered plants: Chorizanthe pungens var. hartwegiana and Erysimum teretifolium. These species are preferentially found in open areas away from sparsely distributed trees (Pinus ponderosa and Quercus agrifolia) and dense cover of European annual grasses and forbs in a savanna community known as sand parkland.; Analysis of historical aerial photographs showed that tree cover has increased since the last fire in sand parkland. Using a series of descriptive studies and manipulative experiments, I found that trees restricted the distribution of native and exotic herbs, primarily through their dense tree leaf litter that inhibited germination and seedling survival. Away from trees, exotic plants reduced the survivorship and fecundity of the endangered species and the cover and richness of native herbaceous plants.; Through experimental tests of the effects of reintroducing fire, I showed that surface fires facilitated the endangered species and the larger suite of native herbs by removing accumulated tree leaf litter. Rather than enhancing the abundance of exotic species as originally hypothesized, fire reduced exotic plant establishment, thereby indirectly benefiting the native plant assemblage. Raking caused similar direct and indirect benefits, suggesting this fire surrogate can also promote recovery of the endangered species and enhance diversity of the native plant assemblage.; Patchy soil disturbances caused by water erosion, animal trails, and gophers mounds also reduced litter and exotic plant abundance. Though these disturbances varied in their specific impacts, they tended to enhance the demographic performance of the endangered species and the cover and species richness of the native assemblage. In the absence of fire, soil disturbances provide refugia allowing persistence of the endangered species in this system.; Results showing that disturbances reduced, rather than enhanced, exotic species in the sandhills may have implications for management of disturbance-dependent native plants in other invaded communities.