The impacts of isolation, canopy size, and environmental conditions on patterns of understory species richness in an oak savanna

摘要

Fragmentation and habitat loss have transformative effects on landscapes. Like many savanna communities worldwide, California oak savannas (Quercus spp.) have experienced extensive anthropogenic clearing, increasing the isolation of individual trees. Isolation and tree size may have consequences for populations of understory plants, but this has never been evaluated in a temperate savanna community. I address two questions: (1) How does the presence of oak trees affect site-level species richness? (2) What landscape and/or local attributes are associated with trees that contribute many species to a site? Vegetation and environmental attributes in the understory of isolated oaks and in surrounding grassland were surveyed at four sites in California, USA. I counted the number of species occurring in plots underneath and at the edge of each tree that were absent from surrounding plots in open grassland ('oak-associated species'). I used species-area curves to evaluate the contribution of oaks to site-level species richness. The importance of crown area, isolation, environmental heterogeneity, and understory soil fertility on species occurrences was assessed using linear mixed models. At two of the four sites, oaks increased site species richness by 23-57 %. At the southernmost site, understory soil fertility and isolation were negatively related to oak-associated species. Across all four sites, trees with larger crown area had more oak-associated species. Savanna oaks are important structuring forces for landscape plant species richness, especially in dry locations. Large trees may be especially important for conserving diverse understories. These positive effects, however, are not always negatively influenced by isolation.