Heterogeneity in recruitment and habitat patterns of valley oak (Quercus lobata Née) at the site and landscape scale in the Santa Monica Mountains, California, USA

Valley oak, Quercus lobata Née, has been a focus of conservation attention due to concerns about regeneration failure, habitat fragmentation and modification, and potential range contraction associated with climate change. A drawback of previous studies of valley oak stand structure and the regeneration problem has been their emphasis on one site or spatial and temporal scale. Generalization about valley oak dynamics from such studies is problematic because demographic parameters are not likely to be constant across sites, but heavily influenced by site-specific conditions. This study examines site-to-site variability across three locations in the Santa Monica Mountains National Recreation Area, California, USA. Variability in measures of stand structure and spatial pattern was examined across sites and within the landscape context. Estimates of expected landscape-scale evenness were derived by repeated random sampling of the data set and used as benchmarks for comparison with site-scale measures. Saplings were more abundant than expected at all three sites with sapling:adult ratios ranging from 0.5 to >1.0. Size-class structure suggested past recruitment problems at two sites, but recruitment at one site appears to have been steady for some time. Spatial distribution of adult stems at each site roughly corresponded to the estimated landscape pattern, but sapling establishment has shifted to north-to-east aspect hillsides and riparian-adjacent positions. In hilltop and swale habitats, adult valley oak are senescing without sapling recruitment. Shifting habitat-recruitment associations may lead to increasingly uneven distribution of stems across the landscape, altering landscape patterns and ecological processes.