PREMISE OF THE STUDY: Although long-distance pollen movement is common in wind-pollinated trees, barriers to gene flow may occur in species that have discontinuous ranges or are confined to certain habitat types. We investigated the genetic structure of Quercus lobata Née populations throughout much of their range in California. We assessed the connectivity of populations and determined if barriers to gene flow occurred, and if so, if they corresponded to landscape features.METHODS: We collected leaf samples from 270 trees from 12 stands of Quercus lobata and genotyped these trees using eight polymorphic microsatellite loci. Genetic structure and clustering was evaluated using genetic distance methods, Bayesian clustering approaches, and network analysis of spatial genetic structure.KEY RESULTS: The southernmost population of Quercus lobata sampled from the Santa Monica area comprised a separate genetic cluster from the rest of the species, suggesting that Transverse Ranges such as the San Gabriel Mountains limit gene flow. Population differentiation among the other sites was small but significant. Network analysis reflected higher connectivity among populations along the Central Coast range, with few connections spanning the dry, low Central Valley.CONCLUSIONS: While long distance pollen movement has been shown to be common in oaks, on larger spatial scales, topographic features such as mountain ranges and the large, flat Central Valley of California limit gene flow. Such landscape features explain gene flow patterns much better than geographic distance alone.