Premise of the study: Evolutionary and biogeographic history, including past environmental change and diversification processes, are likely to have influenced the expansion, migration, and extinction of populations, creating evolutionary legacy effects that influence regional species pools and the composition of communities. We consider the consequences of the diversification process in shaping trait evolution and assembly of oak-dominated communities throughout the continental U.S. Methods: Within the US oaks, we tested for phylogenetic and functional trait patterns at different spatial scales, taking advantage of a dated phylogenomic analysis of American oaks and the US Forest Service Forest Inventory Analysis. Key Results: We find 1) phylogenetic overdispersion at small grain sizes throughout the US across all spatial extents and 2) a shift from overdispersion to clustering with increasing grain sizes. Leaf traits have evolved in a convergent manner, and these traits are clustered in communities at all spatial scales, except in the far west, where species with contrasting leaf types co-occur. Conclusions: Our results support the hypotheses that 1) interspecific interactions were important in parallel adaptive radiation of the genus into a range of habitats across the continent and 2) that the diversification process is a critical driver of community assembly. Functional convergence of complementary species from distinct clades adapted to the same local habitats is a likely mechanism that allows distantly related species to coexist. Our findings contribute to an explanation of the long-term maintenance of high oak diversity and the dominance of the oak genus in North America.