Premise of the study: We use spatial phylogenetics to analyze the assembly of the Wisconsin flora, linking processes of dispersal and niche evolution to spatial patterns in floristic and phylogenetic diversity, and testing whether phylogenetic niche conservatism can account for these patterns. Methods: We use digitized records and a new molecular phylogeny for all vascular plants in Wisconsin to estimate spatial variation in species richness and phylogenetic  and  diversity in a native flora shaped mainly by post-glacial dispersal and response to environmental gradients. We develop distribution models for all species and use these to infer fine-scale variation in potential diversity, phylogenetic distance, and interspecific range overlaps. We identify 11 bioregions based on floristic composition, map areas of neo- and paleo-endemism to establish new conservation priorities, and predict how community-assembly patterns should shift with climatic change. Key results: Spatial phylogenetic turnover most strongly reflects differences in temperature and spatial distance. For all vascular plants, assemblages shift from phylogenetically clustered to over-dispersed northward, contrary to most other studies. This pattern is lost for angiosperms alone, illustrating the importance of phylogenetic scale.Conclusions: Species ranges and assemblage composition appear driven primarily by phylogenetic niche conservatism. Closely related species are ecologically similar and occupy similar territories. The average level and geographic structure of plant phylogenetic diversity within Wisconsin will greatly decline over the next half-century, while potential species richness will increase throughout the state. Our methods can be applied to allochthonous communities throughout the world.