Environmental conditions and biotic interactions are generally thought to influence local species richness. However, immigration and the evolutionary and historical factors that shape regional species pools should also contribute to determining local species richness because local communities arise by assembly from regional species pools. Using the European tree flora as our study system, we implemented a novel approach to assess the relative importance of local and regional mechanisms that control local species richness. We first identified species pools that tolerate particular local environments and quantified the proportion of the pool that is present locally, i.e. the realized/potential (R/P) richness ratio. Because no consensus exists on how to estimate potential richness, we estimated it using three different approaches. Using these three estimates separately and in a combined ensemble estimate, we then analyzed the effects of potential drivers on R/P richness ratios. We predicted that the R/P richness ratio would 1) increase with decreasing distance from glacial refugia (accessibility), 2) and be generally low in geographically fragmented southern Europe because of dispersal limitation; 3) increase with actual evapotranspiration because greater availability of water and energy promotes local population persistence; and 4) increase with topographic heterogeneity because it promotes local species coexistence and facilitates long-term species survival. There was considerable variation among the three R/P richness ratio estimates, but we found consistent support for a negative effect of regional geographic fragmentation and a positive topographic effect. We also identified fairly broad support for the predicted effect of accessibility. We conclude that local tree assemblages in Europe often fail to realize a large proportion of the potential richness held in the regional species pool, partially reflecting their geographical, historical, and environmental circumstances. The dispersal-related effects of geographic fragmentation and accessibility exemplify regional controls that combine with local ecological sorting to determine local species richness.