the results are likely confounded by underlying gradients in climate and phylogenetic composition. We take an alternative approach that examines functional richness across a tree species richness anomaly where species richness doubles from Europe to eastern North America. The results demonstrate the functional richness on both continents saturates at a similar point as species richness increases and that the packing of functional space becomes tighter. Further, the species richness anomaly is driven primarily by genera unique to North America, but those genera contribute less than expected functional richness to the region, indicating a high level of redundancy with genera shared between the continents. Taken together, the results indicate that the species richness anomaly is associated with diversification within a climatically constrained trait space. More generally, the work demonstrates the power of utilizing species richness anomalies in biodiversity research particularly when they are coupled with information regarding organismal function. Introduction Gradients in species diversity are ultimately the result of differential speciation, extinction, immigration and extinction rates through space and time (Mittelbach et al. 2007; Swenson 2011). The spatial distribution of functional diversity may provide a key to unraveling the relative contribution of these mechanisms in producing regional-scale species diversity and how this influences local scale diversity (Swenson and Enquist 2007; Swenson et al. 2012). For example, a classic question in ecology has been whether strong inter-specific interactions and limiting similarity (MacArthur and Levins 1967) results in a greater range of phenotypic space being occupied as species richness increases (Ricklefs and O'Rourke 1975; Ricklefs and Travis 1980). Alternatively, abiotic
the results are likely confounded by underlying gradients in climate and phylogenetic composition. We take an alternative approach that examines functional richness across a tree species richness anomaly where species richness doubles from Europe to eastern North America. The results demonstrate the functional richness on both continents saturates at a similar point as species richness increases and that the packing of functional space becomes tighter. Further, the species richness anomaly is driven primarily by genera unique to North America, but those genera contribute less than expected functional richness to the region, indicating a high level of redundancy with genera shared between the continents. Taken together, the results indicate that the species richness anomaly is associated with diversification within a climatically constrained trait space. More generally, the work demonstrates the power of utilizing species richness anomalies in biodiversity research particularly when they are coupled with information regarding organismal function. Introduction Gradients in species diversity are ultimately the result of differential speciation, extinction, immigration and extinction rates through space and time (Mittelbach et al. 2007; Swenson 2011). The spatial distribution of functional diversity may provide a key to unraveling the relative contribution of these mechanisms in producing regional-scale species diversity and how this influences local scale diversity (Swenson and Enquist 2007; Swenson et al. 2012). For example, a classic question in ecology has been whether strong inter-specific interactions and limiting similarity (MacArthur and Levins 1967) results in a greater range of phenotypic space being occupied as species richness increases (Ricklefs and O'Rourke 1975; Ricklefs and Travis 1980). Alternatively, abiotic