Functional diversity of North American broad-leaved trees is codetermined by past and current environmental factors

Modern and historical climate are known to codetermine broad-scale species richness and composition patterns in temperate regions. Nonetheless, is poorly understood the extent to which these effects individually or in combination determine functional diversity, many studies have simply assumed equilibrium between current climate and functional diversity. We estimated functional richness (F-Rich) and dispersion (F-Disp) of North American broad-leaved trees by combining distribution and trait information. Then, we determined if contemporary water-energy availability, current topographic variability, historical climatic stability, and lagged immigration from glacial refugia co-determined the functional diversity of North American broad-leaved trees. We did this by assessing the directionality, magnitude, and relative importance of various contemporary and historical environmental factors know to affect species diversity. Contrasts were performed across all North America (Mexico, United States, and Canada), and areas within this region that were glaciated or ice-free during the Last Glacial Maximum (similar to 21 000 yr ago). F-Rich and F-Disp showed distinct geographic patterns that are strongly associated with both contemporary environmental conditions and glacial-interglacial climate change. Model averaged regression coefficients and AIC-based variable relative importance estimates show that contemporary productivity (F-Rich-w(AIC): 1.0; F-Disp-w(AIC): 1.0), annual precipitation (F-Rich-w(AIC): 0.81; F-Disp-w(AIC): 1.0), and accessibility to glacial refugia (F-Rich-w(AIC): 0.92; F-Disp-w(AIC): 1.0) have the strongest associations to FRich and FDisp. Furthermore, the association of functional diversity with topographic heterogeneity showed steeper slopes in ice-free regions. These findings suggest that, contrary to the expectation climate-diversity equilibrium, functional diversity of North America broad-leaved trees is codetermined by current climate and lagged immigration from glacial refugia.