Greater tree species richness in eastern North America compared to Europe is coupled to denser, more clustered functional trait space filling, not to trait space expansion

Aim
Determine if differences in species richness between currently climatically similar regions correlate to dissimilarities in functional diversity.
Location
Europe and eastern North America.
Time period
Neogene and Quaternary.
Major taxa studied
Temperate trees of Europe and eastern North America.
Methods
Using classic functional traits defining distinct ecological strategies, we described the trait spaces of European and eastern North American tree floras. The trait space of each region was described based on the occupied area, species accumulation pattern, and the trait‐space size, dispersion and clustering. We then evaluated if the species richness differences between these two climatically similar regions correlate to dissimilarities in both occupied areas and species accumulation patterns; as well as differences in the trait‐space size, dispersion and clustering.
Results
Differences in species richness between climatically similar regions do not necessarily result in functional dissimilarities. Regardless of eastern North American species having a larger trait‐space, occupied areas and species accumulation patterns converged. Although in both regions species clumped towards the trait‐space centroid, we observed statistically significant differences between Europe and eastern North America in the dispersion and clustering but not in size of the trait space. Moreover, traits from species in genera still present in eastern North America but lost from Europe during the Neogene and Quaternary were fully contained within the currently occupied European trait space.
Main conclusions
Positional convergence between European and eastern North American trait spaces suggests that this species richness anomaly does fully translate to the functional space. Our results suggest that species accumulation within a region occurs within a climatically restricted trait space, and not via trait‐space expansion. Moreover, the consistent aggregation of species towards the trait‐space centre aligns with the idea of a directional section towards a generalized morphology, which might provide the best way to interact with a broad array of environmental conditions.