Are plant communities converging in functional traits as well as species composition?

Background/Question/Methods

Biotic homogenization - the tendency for communities to converge in species composition – appears to be occurring across many ecosystems, creating management challenges. Given that species’ presences and abundances in any given community often reflect species traits, we expect that community convergence in species composition to be mirrored by convergence in trait composition ("functional homogenization"). To test this conjecture, we examined changes in beta functional diversity through time across multiple Wisconsin forest sites, expecting declines similar to those already documented for species beta diversity. In particular, we used ecological surveys of the same Wisconsin forest sites from both the 1950s and the 2000s. We measured 14 traits on 240 species and calculate the beta functional diversity during both periods. Two of the methods we used to estimate beta functional diversity are based on pairwise comparisons of sites while the third one represents a translation of the additive partitioning of diversity in the context of functional diversity. We compared these beta functional diversity values using paired t-tests between periods and appropriate null models.

Results/Conclusions

The additive partitioning of functional diversity in Wisconsin forests suggests that regional functional diversity is mostly driven by local patterns in α functional diversity, beta functional diversity being low. All three approaches suggest that beta functional diversity did not decrease significantly over the last 50 years despite significant declines in species beta diversity. However, local functional diversity within these communities (FD_α) has decreased significantly over the last 50 years.

Together these results demonstrate that classic biotic homogenization involving convergence in the species composition of communities does not necessarily imply a parallel convergence in functional traits. Rather, these metrics measure different aspects of community change. The high fraction of alpha diversity supports the idea that niche partitioning may act to sustain local trait diversity. Comparing these approaches across wider sets of communities and different beta functional diversity metrics should provide a fuller picture of how trait variation is shifting within and among plant communities.