Does variance in ecologically-important traits reflect evolutionary divergence? The answer to this question greatly influences the interpretation of recent work showing that phylogenetic diversity (PD) strongly predicts biodiversity effects on ecosystem functioning. If yes, PD is a good proxy for functional diversity (FD) and can represent niche breadth. If no, FD should predict ecosystem functioning better than PD. We addressed this question in 29 grassland plant experiments previously used for a meta-analysis by Cadotte et al. (2008, PNAS). Grassland biodiversity-ecosystem function experiments represent the best test case for a comparison of FD and PD, since detailed trait data are available for many species and aboveground net primary production as an ecosystem functioning is well-understood.
Results/Conclusions

Functional trait variation was largely unrelated to phylogenetic distances between species, and the resulting FD values only weakly correlated with PD. Few significant differences arise in trait variation at the level of family or higher, so that with the exception of N-fixation, none of functional traits examined here clearly reflected phylogenetic divergence. Despite these differences, FD and PD predicted biodiversity effects across all experiments with similar strength, including in subsets excluding plots with legumes and focusing on fertilization experiments. The similar predictive power of FD and PD is surprising because the two indices are based on entirely different information, ecophysiological and morphological traits for FD versus gene sequences for PD. These results suggest that PD captures variation in important traits not included in the FD analysis. Two- and three-trait combinations of the five traits used here (percent leaf nitrogen, height, specific root length, leaf mass per unit area, and N-fixation) resulted in the FD values with the greatest predictive power. FD can be the best predictor of the effect of biodiversity on ecosystem functioning, when trait data are available and the ecosystem function is well-understood, such as in grassland biodiversity-ecosystem functioning studies.