Are differences in functional diversity among plant communities on Mediterranean coastal dunes driven by their phylogenetic history?

Question: Variability in species function is often studied by using phylogenetic relatedness as a surrogate for functional similarity between species, rather than by measuring functional traits directly. The phylogenetic-based method is far less data-intensive than trait-based methods. However, to what extent community-level variability in species function is driven by phylogenetic history has rarely been explored in terms of composition and diversity. In this paper we test this empirically by asking: do differences in the functional composition and diversity of species assemblages (i.e. plots) along an environmental gradient mirror differences in phylogenetic structure? Location: Coastal dune communities of the Tyrrhenian coast of Italy. Methods: We calculated fuzzy-weighted mean trait values and the Rao index for dominant species in 405 plots using 16 functional traits that were measured in the field or from other sources. Based on a phylogenetic distance matrix among species obtained from an aged phylogenetic supertree, we applied matrix correlation tested against appropriate null models to calculate how much of the plot-to-plot variability in functional composition (measured through fuzzy weighting) and diversity (measured with the Rao index) is predicted by the corresponding phylogenetic metrics. Results: At the species pool level there was evidence for a phylogenetic signal in trait variation. Furthermore, we found that differences in species functional diversity among plots were closely related to their phylogenetic variability, but this was not true for functional composition. Conclusions: The results show that even when there is evidence of phylogenetic trait conservatism at the species pool level, phylogeny may be unable to capture all aspects of functional community structure. This emphasizes the need for caution when interpreting measures of phylogenetic community structure as proxies of functional community structure.