Disturbance and stress gradients result in distinct taxonomic, functional and phylogenetic diversity patterns in a subtropical riparian tree community

Abstract

Questions: What are the key environmental variables that influence the structure of riparian forests? Do disturbance (flooding) and stress (soil fertility) gradients result in functionally and phylogenetically different riparian forests? Do patterns of functional and phylogenetic diversity follow patterns of taxonomic diversity? Location: Subtropical riparian tree community, Turvo State Park, Rio Grande do Sul state, southern Brazil. Methods: We collected data on nine functional traits and assembled a phylogeny for all tree species found in a sample with 100 plots. We then modelled species richness and evenness, and functional and phylogenetic richness, evenness and divergence as functions of flooding frequency and soil fertility. Next, we used a three-table ordination method to look for associations between environmental conditions, the spatial locations of sampling sites and the distributions of species, trait values and lineages. We conducted these analyses for the entire area and separately for three habitats with different flooding frequencies. Finally, we tested for phylogenetic signal in functional traits. Results: Species richness and evenness were highest under intermediate flooding frequency. In contrast, functional richness was negatively correlated with soil fertility, while phylogenetic richness increased with both flooding frequency and soil fertility (with phylogenetic evenness following a similar trend). Functional evenness was lowest under intermediate flooding frequency, and functional and phylogenetic divergence correlated in a complex quadratic way with both flooding and soil fertility. Functional traits were generally conserved in the phylogeny, although pioneer establishment strategy and resprouting capacity were convergent and strongly associated with frequent flooding. In contrast, the distribution of lineages was not associated with either environmental variables or site location. Conclusions: There was a mismatch between taxonomic, functional and phylogenetic diversity patterns along disturbance and stress gradients. This mismatch likely occurs because functional traits involved in species sorting were either convergent or conserved in the phylogeny and because disturbance and stress gradients seem to differently affect each of the assessed diversity components. Even though disturbance seems to be the key driver of the structure of riparian tree communities, stress gradients (soil fertility) are the key predictors of important diversity components, especially functional richness. Thus, disturbance and stress gradients result in functionally and phylogenetically different riparian tree communities, leading to differences that cannot be predicted from differences in species richness and composition alone.