Disturbance drives phylogenetic community structure in coastal dune vegetation

Question Coastal dunes are a globally distributed ecosystem characterized by roughly linear geography, strong habitat connectivity, and strong small-scale internal gradients in disturbance and stress. Disturbance, stress and competition are often considered the main ecological forces driving plant community assembly, but their relative importance in coastal dunes is not well elucidated. Phylogenetic community structure reflects the processes controlling community assembly across ecological and evolutionary time scales. We investigate for the first time how disturbance, competition and stress gradients shape phylogenetic community structure in coastal dune plant assemblages. Location Europe's largest coastal dune area (1450km2), west coast of Jutland, Denmark. Methods As a measure of phylogenetic community structure we calculated net relatedness index (NRI) for 2702 dune plots containing a total of 483 vascular plant species. To assess the assumption that closely related species resemble each other more than at random, we tested for phylogenetic signal in plant adaptations to stress, competition and disturbance, as represented by Grime's C-S-R values. We then used regression analyses of NRI against mean Grime C-S-R plot values to assess the influence of disturbance, competition and stress gradients on phylogenetic community structure. Finally, we assessed if these relationships reflected anthropogenic factors by computing species' anthropogenic affinity and then regressing mean anthropogenic affinity for each plot on NRI and Grime's C-S-R plot values. Results Grime's C-S-R values all showed a phylogenetic signal. Phylogenetic community structure was predominantly clustered at both local and regional scales. This clustering was related to disturbance, as high clustering was consistently associated with high dominance of disturbance-adapted species (high Grime's R scores). Importantly, a strong correlation between these and mean anthropogenic affinity suggests that the disturbance effect on phylogenetic community structure was at least partially anthropogenic. Conclusion Dune plant phylogenetic community structure is shaped by the phylogenetic signal in plant adaptations to the main ecological gradients in coastal dunes, with filtering by human-associated disturbance playing the strongest role. Our study supports previous findings of a linkage between anthropogenic disturbance and phylogenetic clustering of species, and suggests that this may be a more general pattern.