Species sorting dominates plant metacommunity structure in coastal dunes

It has long been thought that environmental factors determine plant community assembly, but it is now increasingly argued that geographic spatial processes such as dispersal may also matter. Notably, the metacommunity framework considers local communities to be linked by dispersal and different theories hereunder assign varying importance to dispersal limitation and local environmental species sorting. At present the relative importance of these factors across habitats, geographic regions, and spatial scales remains unclear. The present study assessed the relative importance of species sorting by the local environment and broader-scale geographic spatial processes for local plant species composition using a data set of 3924 plots from coastal dunes across a large region (Denmark). We used ordination to identify the main gradients in species composition, and Linear Mixed-Effects modelling (LME) to estimate the relative importance of local environment and multi-scale random geographic factors as determinants of floristic gradients. In addition, we assessed the dependence of species composition on local environment and geographic distance using Mantel tests. The LME analyses found local species sorting to be the dominant determinant in this metacommunity system, with soil moisture, pH, and fertility requirement patterns explaining ≥77% of the compositional gradients, while geographic factors accounted for ≤2%. Partial Mantel tests confirmed this finding, with 31.6% of the variation in species dissimilarity explained by environmental species sorting and just 1.6% by geographical distance. The apparently limited impact of dispersal limitation or other geographic spatial processes may reflect high habitat continuity and efficient dispersal by strong winds and ocean currents in the Danish coastal-dune metacommunity system.