Co-occurrence patterns and abiotic stress in sand-dune communities: Their relationship varies with spatial scale and the stress estimator

The analysis of co-occurrence patterns between plant species is being increasingly employed to study biotic interactions at the community level. We investigated changes in such patterns along two different abiotic stress gradients in eight sand-dune plant communities of the western coast of Portugal. We sampled a total of 72 transects consisting on 25 1 m2 quadrats each, and we calculated the standardized effect size for each transect (SES), i.e. the differences between the observed co-occurrence patterns and simulated patterns for a random co-occurrence. The relationships between the average SES values for each site and the abiotic stress gradients, defined by the north-south geographic gradient (a surrogate of the climatic stress) and total plant cover (a surrogate of productivity), were analyzed. The factors driving co-occurrence patterns at smaller spatial scale were also investigated by a stepwise multiple regression analysis. Average SES values were significantly higher than zero in five of the studied sites, indicating lower species co-occurrence than expected by chance. These results suggest that the studied sand-dune communities studied are structured mainly by competitive interactions. At the site scale, SES values were not related to plant cover or the location along the north-south gradient. However, at the transect scale, SES values were inversely explained by different abiotic factors (the geographic position along the north-south gradient and the plant cover). Our results indicate that the magnitude and the shifts in co-occurrence patterns along environmental gradients varied with the spatial scale and the surrogate of abiotic stress considered.