Changes in assembly rules along a stress gradient from open dry grasslands to wetlands

A central issue of community ecology is finding rules that explain the composition and abundance of coexisting species. Nowadays two main processes, environmental filtering and limiting similarity, are thought to play the main roles in structuring communities. Their relative importance under different environmental conditions, however, is still not properly clarified. We studied the strength and the effect of environmental filtering (causing convergence) and limiting similarity (causing divergence) in 137 sample plots along an extremely long environmental gradient ranging from open sand grasslands to highly productive marshes, using a trait-based approach. The main environmental gradient (i.e. productivity) was characterized by the Normalized Difference Vegetation Index, an indicator of above-ground live biomass. Cover of the plant species was estimated visually. Values of 11 plant traits were collected from field measurements and data bases. Mean and dispersion of the trait values of the plots were quantified by community-weighted means and Rao's quadratic entropy. Trait convergence and divergence were tested by randomization tests, followed by the study of changes in effect size along the productivity gradient by fitting generalized additive mixed models (GAMM). For vegetative traits we found mainly convergence, indicating the filtering effect of environmental constraints, while traits related to regeneration showed divergence. The strength of convergence in vegetative traits generally decreased as productivity grew, indicating that while under harsh conditions environmental constraints strongly limit the possible trait values, under more benign conditions various water and nutrient use strategies are adaptable. At high productivity, the strength of divergence in regenerative traits decreased. Since the larger diversity of vegetative traits found here reduces competition, the importance of diverse reproductive strategy is probably lower.Synthesis. Our results partly support the stress-dominance hypothesis, but reveal that assembly rules are more complex. The relative importance of environmental filtering and limiting similarity depends on the trait and on the environmental conditions of the habitat. Traits related to resource use are generally limited by environmental filtering, and this restriction is weakening as conditions become more favourable, while traits related to regeneration are constrained by limiting similarity and are more diverse under harsh conditions.