Assembly rules in a resource gradient: competition and abiotic filtering determine the structuring of plant communities in stressful environments

Abstract

The relative importance of different community assembly mechanisms varies depending onthe environment. According to the stress-dominance hypothesis (SDH), assembly mecha-nisms range from strong abiotic filtering to competition as the environment becomes morefavourable. Most evidence for the SDH comes from studies in gradients of conditions (i.e.abiotic environmental factors that influence the functioning of organisms but are not con-sumed by them). However, we hypothesized that in resource gradients, competitionincreases as abiotic filtering becomes stronger. To test our hypothesis, we set up eight plotsat different sites along an abiotic severity gradient in the Brazilian semi-arid region (BSAR).In each plot, we identified and measured each woody plant species found, and we recorded11 functional traits of the main species, dividing the traits into alpha (competition effects)and beta (abiotic filtering effects). We investigated the presence of phylogenetic signal inthe traits, the community phylogenetic and phenotypic patterns, and associated the variationin these patterns with the availability of water and soil nutrients. We found phylogenetic sig-nal for most (91%) of the traits analysed. The phylogenetic patterns varied from clustered instressful sites to random or overdispersed in favourable sites, and we concluded that thesephylogenetic patterns were the result of historical processes influencing community assem-bly in different environments in the BSAR. In general, the phenotypic patterns varied fromclustered at the most stressful end to random at less stressful sites. Our results show that inresource gradients, any restriction of the resource (hydric or edaphic) intensifies abiotic fil-tering and, at the same time, increases the competitive hierarchy among species. On theother hand, stochastic processes seem to have a stronger influence under more favourableabiotic conditions, where abiotic filtering and competition are weaker. Thus, we concludethat the SDH is not supported in resource gradients