Abstract:

Aims Community assembly rules are poorly understood, especially in subalpine areas. Plant communities are thought to be organized from a regional species pool through environmental filtering and competition. Environmental filtering will result in the formation of groups of species with similar traits, while competition will limit similarity of coexisting species. Environmental filtering and competition can also affect community phylogenetic structure when species niches are phylogenetically conservative. Our object was to uncover how these processes affected community assembly and their responses to grazing.
Methods We sampled plant communities in six sites across a grazing gradient and calculated functional group evenness to test effects of environmental filtering and competition. For investigating community phylogenetic structure, we calculated nearest taxon index and net relatedness index using phylocom software. We examined community phylogenetic structures and functional group evenness under a grazing gradient using null model and phylogenetic indexes.
Important findings We found strong nonrandom patterns in species richness distributions among functional groups in non-grazed plots, and functional group evenness significantly decreased with increased grazing pressure. There was a significant phylogenetic cluster in intensively grazing communities and this gradually became dispersion toward primary forest. As a filtering effect, grazing increased species richness of a few lineages and decreased others. This process resulted in community phylogenetic cluster or reductions in functional group evenness. Functional group evenness increasing and phylogenetic over-dispersion might stem from competition, which could limit similarity of coexisting species. Results indicated that both habitat filter and competition simultaneously shaped community structure. Findings suggest that grazing affected functional group evenness and phylogenetic structure through changing the balance between environmental filtering and competition in communities. The negative relationship between functional group evenness and species richness suggests that competition might suppress plant diversity in local communities. Results support that species functional divergence and their competition have important roles in community assembly.

Key words: assembly rules, functional group composition, grazing gradient, phylogenetic structure, plant community