Abstract: Aims Understanding how environmental factors and human activity drive plant community assembly remains a major challenge in community ecology. Two opposing processes, namely deterministic environmental filtering and nondeterministic elements such as interspecies competition, can be quantified by analyzing trait distributions in the community-assembly process.
Methods We examined the trait-mediated effects of environmental filtering and stochastic process and the distribution over time of nine traits related to vegetative growth, regenerative phase, dispersal capability, decomposition and interspecific competition in plant communities along a degradation gradient in the Xilin River Basin, Inner Mongolia, China. We analyzed the turnover of environmental trait filtering and the divergence/convergence of different traits along the degradation gradient.
Important findings Our results showed the following. (i) The patterns of trait distribution and filtering were strongly dependent upon the degradation gradient and trait types. Most traits were filtered intensely in degraded grasslands. (ii) Plants with two different strategies showed contrasting trait-distribution patterns. The traits that were related to biological matter cycling showed divergent patterns in highly degraded grasslands, while convergent patterns along the overall gradient were demonstrated in traits associated with other plant strategies. This suggests that the coexistence of multiple 'biological matter cycling-related niches' might be a basic structuring pattern of plant communities in our study area. (iii) The simultaneous occurrence of strong filtering and divergence revealed that environmental filtering does not necessarily prevent competition, and that different traits show different signatures.

Key words: community assembly, convergence, degradation, grazing intensity, plant traits, typical grassland