Using functional trait diversity to infer community assembly mechanisms: an exclosure experiment as an example

The Qinghai-Tibetan Plateau has a mean altitude exceeding 4000 m and covers about 2.5 million km(2). More than 60% of this area is alpine grassland. Exclosures have been widely used in this region to study the sustainable use of grassland resources. We used patterns of functional trait diversity to infer the effects of exclosures on community assembly in alpine meadows. We studied functional diversity using five traits under grazing and three enclosed (exclosure) plots (3, 8, and 18 years old) in an alpine meadow on the Qinghai-Tibetan Plateau. We quantified the strength of the community assembly processes by comparing the observed functional trait diversity with a null model that assumes random community assembly. We found evidence for deterministic assembly processes for plant communities in exclosures. The changes in CWM of the five traits from grazing land to 18-year exclosure indicated that environmental filtering occurred due to the exclosures. Multivariate functional diversity (MFDis and MPDses), and functional diversity of individual traits, including that of leaf area, leaf weight and aboveground biomass (FDis of leaf area, leaf weight, and aboveground biomass), increased gradually from grazing land to the 18-year exclosure, and the values of the 18-year exclosure were significantly greater than null expectation. This can be interpreted to indicate that exclosures resulted in greater competitive interaction between species. These results suggest that the effect of exclosures on community assembly is more deterministic than stochastic in this meadow.