Research On Trait-based Community Assembly

The community assembly mechanism is not only one of the core topics of ecological research,but also the key theoretical basis of conservation biology.Current analysis approaches based on functional traits and phylogenetic dimensions determine the main ecological processes of dominant community construction by judging the comparison of phylogenetic and trait distribution patterns with random null models.In addition,based on the methods of phylogeny and functional traits,we can also resolve the phylogenetic signal patterns and biodiversity levels of the community,and then to verify the ecological processes of community assembly from different aspects.In this paper,we conducted our research at the subtropical mountain evergreen and deciduous broad-leaved mixed forest in the Badagongshan National Nature Reserve of Hunan province.Within the 25 hectares of forest dynamic plots,we resolved the intra-species variability of leaf traits and their sources,the constructed phylogenetic and functional trees to interpret the trade-off relationship between each functional trait at both species-level and community-level,and their response to the environment.We also analyzed the phylogenetic signals and evolution patterns of the main foliar functional traits to reveal the community assembly mechanism based on the distribution of traits and phylogenetic structure at the local scale,so as to provide a scientific basis for the study of the biodiversity pattern and maintenance mechanism of evergreen and deciduous broad-leaved mixed forests in subtropical mountains.The main results of this study were as follows:(1)On the plot-scale,inter-specific variability was the main source of trait variation,but intra-specific trait variation also accounts for a large proportion,and there were significant differences in the sources of variation of different plant functional traits.The difference in leaf area(LA)originated from species to species and was mainly limited by genetic factors.Compared to leaf area(SLA)and leaf phosphorus content(LPC),the contribution of intra-specific variation was greater than the contribution of inter-specific variation because of the environment filtering might cause species with similar functional traits to coexist in local habitats,and these coexisting species would show some divergence of certain ecological strategies through intra-specific variation,thereby reducing the intensity of biological competition,which also showed that environmental filtering and limiting similarity played an important role in the process of community assembly.In addition,in the subtropical mountain evergreen and deciduous broad-leaved mixed forest,the specific leaf area(SLA),leaf area(LA),and leaf nitrogen content(LPC)of evergreen trees were significantly lower than those of deciduous tree species,while the leaf thickness of evergreen tree species(LT)was significantly higher than deciduous tree species.Generally,deciduous tree species apted to adopt an ecological strategy of high growth rate,while evergreen plants adopted a relatively conservative strategy to compete for scarce resources.Evergreen tree species and deciduous tree species co-existed by using different ecological strategies.(2)At the species-level,there was a clear synergy or trade-off relationship between the main leaf functional traits in the Badagongshan forest plot.Among them,the specific leaf area(SLA)and leaf area(LA),leaf nitrogen and phosphorus content(LNC and LPC)had a significant positive correlation between the three traits,and the leaf carbon content(LCC)was significantly negatively correlated;while the leaf thickness(LT)was significantly negatively correlated with specific leaf area(SLA)and leaf nitrogen and phosphorus content(LPC).The relationship between leaf traits reflected the life strategy of the leaf,and was a trade-off between structural composition and nutrient distribution of metabolic components.In addition,phylogenetic independent contrast analysis(PICs)of functional traits showed that the relationship between trait pairs was consistent in the original data and in the phylogenetic control,but the slopes of the Model ? regression after phylogenetic correction were significantly different.It showed that the functional traits of the species in the study plot were directly or indirectly affected by phylogenetic evolution,and the traits showed different degrees of convergence or divergent evolution.(3)Similar to the results at the species level,there are different degrees of trade-offs between the CWM(Community-weighted means)of different leaf traits in the BDGS plot at the community level.At the 20m×20m scale,LT,LCC and SLA showed significant negative correlation.LA,LPC,LKC and SLA showed extremely significant positive correlation.LNC,LPC,SLA and LCC showed significant negative correlation.LT,LCC and LPC showed a very significant negative correlation,while SLA,LA,LNC and LPC were significantly positive correlation.The combined of these functional traits made the plant better adapt to the changing environment.The results showed that the elevation and slope of the topographic factors had significant impact on the distribution pattern of community functional trait.Soil conditions also affected the distribution of traits at the community level by affecting the dynamic balance of element content in plants.(4)On the local scale,the niche differentiation promoted the coexistence of different life-form plants,and environmental filtering promoted the coexistence of relative species with similar functional traits at all study scales(10m,20m,50m and 100m),showing functional traits and lineage aggregation,revealing the stabilization mechanism of community structure.Niche differentiation(competitive exclusion,etc.)played a limiting role on smaller scale,leading to competitive exclusion of similar functional trait species,reflecting the equalization mechanism of community assembly.In short,the aggregate or divergent distribution of traits and phylogenetic structure indicated that niche differences played an important role in BDGS plot,while stochastic was relatively less important.At the same time,the phylogenetic structure and the distribution pattern of traits changed with scales.Except for the leaf carbon content(LCC),the trait distributions were gradually dispersed with the increase of scale,while the phylogenetic and functional structure of other traits increased with scale,and were more convergent.