Research On Plant Community Assembly Rules Across Alpine Meadow-forest Gradients In Western Sichuan

Both of plant functional traits and phylogenetic relationships may play important roles in species coexistence and community assembly. Classic niche theory suggests that communities are organized from species with limiting ranges in functional traits under environmental filtering effects which can affect species distribution. On the other hand, complete competitors can not coexist. Competition can result in niche partition or character replacement and therefore limiting similarity of coexisting species. By contrast, the neutral theory believes that coexisting species can converge toward a functional equivalent species under an environment condition. To explore community assembly rules, this study investigated the distribution patterns of plant traits and phylogenetic structure in communities across two alpine vegetation gradients. The results showed:(1) Based on clustering analysis, Sorensen similarity indices were higher of similar vegetation types. The woody plant composition was significantly related to herbs composition (P＜0.001).(2) Species richness was significantly associated with vegetation types (P＜0.001), but not soil nutrient (P＞0.05). Community phylogenetic structure and functional group evenness were also not significantly associated with soil available nutrients (P> 0.05).(3) The functional group evenness significantly decreased with grazing press in grazing gradient (P＜0.001) and showed a "U" shape in the timberline ecotone. Totally, the trends in functional group evenness increased from meadows to forests in two gradients.(4) A detrended correspondence analysis using eight traits showed that the congeners were scattered more closely in the DCA diagram. This indicated that species traits were evolutionary conservative among congeners.(5)The ranges in some plant traits showed significant reductions in some plots compared to null model (P＜0.05). This supported that habitat filtering could constrain plant species distribution which were traits-mediated.(6) With constraining the range in traits of null model, the standard deviation of nearest-neighborhood distance in seven out of eight traits showed significantly larger than null model in some plots (P＜0.05) which mean plant traits were uneven spaced in these communities(P<0.05). This result was inconsistent with principle of limiting similarity and supported the theory of Scheffer& van Nes (2006) which suggested that competition would result in a lumpy pattern of species niches.(7) The community phylogenetic structure changed greatly across two gradients. Nearest related index (NRI) increased from coniferous forest to meadow which indicated phylogenetic structure changed from over-dispersed to under-dispersed. NRI in up-part timberline ecotone were significantly higher than null model (P＜0.05) that means cluster in phylogenetic structure. The nearest related indices in deciduous and coniferous forests on grazing gradient were significantly lower than null model (P<0.05) which indicated phylogenetic structures were over-dispersed in less disturbed communities.(8) The functional group evenness was negatively related to species richness (P＜0.001) and NRI (P＜0.005), but positively related to phylogenetic diversity (PD) (P＜0.05). This suggested that competition might depress the species richness in several communities.The results are consistent with Weiher & Keddy (1995) who suggested habitat filtering and competition affect community assembly from two directions. As habitat filtering factors, grazing and environmental press could reduce functional group evenness and the ranges in plant trait which resulted in phylogenetic structure under-dispersed, while competition increased the functional group evenness and ranges in traits which made phylogenetic structure over-dispersed. The uneven spaced traits in our result suggest that niche theory and neutral theory can be bridged together. Species from the same functional groups can be seen as functional equivalent while species from different functional groups can coexist by niche partition.