Patterns of species and phylogenetic diversity in Picea purpurea forests under different levels of disturbance on the northeastern Qinghai-Tibetan Plateau

Abstract

Picea purpurea is a endemic species to China and a main component of subalpine dark coniferous forests on the northeastern Qinghai-Tibetan Plateau (QTP). In this study, twenty-four plots (20 × 30 m) were established in the natural distribution area of P. purpurea and categorized into three disturbance levels (mild, moderate, and severe). The correlations among species richness (SR), phylogenetic diversity (PD) and net relatedness index (NRI) were investigated though ordinary least squares (OLS) regression and the differences in the SR and PD among disturbance levels were analyzed via one-way analysis of variance. The results showed that the SR was positively correlated with the PD (R2 =0.935, p < 0.001) and that the differences in the SR and PD among the different disturbance levels were significant (p < 0.001), with increasing the degree of disturbance, the SR increased firstly and then decreased. The effects of environmental factors (including disturbances) on the PD and phylogenetic structure were analysed with SEM (structural equation modeling) and showed that the PD of all plants and woody plants was affected directly by actual evapotranspiration, slope position, soil total nitrogen and available phosphorus jointly, indirect affected by disturbance with altering soil chemistry, the PD of herbaceous plants was affected mainly by microhabitat features (i.e., slope position). The overall phylogenetic structures of all plants and woody plants showed trends of overdispersion, and environmental factors had little explanatory power with respect to the NRI values, indicating that the phylogenetic structures of these plant groups were not dominated by environmental filtering. But in herbaceous plants, environmental factors had strong explanatory power with respect to the NRI, and environmental filtering played an important role in controlling the phylogenetic structure of herbaceous plants.