Elevational gradient of species and phylogenetic diversity of plant communities in the Nyangchu River Valley, China.

By investigating seed plants at an altitude range of 3800-5100 m in the Nyangchu River Valley, we examined plant community structure and the distribution patterns of species diversity and phylogenetic diversity along the altitudinal gradient, as well as the environmental factors driving these patterns. The results showed that there were three main community types, including the Poa litwinowiana+Elymus brevipes+Stipa purpurea community, the Carex parvula+Blysmus sinocompressus+Argentina anserina community, and the Sophora moorcroftiana+Pennisetum flaccidum community. Species richness varied from 5 to 28, averaging 15.18±5.04. The phylogenetic diversity index (PD) ranged from 621.45 to 2315.96, averaging 1441.44±348.83. The species and phylogenetic diversity monotonically increased with altitude. In 65% of the plots, both the net relatedness index (NRI) and net nearest taxa index (NTI) were greater than zero. The main soil drivers of species and phylogenetic diversity were total nitrogen and hydrolyzable nitrogen, with contribution rates of 23.4% and 22.8%, respectively. In terms of climatic drivers, potential evapotranspiration and mean annual temperature negatively correlated with species and phylogenetic diversity, while moisture negatively correlated with NRI and NTI. Altitude and latitude were the most significant geographical drivers of species richness and phylogenetic diversity. Results of the Mantel tests confirmed that species β-diversity was significantly positively correlated with geographic distance and environmental factors such as altitude, while phylogenetic β-diversity showed comparatively weaker correlations with geographic distance. Community assembly in the Nyangchu River Valley was jointly driven by geographical isolation and environmental heterogeneity. The phylogenetic structure had a clear clustering. Higher phylogenetic diversity was found in mid to high-altitude zones, with environmental heterogeneity showing higher explanatory power. Increasing drought induced by climate change would threaten plant diversity in this region.