Precipitation and Soil Nitrogen as Key Drivers of Species and Phylogenetic Diversity Across Growth Forms in Picea purpurea Forests of the Eastern Qinghai–Tibet Plateau

Picea purpurea forests are a fundamental part of the coniferous forests of the Qinghai–Tibet Plateau. These forests support an extremely rich diversity of organisms while providing crucial ecological functions. Despite their importance, the species and phylogenetic diversity (PD) of these forests remain understudied. The extent to which different plant growth forms in the forest are influenced by the environment varies according to differences in the evolutionary history and characteristics of these taxa. Here, we investigate the variations in all, woody, and herbaceous plant species diversity (SD) and PD in the eastern Qinghai–Tibet Plateau's Picea purpurea forests, along with the associated environmental factors influencing these patterns. Our results show that both SD and PD exhibit similar patterns across all, woody, and herbaceous plants. Notably, mean annual precipitation, total nitrogen, and altitude were identified as the primary factors accounting for the majority of variance in SD and PD within plant communities. Specifically, we found positive correlations between SD and PD with soil total nitrogen content, while negative correlations were observed with mean annual precipitation and altitude. Furthermore, we find that the influence of environmental variables on all plants was found to be more pronounced compared to woody and especially herbaceous plants in terms of both SD and PD. Environmental factors had a direct effect on SD and PD in all woody and herbaceous plants. However, the strength of these effects varied. Importantly, our results indicate that environmental factors influence SD independently of PD. Our findings suggest that precipitation and total soil nitrogen were the most influential environmental factors affecting plant diversity across different growth forms in Picea purpurea communities. However, inferring SD from PD remains a challenging task. These findings of our study will provide guidance for forest biodiversity conservation and sustainable forest management in the Qinghai–Tibet Plateau.