The mechanism of species coexistence and diversity maintenance along aspects in the northeast of the Qinghai–Tibetan plateau

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To examine the role of plant functional traits and phylogenetic relationships in predicting plant community species coexistence and diversity maintenance, we measured 73 species and six functional traits along a slope aspect gradient on the Qinghai–Tibetan plateau. We calculated the net relatedness index (NRI), the nearest taxon index (NTI), phylogenetic diversity (PD), functional diversity, and analysed phylogenetic signals. The results show that the species richness, plant composition, and PD changed substantially from northern to southern aspects, and the phylogenetic structure of the community changed from clustering to over-dispersion. Weak phylogenetic signals in plant height, leaf nitrogen content, and leaf potassium content were recorded. We conclude that the influencing factor(s) of species coexistence on northern and north-western aspects is limiting similarity (interspecific competition), whereas on southern and south-western aspects, habitat filtering (environmental effect) is predominant. On western aspects, the influencing factors are driven by three processes: limiting similarity, habitat filtration, and random processes. Results suggest that niche processes (including habitat filtration and limiting similarity) are the main mechanisms for species coexistence and diversity maintenance on aspects of the alpine meadow in the northeast of the Qinghai–Tibetan plateau, while random processes appear at the transitional zone (the western aspect in our study) between aspects.