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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.

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To analyse the variation of species abundance distribution (SAD) patterns with scale, and consider its formation mechanism and ecological process in alpine meadow in the eastern Qinghai–Tibet Plateau of China, study areas were established, nine permanent plots were randomly selected, and 72 quadrats were investigated. Four diversity indices were selected, including species richness, the Margalef, Shannon–Wiener and Simpson indices and relative importance values to quantify community assembly and biodiversity. Species accumulation curves were used to describe the SADs of the alpine meadow, and three model types used to fit the SADs. The results showed the following: (1) the Cyperaceae families were the respective dominance family in alpine meadow in the eastern Qinghai–Tibet Plateau of China; (2) at sampling scales from 0.25 to 2 m2, the total species richness and diversity indices increased; (3) total SADs were scale-dependent; the statistical model could not fit the SADs; the neutral theory model could explain SADs patterns at smaller spatial scales (0.25–1 m2), and the niche process gradually replaced the neutral process in importance and became the main mechanism for determining SADs with the increase of sampling scales (1.25–2 m2); (4) for common species, the niche model (geometric series model) could fit the SAD of common species at different sampling scales, which indicates that the niche process is the main mechanism to determine the SAD of common species; and (5) for rare species, with the change of sampling scale, the fitting model of rare species is different, which indicates that the SAD of rare species is affected by a variety of uncertain factors. In general, niche differentiation processes had a major effect on the biodiversity patterns of alpine meadow in Qinghai–Tibet Plateau.