Successional chronosequences provide a unique opportunity to study the effects of multiple ecological processes on plant community assembly. Using a series of 0.5 × 0.5 m2 plots (n = 30) from five successional sub‐alpine meadow plant communities (ages 3, 5, 9, 12, and undisturbed) in the Qinghai‐Tibetan Plateau, we investigated whether community assembly is stochastic or deterministic for species and functional traits. We tested directional change in species composition, functional trait composition, and then functional trait diversity measured by Rao's quadratic entropy for four traits – plant height, leaf dry matter content, specific leaf area, and seed mass – along two comparable successional chronosequences. We then evaluated the importance of species interactions, habitat filtering and stochasticity by comparing with random communities and partitioning the environmental and spatial components of Rao's quadratic entropy. We found no directional change in species composition, but clear directionality in functional trait composition. None of the abiotic environmental variables (except P) showed linear change with successional age, but soil moisture and nitrogen were positively related to functional diversity within meadows. Functional trait diversity increased significantly with the increase in successional age. Comparison with random communities showed a significant shift from trait divergence in early stages of succession (3‐ and 5‐yr) to convergence in the later stages of succession 9‐, 12‐yr and undisturbed). The relative importance of abiotic variables and spatial structure for functional trait diversity changed in a predictable manner with successional age. Stochasticity at the species level may indicate dispersal limitation, but deterministic effects on functional trait distributions show the role of both habitat effects and biotic interactions.

SummaryWithin a community, species may germinate at different times so as to mitigate competition and to take advantage of different aspects of the seasonal environment (temporal niche differentiation). We illustrated a hypothesis of the combined effects of abiotic and biotic competitive factors on germination timing and the subsequent upscale effects on community assembly.We estimated the germination timing (GT) for 476 angiosperm species of the eastern Tibetan Plateau grasslands under two light treatments in the field: high (i.e. natural) light and low light. We also measured the shift in germination timing (SGT) across treatments for all species. Furthermore, we used phylogenetic comparative methods to test if GT and SGT were associated with seed mass, an important factor in competitive interactions.We found a significant positive correlation between GT and seed mass in both light treatments. Additionally, small seeds (early germinating seeds) tended to germinate later and large seeds (late germinating seeds) tended to germinate earlier under low light vs high light conditions.Low light availability can reduce temporal niche differentiation by increasing the overlap in germination time between small and large seeds. In turn, reduced temporal niche differentiation may increase competition in the process of community assembly.

Although trait-based processes of community assembly during secondary succession invokes multiple factors that ultimately determine the presence or absence of a species, little is known regarding the impacts of functional traits on species abundance in successional plant communities. Here in species-rich subalpine secondary successional meadows of the Qinghai-Tibetan Plateau, we measured photosynthesis rate and leaf proline content that are related to plant growth and abiotic stress resistance, respectively, and seed germination rate that is closely correlated with plant germination strategy to test their influence on species abundance during succession. We used a linear mixed effects model framework to examine the shifts in trait-abundance relationships and the correlations among these three traits in successional communities. We observed significant shifts in trait-abundance relationships during succession, e.g., abundant species in early-successional meadows exhibited relatively high photosynthesis rates and leaf proline content, but showed low seed germination rates, whereas the converse were true in late successional communities. However, the correlations among the three traits were insignificant in most meadow communities. Our results show that functional traits associated with plant growth, stress resistance, and reproduction impose strong influence on species abundance during secondary subalpine meadow succession in the Qinghai-Tibetan Plateau.

Variation in abiotic conditions along altitudinal gradients may sort plant species from regional species pools according to their seed mass. With increasing elevation, seed mass is expected to be either larger for its advantage during seedling establishment in stressful high‐elevation environments (‘stress‐tolerance’ mechanism), or smaller owing to energy constraints. Using a large trait database involving 1355 species from the northeastern verge of the Tibetan Plateau, we found that, overall, these two opposing mechanisms balanced out one another, resulting in non‐significant seed mass–elevation relationship across all species after controlling for phylogeny. At the same time, we found that the influence of energy constraints on seed mass was indirect and mediated by the variation in plant height. Moreover, our results revealed a mass‐dependent seed mass variation along elevation gradients: with increasing elevation small seeds tended to increase ( supporting stress‐tolerance mechanism) but large seeds tended to decrease (supporting energy‐constraints mechanism). Finally, the seed mass–elevation relationships were significantly different among species with different life forms or different dispersal modes, but statistically similar for anemophilous and entomophilous species. This implies that life‐history cycle, resource allocation pattern and availability of dispersals agents, rather than pollination efficiency, can affect the responses of seed mass to elevation. Together our results suggest that a comprehensive perspective is necessary when interpreting geographic distribution of even a single trait. Synthesis With increasing elevation, seed mass may be either larger for its advantage during seedling establishment (‘stress‐tolerance’ force), or smaller owing to energy constraints. Our paper shows some novel and importance results in the seed mass–elevation relationship in a northeastern Tibetan flora. Firstly, these two opposing forces operate simultaneously but overall balance out one another. Secondly, the balance tends to shift toward increased energy‐constraints (stress‐tolerance) with the increase (decreased) in average seed mass. Thirdly, energy constraints on seed mass is indirect and mediated by the variation in plant height. Finally, plant resource allocation pattern, life‐history cycle, and availability of dispersal agents can affect the responses of seed mass to elevation.

Summary Soil functions and processes are driven by complex microbial interactions. It is, therefore, critical to understand the coexistence patterns of soil microbiota, especially in fragile alpine ecosystems. We identified biogeographic patterns in the network‐level topological features of the soil microbial co‐occurrence network in the Tibetan alpine grasslands, based on high‐throughput sequencing. We verified that soil pH was the most important environmental variable for predicting network‐level topological features of soil microbial co‐occurrence networks. Associations among soil microbiota were enhanced with increasing pH (5.17–8.92), and the network was the most stable at neutral pH. Moreover, node‐level topological features suggested that the archaeal operational taxonomic units, compared with bacterial operational taxonomic units, hold a central role in the co‐occurrence network. Network‐level topological features revealed closer connections among soil microbiota in the steppe ecosystem than in the meadow ecosystem. Therefore, our study demonstrated that soil pH served as a critical environmental filter that influenced the potential associations and ecological signature of soil microbiota in the Tibetan alpine grasslands. These findings provide a new perspective on the distinct biogeographic patterns of co‐occurrence networks, to explore the ecological role of soil microbiota and thus help manage soil bacterial and archaeal communities for provisioning alpine ecosystem services.