Slope aspect modulates the spatial homogeneity of plant communities by altering species coexistence mechanisms. Modern coexistence theory posits that stable coexistence depends on the balance between niche differences (stabilizing mechanisms) and fitness differences (competitive asymmetry). How topographic factors like slope aspect modulate these mechanisms to influence community spatial patterns in alpine ecosystems remains unclear. We hypothesis that slope aspect regulates the spatial homogeneity of alpine meadow communities primarily by altering niche differences, and that this mechanism is most pronounced on environmentally stressful aspects. We surveyed community across four slope aspects (east, south, west, north) in Maqin County, Qinghai-Tibetan Plateau. We developed proxy indices to quantify niche difference (from phylogenetic diversity and community integrity) and fitness difference (from species evenness). Community spatial homogeneity was assessed as the inverse of spatial variability in biomass and species composition. Slope aspect significantly influenced niche differences but not fitness differences. Accordingly, homogeneity in species composition varied with aspect. Critically, variance partitioning revealed that niche difference was the primary regulator of both biomass and species composition homogeneity, but this regulatory role was significant only on the environmentally stressful west-facing slope. Our findings demonstrate that slope aspect shapes community assembly via niche-based filtering. The predominance of niche difference in driving spatial homogeneity specifically under the stringent conditions of west-facing slopes underscores the context-dependency of coexistence mechanisms and highlights the critical role of stabilizing mechanisms in maintaining community structure across topographic gradients.

Plant diversity and dominant species both influence the stability of soil aggregates, but whether their underlying mechanisms are internally consistent remains unclear. We assessed how the plant diversity and dominant species influence the soil aggregates of alpine meadows between sunny and shady slopes in southeastern Qinghai, China. The plant diversity and dominant species did not differ significantly between the slope aspects nor did the soil aggregates. Both the dry- and wet-sieved macroaggregates >0.25 mm on the sunny slopes were driven by the dominant species. However, they were driven by plant richness on the shady slopes, and the effect of plant richness depended on the dominant species. This study revealed that the diversity effect is regulated by the dominant species, which provides a new perspective on the long-standing unresolved issues of how to address the relationship between diversity and dominant species in the conservation of diversity and maintenance of ecosystem functions.

Alpine meadow degradation, usually involving decreased soil N and patchy landscapes, is challenging for natural restoration. However, the mechanism underlying plant species coexistence during degradation is unclear. In this study, we evaluated plant N niche complementarity in degraded alpine meadows by a 15N-labeling (15NO3-, 15NH4+ and 15N-glycine) experiment. At the community level, the degraded alpine meadow showed larger root and all plant 15N concentrations and preferred glycine over NO3- compared with the undegraded alpine meadow. At the species level, dominant species in the undegraded alpine meadow consistently preferred NO3-. For the degraded alpine meadow, generalist species, common to both meadows, showed diverse preferences, while unique species generally preferred glycine, among which the uneven distribution could reduce glycine competition. We observed that differentiation in N sources and the uneven distribution of unique species may explain the stability of degraded alpine meadows. Our results suggested that plant spatial distribution could be powerful for community stability and emphasized the importance of considering fine-scale perspectives in studies of niche theory. This study have important implication for restoration of degraded alpine meadows.