Slope Aspect Modulates Alpine Meadow Spatial Homogeneity Through Niche Differences, Not Fitness Differences.

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 hypothesize 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 the 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.