Fertilization and herbivore exclusion promote more deterministic processes in plant community assembly on the Qinghai-Tibet Plateau.

Fertilization and herbivore exclusion are two key forces influencing community structure and species richness in grassland ecosystems. However, the extent to which these factors influence the relative contributions of deterministic and stochastic processes in community assembly remains poorly understood. In this study, we conducted a five-year field experiment that manipulated nutrient availability and herbivore exclusion in an alpine meadow on the eastern Qinghai-Tibet Plateau. The results showed that fertilization, herbivore exclusion, and their combination significantly decreased species richness and drove Raup-Crick dissimilarity close to - 1 relative to the control. In the final year of the experiment, these treatments drove a distinct directional shift in community composition, diverging significantly from the control. Furthermore, all treatments significantly increased community-weighted mean (CWM) height and specific leaf area (SLA), and these shifts in fast resource-acquisition traits were significantly correlated with the intensification of deterministic processes (i.e., Raup-Crick dissimilarity approaching - 1). Consequently, we conclude that in natural alpine meadows (control), community assembly is primarily driven by stochastic processes. In contrast, the experimental treatments diminished the influence of stochasticity, establishing the dominance of deterministic processes through environmental filtering. This shift in assembly patterns was largely mediated by plant functional traits. Because this filtering strongly selected for dominant species possessing fast resource-acquisition traits and exceptional competitive abilities for light, it ultimately led to community homogenization and a decline in species richness. Notably, the combined treatment did not yield a stronger deterministic effect than either treatment alone, indicating a lack of an additive effect.