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

Nutrients 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 grassland on the eastern Qinghai–Tibet Plateau. The results showed that, compared to the control, species richness significantly decreased under fertilization, herbivore exclusion, and their combined treatment, with the greatest decline in species richness observed in the final year of the experiment. Furthermore, stochastic processes predominated in the control. In contrast, the fertilization, herbivore exclusion, and fertilization + herbivore exclusion treatments were primarily driven by deterministic processes. The community assembly processes of graminoids under each treatment were largely consistent with the overall community changes. The results showed that herbivore grazing can strengthen stochastic processes in plant community assembly. Regardless of the herbivores, fertilization reduced the importance of stochasticity and strengthened the deterministic environmental filter process that homogenized communities through the selection of graminoid species with greater competitive ability for light.