Shrub Expansion Mediates the Relationship Between Alpine Meadow Biodiversity and Ecosystem Multifunctionality

The expansion of alpine meadow shrubs driven by global change is profoundly affecting biodiversity and ecosystem function, but how it shapes the dynamic relationship between biodiversity and ecosystem multifunctionality (EMF) remains unclear, which limits understanding of the ecological succession law on the Tibetan Plateau. In this study, the relationship between plant species richness (PSR), functional richness (FRic), phylogenetic diversity (PD), soil biodiversity (ciliate, fungal, and bacterial richness), and EMF was investigated in an alpine meadow under the shrub expansion on the Tibetan Plateau using multi‐threshold, moving window analysis, and structural equation modeling methods. The results showed that (1) shrub expansion significantly reduced biodiversity and EMF levels, and biodiversity and ecosystem functioning differed significantly ( p < 0.05) among different levels of shrub expansion; (2) both plant and soil biodiversity showed significant effects on EMF under shrub expansion ( p < 0.01), and plant diversity was the more prominent driver, effectively predicting EMF over a broad threshold range (29%–98%) with a peak effect of 0.38; (3) the biodiversity‐EMF relationship shifted along the shrub expansion gradient, with a turning point at approximately 61% shrub cover. Plant diversity was more strongly associated with EMF under low shrub cover (< 58%), whereas soil biodiversity became relatively more important for explaining EMF under high shrub cover (> 65%). This study quantifies the dynamic relationship between biodiversity and EMF during the expansion of alpine meadow shrubs, providing a new basis for understanding the response mechanisms of alpine ecosystems to global change.