Evaluating Shrub Encroachment in Alpine Grasslands on the Tibetan Plateau Under Different Stocking Rates

Grassland ecosystems provide critical ecological functions. In recent years, the alpine grasslands of the Tibetan Plateau have experienced severe degradation accompanied by shrub encroachment, with overgrazing likely being the primary driving force. In this study, we focused on the alpine grasslands of the plateau and used a terrestrial ecosystem process-based model that incorporates grazing dynamics to simulate regional net primary productivity (NPP) changes under varying stocking rate scenarios. The aim was to examine how stocking rate alters the timing, magnitude, and spatial patterns of shrub encroachment. Our results show that increasing stocking rate induces a pronounced divergence in NPP patterns, characterized by persistent declines in grassland NPP and concurrent increases in shrub NPP. Shrub encroachment occurs primarily in areas where grassland NPP has already substantially declined. Across the stocking rate gradient, the primary response time of shrub NPP consistently exceeds that of grassland NPP, indicating a temporal lag of shrub responses relative to grassland degradation. In terms of magnitude, NPP change increases most rapidly within stocking rate of 1.5–4.0 sheep unit·ha⁻¹, but shows diminishing additional change at stocking rate greater than 9.5 sheep unit·ha⁻¹, indicating that the net long-term NPP response stabilizes under higher grazing pressure. Even under extreme grazing condition, the maximum increase in shrub NPP compensates for only about 50% of the pregrazing grassland NPP. Spatially, this transition advanced progressively from the northwest to the southeast with increasing stocking rates, a pattern closely linked to regional hydrothermal gradients. These findings indicate that shrub encroachment is a passive response to grassland degradation rather than the result of enhanced shrub competitiveness.