Response of vegetation productivity and resilience to extreme climate events under a grazing pressure gradient on the Qinghai-Tibetan Plateau.

With the intensification of climate change and human activities, the frequency and intensity of extreme climate events (ECEs), along with grazing pressure, have increased significantly. Understanding how vegetation responds to ECEs and grazing is essential for maintaining ecosystem stability. However, the quantitative effects of ECEs on vegetation dynamics under varying grazing pressures, and their underlying mechanisms, remain poorly understood. This study evaluated vegetation dynamics on the Qinghai-Tibet Plateau (QTP) using two key indicators: net primary productivity (NPP) and vegetation resilience (VR). We quantified VR based on the theory of 'critical slowing down,' which posits that recovery from minor perturbations slows as a system loses resilience. This was measured by calculating the temporal autocorrelation (TAC) in the normalized difference vegetation index (NDVI) residuals. We systematically evaluated how vegetation dynamics, characterized by NPP and VR, respond to extreme climate events (ECEs) under varying grazing intensities, focusing on spatial response patterns and nonlinear threshold behaviors. Results showed that from 2005 to 2019, NPP exhibited a slight increase at a rate of 3.62 gC·m^-2·yr^-1, while VR declined significantly at a rate of 0.029 ·10yr^-1. Compared to extreme temperature events (ETEs), extreme precipitation events (EPEs) had a stronger influence on both NPP and VR, with contribution rates of 65.68 % and 75.94 %, respectively. Our findings reveal a critical threshold where moderate grazing pressure (peaking at 2.4) maximizes vegetation's adaptive response to climate extremes. Crucially, exceeding this threshold diminishes resilience and amplifies climate-driven damages. These results provide quantitative, actionable guidance for sustainable ecosystem management, demonstrating that policies enforcing moderate grazing are essential for enhancing the climate resilience of alpine grasslands.