Heterogeneous impacts of human activities and climate change on transformed vegetation dynamics on the Qinghai-Tibet Plateau

In the context of intensifying global environmental change, elucidating vegetation dynamics and their driving mechanisms is vital for sustainable ecosystem management. The Qinghai-Tibet Plateau (QTP), a region known for its sensitivity and vulnerability, exhibits a high degree of responsiveness to climate change and human activities. The region's pronounced spatiotemporal heterogeneity renders it an optimal area for investigating vegetation changes and their driving mechanisms. However, existing research predominantly emphasizes natural factors, with insufficient systematic analysis of human activities, thereby constraining a comprehensive understanding of driving mechanisms. This study utilizes MODIS NDVI data to systematically analyze GNDVI (the average NDVI during the growing season) trends and their driving mechanisms across the QTP from 2000 to 2018, integrating natural factors (temperature, precipitation, potential evapotranspiration, snow depth, elevation, slope) and human factors (roads, population, grazing intensity) from multi-scale and vegetation-type perspectives. Results reveal a significant increasing trend in GNDVI (p < 0.05) across the QTP, with pronounced improvements in the northeast and degradation in specific regions of the southwest and southeast. From 2000 to 2018, 79.19% of QTP vegetation exhibited improvement (39.52% significantly), while 14.28% experienced degradation (2.78% significantly). During the study period, the QTP climate exhibited a warming and moistening trend, which generally benefited vegetation growth. The impacts of natural and human factors on vegetation changes vary significantly across different spatial regions. Population density and grazing intensity have obvious threshold effects on vegetation dynamics: when population density exceeds 17 pop/km², their effects on vegetation change rate approach marginal effects, while grazing intensity exceeds 250 SU/km², resulting in a significant decrease in GNDVI change rate. Notably, grazing and tourism activities near roads and lakes negatively impacted GNDVI in the southwest and southeast, partially offsetting the positive effects of climate change and contributing to degradation. Based on these findings, the study recommends establishing a long-term investment mechanism for ecosystem protection, implementing differentiated regional management strategies, and enhancing regulatory oversight of human activities. Specifically, strict controls on grazing and tourism development in ecologically sensitive areas are necessary to mitigate their impacts on fragile ecosystems. This study constructs a framework that integrates the spatiotemporal heterogeneity of natural and human factors, overcoming the limitations of traditional methods. It advances the theoretical understanding of the driving mechanisms behind vegetation dynamics in alpine ecosystems and provides a scientific foundation for formulating differentiated ecological protection policies and sustainable management strategies.