Disentangling Topographic Constraints and the Climatic and Anthropogenic Pathways Shaping Vegetation Greenness in the Southeastern Qinghai–Xizang Plateau

Vegetation greenness reflects ecosystem responses to climate change and human activities. This study quantified spatiotemporal variation in the normalized difference vegetation index (NDVI) in Nyingchi, a representative alpine-gorge region of the Southeastern Qinghai–Xizang Plateau, using the MODIS NDVI for 2004–2024 and environmental and human-activity datasets with variable-specific temporal coverage. Theil–Sen–Mann–Kendall trend analysis, Breaks for Additive Season and Trend, the optimal parameter-based geographical detector, and partial least squares structural equation modeling were used to characterize temporal dynamics and spatial associations. The NDVI was higher in summer and autumn and in river valleys and low- to mid-elevation areas, but lower in spring and winter and at high elevations. The annual mean NDVI increased significantly by 0.0014 yr−1, and detected breakpoints were concentrated during 2008–2019. Temperature and elevation had the highest single-factor explanatory power for spatial NDVI variation, while several elevation-containing joint stratifications yielded higher q values. In the path model, the terrain–NDVI coefficient was dominant, although the terrain score was strongly controlled by elevation. The results distinguish long-term greening from model-conditional spatial associations and provide regional evidence for vegetation monitoring in complex alpine-gorge landscapes.