Topographic effects on vegetation phenology in response to climate change on the southern slope of Qilian Mountains, Northwest China.

The southern slope of the Qilian Mountains is a typical mountain ecosystem. To understand the vegetation phenological response mechanisms under the interaction between complex terrain and climate change is of significance in revealing ecological adaptability laws. Based on MODIS NDVI data from 2002 to 2020, combined with topographic and meteorological datasets, we used correlation analysis and structural equation modeling to quantitatively assess the role of topography in mediating vegetation phenological responses to climate change. The results showed that the start of the growing season, end of the growing season, and length of the growing season in the study area advanced, delayed, and extended at rates of 0.33, 0.19, and 0.51 d·a-1, respectively, with spatial differentiation exhibiting a distinct northwest-southeast gradient. Regional vegetation phenology displayed spatial heterogeneity in its responses to the changes in temperature and precipitation, modulated by topographic factors and jointly governed by diverse environmental conditions and hydrothermal regimes. Elevation had the most pronounced influence on vegetation phenology, followed by aspect. Elevation and aspect exerted cumulative effects on vegetation phenology through meteorological factors, while the influence of slope on vegetation phenology through meteorological factors was offset by each other.