Vegetation Growth Status and Topographic Effects in Frozen Soil Regions on the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau (QTP), which is known as Earth's "Third Pole", is a driver of global climate change, and long-term monitoring of QTP vegetation can reveal changes attributable to climatic and human influences. Previous research monitoring vegetation on the QTP focused primarily on spatiotemporal variations of vegetation indices, while few studies have considered fractional vegetation cover (FVC) in relation to topographic and frozen soil factors. We used MODIS-EVI, digital elevation models, and frozen soil data to investigate topographic effects on vegetation growth status in different soil types on the QTP during 2000-2020. (1) FVC showed a trend of increase during 2000-2020, and the FVC on the QTP decreased from the southeast to the northwest in spatial distribution. FVC in permafrost regions was the lowest, followed by seasonal frozen soil areas; FVC in unfrozen areas was the highest. (2) With increasing elevation, FVC of permafrost, seasonal frozen, and unfrozen soil areas showed downward trends for each aspect. In seasonal frozen soil areas, at elevation <= 4000 m (>4000 m), FVC of sunny (shady) slopes was greater than that of shady (sunny) slopes. In permafrost regions, except at elevations of 3000-4000 m, FVC of shady slopes was greater than that of sunny slopes. In unfrozen soil areas, at elevation >4000 m, FVC of sunny slopes was obviously greater than that of shady slopes. (3) With increasing slope, FVC in seasonal frozen and permafrost soil (unfrozen soil) regions showed a trend of increase (decrease). In seasonal frozen soil areas, FVC of sunny (shady) slopes was greater than that of shady (sunny) slopes on slopes <= 6 degrees (>6 degrees). In permafrost regions, FVC of sunny slopes was less than that of shady slopes. With increasing slope, the influence of aspect became more obvious. In unfrozen soil areas, FVC of sunny slopes was slightly greater than that of shady slopes. Topographic effects especially the elevation and slope effects might significantly affect the spatiotemporal variations of vegetation growth status in frozen soil regions on the QTP.