Precipitation and anthropogenic activities regulate the changes of NDVI in Zhegucuo Valley on the southern Tibetan Plateau

Whether climate change or anthropogenic activities play a more pivotal role in regulating vegetation growth on the Tibetan Plateau is still controversial. A better understanding on grassland changes at a fine scale may provide important guidance for local government policy and grassland management. Using two of the most reliable satellite NDVI products (MODIS NDVI and SPOT NDVI), we evaluated the dynamic of grasslands in the Zhegucuo valley on the southern Tibetan Plateau from 2000 to 2020, and analyzed its driving factors and relative influences of climate change and anthropogenic activities. Here, the key indicators of climate change were assumed to be precipitation and temperature. The main results were: (1) the grassland NDVI in Zhegucuo valley did not reflect a significant temporal change during the last 21 years. The variation of precipitation during the early growing season (GSP) resembled that of NDVI, and the GSP was positively correlated with NDVI. At the pixel level, the partial correlation analysis showed that 37.79% of the pixels depicted a positive relationship between GSP and NDVI, while 11.32% of the pixels showed a negative relationship between temperature during the early growing season (GST) and NDVI. (2) In view of the spatial distribution, the areas mainly controlled by GSP were generally distributed in the southern part, while those affected by GST stood in the eastern part, mainly around the Zhegucuo lake where most population in Cuomei County settled down. (3) Decreasing NDVI trends were mainly occurred in alpine steppe at lower elevations rather than alpine meadow at higher elevations. (4) The residual trend (RESTREND) analysis further indicated that the anthropogenic activities played a more pivotal role in regulating the annual changes of NDVI rather than climate factors in this area. Future studies should pay more attention on climate extremes rather than the simple temporal trends. Also, the influence of human activities on alpine grassland needs to be accessed and fully considered in future sustainable management.