Grassland phenological dynamics and its response to driving factors on the Qinghai-Tibet Plateau

The phenological dynamics of vegetation are the result of multiple factors, especially in the alpine grassland ecosystem of the Qinghai-Tibet Plateau (QTP), which is sensitive to external disturbances. In this study, we used the Normalization Difference Vegetation Index (NDVI) data generated by MODIS from 2000 to 2020 and the dynamic threshold method to extract the vegetation phenology in the QTP, and explored the dynamic change of grassland phenology and its response to driving factors. The results showed that: 1) From 2000 to 2020, the start of the growing season on the QTP showed an earlier trend from northwest to southeast, with an annual advance of 0.19 d, and the overall trend was delayed by the same amount at the end of the growing season. The length of the grassland growing season increased gradually from northwest to southeast. 2) Wind speed and yearly maximum five-day precipitation (RX5day) were the main factors influencing the grassland length of the growing season (LOS), and the pairwise interaction between the factors was stronger than the influence of a single factor on vegetation phenology. RX5day and wind speed both interacted with temperature. 3) The regression coefficients of grassland LOS with wind speed and Rx5day showed obvious spatial heterogeneity. The grassland LOS in the northern Tibetan Plateau increased with wind speed and decreased with RX5day, while that in the Qinghai Plateau and South Tibet Valley became shorter with the increase of wind speed. The grassland LOS increased with the increase of RX5day in the Sichuan-Tibet alpine valley area. This study provides a scientific basis for ecosystem protection and the development of animal husbandry in the study area.