Spatiotemporal dynamics of alpine grassland phenology on the Tibetan Plateau

Vegetation phenology is an important sensor for ecosystem feedbacks on climate change. The study of phenological changes of alpine grassland vegetation on the Tibetan Plateau is of great scientific significance, to reveal the response mechanisms of alpine ecosystems to global climate change. In this study, we selected the 16-day maximum synthetic product MOD13A1 of the MODIS Vegetation Index (VI) from 2001 to 2015. Based on the TIMESAT 3.2 platform, the threshold method was used to extract the vegetation phenological period of the alpine grassland on the Tibetan Plateau, including the start of growth season (SOG), end of growth season (EOG), and length of growth season (LOG). We analyzed the temporal and spatial variations of the vegetation phenology and its driving forces in an alpine grassland of the Tibetan Plateau from 2001 to 2015. The conclusions are summarized as follows: 1) From southeast to northwest, with the deterioration of water and heat conditions and the uplift of topography, the SOG became gradually more delayed, from the 110 days to 170 days. The EOG became gradually more advanced, from the 300 days to 260 days. Finally, the LOG became gradually shorter, from the 300 days to 260 days. However, major differences were noted for the different grassland types. 2) The interannual variation of alpine grasslands showed a tendency for the SOG and EOG to advance, but the LOG increased on the Tibetan Plateau. 3) Altitude is the main factor that affects the phenological heterogeneity of the different grassland types on the Qinghai-Tibet Plateau. Phenology is closely related to altitude between 3 500 m and 5 000 m. With increasing elevation, the SOG of different grassland types became gradually more delayed, the EOG became gradually more advanced, and the LOG became gradually shorter. However, below 3 500 m, the alpine grassland phenology fluctuated greatly with elevation, with no obvious regularity.