Climatic controls on spring onset of the Tibetan Plateau grasslands from 1982 to 2008

Understanding environmental controls on vegetation spring onset (SO) in the Tibetan Plateau (TP) is crucial to diagnosing regional ecosystem responses to climate change. We investigated environmental controls on the SO of the TP grasslands using satellite vegetation index (VI) from the 3rd Global Inventory Modeling and Mapping Studies (GIMMS3g) product, with in situ air temperature (T_a) and precipitation (Prcp) measurement records from 1982 to 2008. The SO was determined using a dynamic threshold method based on a 25% threshold of seasonal VI amplitude. We find that SO shows overall close associations with spring T_a, but is also subject to regulation from spring precipitation. In relatively dry but increasingly wetting (0.50 mm· year^-1, p < 0.10) grasslands (mean spring Prcp = 22.8 mm; T_a = 3.27 °C), more precipitation tends to advance SO (0.146 day· mm^-1, p = 0.150) before the mid-1990s, but delays SO (0.110 day·mm^-1, p = 0.108) over the latter record attributed to lower solar radiation and cooler temperatures associated with Prcp increases in recent years. In contrast, in relatively humid TP grasslands (73.0 mm; 3.51 °C), more precipitation delays SO (0.036 day·mm^-1, p = 0.165) despite regional warming (0.045 °C^-1 year1, p < 0.05); the SO also shows a delaying response to a standardized drought index (mean R = 0.266), indicating a low energy constraint to vegetation onset. Our results highlight the importance of surface moisture status in regulating the phenological response of alpine grasslands to climate warming.