Climatic Controls on Spring Onset of the Tibetan Plateau Grasslands from 1982 to 2008

U nderstanding environm ental controls on vegetation spring onset (SO) in the Tibetan Plateau (TP) is crucial to diagnosing regional ecosystem responses to climate change. We investigated environm ental controls on the SO of the TP grasslands using satellite vegetation index (VI) from the 3rd Global Inventory M odeling and M apping Studies (GlMMSSg) product, w ith in situ air tem perature (Ta) and precipitation (Prep) m easurem ent records from 1982 to 2008. The SO w as determ ined using a dynamic threshold m ethod based on a 25% threshold of seasonal VI am plitude. We find that SO shows overall close associations w ith spring Ta, bu t is also subject to regulation from spring precipitation. In relatively dry bu t increasingly w etting (0.50 m m -year^^, p < 0.10) grasslands (mean spring Prep = 22.8 mm; Ta = —3.27 °C), more precipitation tends to advance SO (—0.146 day-m m ^^, p = 0.150) before the mid-1990s, but delays SO (0.110 daym m ^^, p = 0.108) over the latter record attributed to lower solar radiation and cooler tem peratures associated w ith Prep increases in recent years. In contrast, in relatively hum id TP grasslands (73.0 mm; —3.51 °C), m ore precipitation delays SO (0.036 daym m ^^, p = 0.165) despite regional w arm ing (0.045 °C-year^^, 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. O ur results highlight the im portance of surface moisture status in regulating the phenological response of alpine grasslands to climate warm ing.