Decreased Sensitivity of Grassland Spring Phenology to Temperature on the Tibetan Plateau

Spring phenology is a critical indicator to characterize vegetation dynamics and their responses to climate change. Spring phenology on the Tibetan Plateau (TP) has received extensive attentions as it has experienced one of the most rapid warmings. Warming-induced advancement of spring phenology has been revealed by many studies, however, the underlying mechanisms remain obscure. In this article, we derived the start of growing season (SOS) from the satellite solar-induced chlorophyll fluorescence (SIF) and investigated the spatial and temporal variations of SOS over grasslands on the TP during 2001–2020. The temperature sensitivity (St) of SOS was then analyzed, i.e., the slope of a linear regression between the advanced SOS and preseason air temperature. Results showed an average advanced trend of 0.29 days per decade of SOS, although not statistically significant. Spatially, grasslands in eastern TP showed an earlier trend of SOS whilst those in western TP showed a later trend of SOS. The spatial distribution of St was much more affected by precipitation and air temperature, i.e., a 1 mm decrease of precipitation and 1 °C warming incur a decrease in St of 0.02 and 0.54 day/°C, respectively. Temporally, St showed a significant decrease with an average speed of 0.14 day/°C per year during 2001–2020, and the climate controllers show a high spatial heterogeneity. These findings improved our understanding of grasslands spring phenology responses to warming and help us clarify future global water and energy cycles.