Spatiotemporal variations of the start of thermal growing season for grassland on the Qinghai-Tibetan Plateau during 1961–2014

The start of thermal growing season (STGS) is an important indicator for climate change effects on regional plant growth and development. This study comprehensively investigated and compared the spatiotemporal variations in STGS at 0 °C (STGS_0), 5 °C (STGS_5), and 10 °C (STGS_10) thresholds for grassland on the Qinghai-Tibetan Plateau (QTP) during 1961–2014. Although elevation was the predominant influencing factor of the spatial variations of STGSs, the effect of latitude should not be ignored at the low-elevation regions, especially for the STGS with a low-temperature threshold (e.g., STGS_0). With the decrease of temperature thresholds, the effect of elevation became weaker, while the effect of latitude became stronger. Significant advancing trends were observed in all the three STGSs, with greater advancing rate for STGS_0 (0.23 days·year−1) than that of STGS_5 (0.15 days·year−1) and STGS_10 (0.16 days·year−1). More obvious advancing trends were found after 1980, which coincided with more rapid climate warming. The advancing trends weakened after 1998 when climate warming hiatus occurred. Since positive and negative impacts may be simultaneously induced by the advanced STGSs, more observations are still needed to analyze their impacts on the growth and development of alpine grassland on the QTP.