Grassland Productivity in Arid Central Asia Depends on the Greening Rate Rather Than the Growing Season Length

Climate change has induced substantial impact on the gross primary productivity (GPP) of terrestrial ecosystems by affecting vegetation phenology. Nevertheless, it remains unclear which among the mean rates of grass greening (RG), yellowing (RY), and the length of growing season (LOS) exhibit stronger explanatory power for GPP variations, and how RG and RY affect GPP variations under warming scenarios. Here, we explored the relationship between RG, RY, LOS, and GPP in arid Central Asia (ACA) from 1982 to 2019, elucidating the response mechanisms of RG, RY, and GPP to the mean temperature (TMP), vapor pressure deficit (VPD), precipitation (PRE), and soil moisture (SM). The results showed that the multi -year average length of greening (LG) in ACA was 22.7 days shorter than that of yellowing (LY) and the multi -year average GPP during LG (GPP l g ) was 38.28 g C m 2 d -1 more than that of during LY (GPP ly ). RG and RY were positively correlated with GPP lg and GPP ly , although the degree of correlation between RG and GPP lg was higher than that between RY and GPP ly . Increases in RG and RY contributed to an increase in GPP lg (55.44 % of annual GPP) and GPP ly (35.44 % of annual GPP). The correlation between RG and GPP lg was the strongest (0.49), followed by RY and GPP ly (0.33), and LOS and GPP was the weakest (0.21). TMP, VPD, PRE, and SM primarily affected GPP by influencing RG and RY, rather than direct effects. The positive effects of TMP during LG (TMP lg ), PRE during LG (PRE lg ), and SM during LG (SM l g ) facilitated increases in RG and GPP l g , and higher VPD during LY (VPD ly ) and lower PRE during LY (PRE ly ) accelerated increases in RY. Our study elucidated the impact of vegetation growth rate on GPP, thus providing an alternate method of quantifying the relationship between vegetation phenology and GPP.