Drought offsets the vegetation greenness-induced gross primary productivity from 1982 to 2018 in China

Recent studies have shown that climate stress may cause asynchronous changes between greenness and productivity, but the mechanism how greenness and productivity respond to drought remains unclear. The significant greening of vegetation in China has attracted worldwide attention; nevertheless, it is unclear whether large-scale greening will translate into productivity in the context of recent droughts. In this study, we used the vegetation index dataset to extract long-term trends in vegetation greenness and productivity from 1982 to 2018, and then chose the multi-scale Standardized Precipitation Evapotranspiration Index (SPEI) index to measure how vegetation greenness and productivity change in response to drought. The results show that significant vegetation greening did not cause a simultaneous increase in productivity during 1982-2018. Vegetation greenness and productivity trends were relatively consistent before 2000 and diverged after 2000, especially during the prolonged drought of 2000-2009. The opposite trend between greenness and productivity in humid ecosystems during drought periods mainly caused this trend difference. In humid ecosystems, short-term drought promotes vegetation greening, while long-term drought has no significant impact on greenness. However, the inhibitory effect of drought on productivity deepens with an increase in drought duration. Therefore, prolonged droughts offset vegetation greenness-induced gross primary productivity. We also found that prolonged drought causes a significant decrease in greenness but has little effect on productivity in arid ecosystems, possibly because vegetation productivity is more sensitive to soil water than atmospheric drought. Our results highlight the complexity of vegetation growth in the context of climate change. Furthermore, it is crucial to pay attention to the impact of climate stress on regional carbon sequestration functions, considering the significant carbon sink potential of forest areas in southern China.