Southwest China was affected by two extreme droughts in the autumn to spring of 2012-2013 and the winter to summer of 2020-2021. These droughts caused water depletion, crop damage, and socio-economic disruption. However, little is known about the accurate representation of the two drought events and the responses of vegetation to the droughts. We used multiple vegetation indices and multi-source climate data to quantify the spatiotemporal variations of the two events. We assessed the different responses of vegetation greenness in Southwest China to the two drought events to determine the underlying mechanisms. Vegetation greenness in Southwest China showed different responses to the two events due to differences in the early hydrothermal conditions. The 2012-2013 autumn-spring drought suppressed vegetation growth in Southwest China, with a total decrease of 0.17 (31.7 %) in the normalized difference vegetation index relative to the baseline conditions in the early stage of the drought. The decrease in precipitation and soil water depletion in late summer 2012 aggravated the decrease in vegetation greenness from winter 2012 to spring 2013. By contrast, during the winter-summer drought in 2020-2021, there was an increase of 0.22 (52.3 %) in the normalized difference vegetation index in January-March 2021 relative to the baseline conditions. Adequate precipitation and soil water in the late summer to autumn of 2020 compensated for water loss due to the extreme drought, and, concurrently, more downward solar radiation and warmer conditions linked to less cloudiness contributed to vegetation greening in spring 2021. These results show that early hydrothermal conditions have a vital role in the different responses of vegetation greenness to extreme drought events. These results will help in water management and ecosystem protection in the current background of more frequent extreme weather and climate events resulting from the global climate crisis.

Southwest China, characterized by its climate sensitivity and ecological fragility, is experiencing heightened vulnerability to recurrent extreme drought due to climate change. However, not all drought events impart identical damage effects on terrestrial ecosystems. The ecosystem's response to drought becomes intricately diverse and is remain poorly understood in this region. Here, we comprehensively distinguish the inhibiting and promoting effects of cumulative drought on vegetation productivity and the modulation of hydrothermal conditions using various remote-sensing data and meteorological observations. Our results show that cumulative drought exerts pronounced inhibiting and promoting effects on vegetation productivity in 57.3% and 25.0% of vegetated area in southwest China, respectively, and shows large discrepancies in different geomorphic settings and different vegetation types. Particularly, vegetation productivity is more easily inhibited by cumulative drought in the karst landform with inadequate water-holding capacity. The croplands suffer the most inhibited effects and hardly benefit from the cumulative drought because that most of croplands are mainly distributed on the karst landform. Productivity for much of the grasslands is most strongly promoted by cumulative drought with relatively low temperature over western Sichuan and northwestern Yunnan, where with rich solar radiation can compensate energy for vegetation growth and less precipitation prevents prolonged waterlogging of plant roots owing to soil water saturation. Forests have well-developed deep root systems and can draw deep groundwater to compensate for water shortages during prolonged droughts, making them the least inhibited by cumulative drought. Savannas are the second weakly inhibited by cumulative drought, ranking below forests owing to their intermediate ecological character, straddling the boundaries between grasslands and forests. Overall, this study significantly advances our knowledge of the effects of cumulative drought on vegetation productivity and the role of hydrothermal conditions, providing valuable insights for efforts to mitigate cumulative drought risk under changing climate conditions.