Water surplus as a constraint on vegetation growth in Mainland Southeast Asia

As global temperatures continue to rise, the intensifying occurrence of extreme weather phenomena constitutes a critical hazard to land-based ecological systems. However, a systematic quantitative assessment of how vegetation growth responds to diverse individual and compound climate extremes remains lacking, particularly in tropical monsoon regions characterized by abundant hydrothermal resources yet intense variability. By integrating meteorological reanalysis products with space-borne vegetation indices (specifically KNDVI and GPP) for the period 2001-2021, we employed event coincidence analysis (ECA) and sensitivity analysis to elucidate the response characteristics of vegetation growth anomalies to climate extremes in Mainland Southeast Asia and their modulation by background hydrothermal conditions. The results indicate that extreme wetness (31.5%) is the primary types of climate extremes inhibiting vegetation growth in this region. Compound cold-wet events constitute the dominant compound stress constraining vegetation growth in the northern mountainous areas. Unlike in arid and semi-arid regions, vegetation in this region exhibits a rare positive sensitivity to water deficit, attributed to the photosynthesis-promoting effect of enhanced solar radiation that typically accompanies drought conditions. Furthermore, we identified a significant nonlinear modulation of background hydrothermal conditions on vegetation sensitivity, characterized by critical climatic thresholds: high temperature and water surplus transition from facilitating factors to inhibiting stressors when the growing-season mean temperature exceeds 26.0 °C or annual precipitation surpasses 1300 mm. Moreover, different vegetation types exhibit divergent vulnerabilities, with croplands showing the highest sensitivity to extreme wetness. This study reveals that vegetation growth in tropical monsoon regions is primarily constrained by water surplus, underscoring the necessity of prioritizing water surplus in future climate change risk assessments.