High vapor pressure deficit (VPD) and low soil moisture (SM) are regarded as atmospheric and soil water stress on the ecosystems respectively, but the two factors are influencing ecosystems through different mechanisms. The relative influences of two sources of water stress during droughts are difficult to separate, especially for flash droughts that develop rapidly with strong land-atmospheric coupling. The dominant factors that expose ecosystems to water stress during droughts are controversial, which hinders the understanding of flash drought impacts and the related adaptation practices. Therefore, here we decouple SM and VPD stress on gross primary productivity (GPP) during flash droughts using hourly observations from 35 global FLUXNET stations with long-term eco-hydro-meteorological records. The results show that 52% of the stations have significant water stress on GPP during flash droughts, where low SM and high VPD are dominant for 46% and 6% of the stations respectively. For subsets of flash droughts with hot conditions that are defined by concurrent high temperature and above normal solar radiation (or defined by concurrent high net radiation larger than 400 W/m²), low SM and high VPD dominate water stress for 37% and 49% (31% and 50%) of the stations respectively. In addition, low SM and high VPD dominate water stress on the GPP normalized by solar radiation for 14% and 80% of the stations respectively, but the role of SM might be underestimated if flash droughts penetrate in deep soil. As compared with forest ecosystem productivity, non-forest ecosystem productivity declines quickly in response to soil water stress during the onset stage of flash droughts. Our results suggest an intensified water stress on ecosystem productivity and the dominance of high VPD stress during flash droughts with hot conditions, although the soil water stress is nontrivial for unconditional flash droughts.

高蒸汽压亏缺（VPD）和低土壤水分（SM）分别被认为是对生态系统的大气和土壤水分胁迫，但这两个因素通过不同的机制影响着生态系统。干旱期间两种水分胁迫源的相对影响难以区分，特别是对于快速发展且具有强烈地气耦合的突发干旱。在干旱期间使生态系统面临水资源压力的主要因素是有争议的，这阻碍了对突发干旱影响和相关适应实践的理解。因此，在这里，我们使用来自具有长期生态水文气象记录的 35 个全球 FLUXNET 站的每小时观测资料，将 SM 和 VPD 对突发干旱期间总初级生产力 (GPP) 的压力解耦。结果表明，52%的台站在突发干旱期间对GPP有显着的水分胁迫，其中低SM和高VPD分别占46%和6%的台站。对于由同时高温和高于正常太阳辐射（或由同时高净辐射大于 400 W/m^如图2所示，低 SM 和高 VPD 分别占 37% 和 49%（31% 和 50%）的站点的水分胁迫。此外，低 SM 和高 VPD 分别在由太阳辐射归一化的 GPP 中占主导地位的水分胁迫分别占 14% 和 80% 的台站，但如果突发干旱渗透到深层土壤中，则 SM 的作用可能被低估。与森林生态系统生产力相比，非森林生态系统生产力在突发干旱初期响应土壤水分胁迫迅速下降。我们的研究结果表明，在炎热条件下的突发干旱期间，生态系统生产力的水分胁迫和高 VPD 胁迫占主导地位，尽管土壤水分胁迫对于无条件的突发干旱来说并非微不足道。