Significant water stress on gross primary productivity during flash droughts with hot conditions

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 eco-systems 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/m(2)), 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.