Decoupling terrestrial and atmospheric water stress on ecosystem productivity during flash droughts

In the context of climate warming, frequent outbreaks of flash droughts are causing serious damage to ecosystems, so there is an urgent need to understand the water stress on ecosystems during flash droughts. High vapor pressure deficit (VPD) and low soil moisture (SM) are regarded as atmospheric and soil water stress on the ecosystem, but their stress mechanisms are different. Their independent influences are difficult to separate during flash droughts that develop fast with a strong land-atmospheric coupling. Therefore, to understand the response mechanism of vegetation gross primary productivity (GPP) to flash droughts, this study uses statistical analysis to decouple the effects of atmospheric and terrestrial water stress on GPP at the site scale and regional scale, respectively. At the site scale, we use the FLUXNET2015 Dataset to decouple the stress of SM and VPD on the GPP during flash droughts, and find that low SM dominants water stress for 55% of the stations, and high VPD dominants water stress for 10%. We further investigate the differences of GPP response to moisture deficit for different ecosystems during different stages of flash droughts. The results show that non-forest ecosystems respond to water stress during the onset stage of flash droughts, while more forests respond during the drought recovery stage. Specifically, for the days that are accompanied by high temperature and intense solar radiation during flash droughts , the water stress dominated by high VPD increase to 41% of the stations. For the regional scale, we use remote sensing data to decouple the effects of water stress over China. Our results show that water stress during flash droughts is dominated by soil moisture deficit over most regions of China, but VPD stress is stronger over northern China than that over southern China.