Drought changes the dominant water stress on the grassland and forest production in the northern hemisphere

Low soil moisture (SM) and high vapor pressure deficit (VPD) have a significant impact on the terrestrial ecosystem productivity through different mechanisms. The sources of relative impact of these two water stress factors are difficult to separate due to the strong interaction between the land atmosphere, especially during drought events with an increasing frequency and intensity, leading to significant controversy over the dominant water stress factors in ecosystems. Here, we analyzed the relative effects of SM and VPD on the Gross Primary Productivity (GPP) during non-drought and drought periods and how drought events alter the dominant water stress of the ecosystems using daily observations from 89 flux sites at the Northern Hemisphere. Our results showed that 84 of 89 sites and 81 of 89 sites were significantly affected by water stress during both non-drought and drought periods, with 43 % and 34 % of sites remaining dominant water stress from the high VPD and low SM during non-drought and drought conditions, respectively. In addition, 19 sites have experienced significant dominant water stress shifts when drought events occurred, including forests (47 %), grasslands (37 %), and shrubs (16 %). The normalized GPP by radiation and temperature also emphasizes that high VPD has a stronger control effect on vegetation than low SM and confirmed that drought events could alter the dominant water stress factors in forests and grasslands. Our results might help to clarify the relative impacts of the SM and VPD on the vegetation, as well as the driving forces of drought events on the shifting role of SM and VPD. The driving mechanism for this shifting varies depending on the biomes and plant functional types Furthermore, our findings could provide a more accurate prediction of the vegetation productivity in different ecosystems, and a deeper understanding of the relationship between the vegetation and the climate.