# Early Evidence That Soil Dryness Causes Widespread Decline in Grassland Productivity in China
## 1 Introduction  
Global climate change has fundamentally altered the inherent patterns of variability of weather phenomena, such as precipitation and temperature, leading to the increasingfrequency of extreme weather events such as droughs, heat waves, and cold waves [1]. Extreme events severely affect the composition and function of terrestrial ecosystems, thus affecting the terrestrial carbon cycle and its feedback to the climate system. Extreme droughs, in particular, are highly anticipated natural disasters because they occur with the highest frequency and exert the greatest influence [2; 3]. The intensity and frequency of droughs are the most limiting factors affecting terrestrial vegetation growth and carbon cycling [4], and droughs reduce the water available to vegetation directly [5], with extensive and profound effects on carbon uptake in terrestrial ecosystems [6]. Recent research pointed out that terrestrial carbon sinks are often strongly influenced by interannual fluctuations in terrestrial water storage. Terrestrial water constraints are also an important limiting factor for the accumulation of ecosystem gross primary productivity (GPP) [7], that is, the largest carbon flux [8]. However, the terrestrial water storage referred to by previous studies includes all water types, such as soil water, groundwater, surface water, and canopy water. In contrast, there is a lack of exploration of the possible effects of water shortage from the soil on ecosystem productivity, as the terrestrial water variability is considerably sensitive [9]. Soil moisture accounts for a relatively large proportion of terrestrial water storage [10; 11]. However, soil moisture is a generalized concept, and it is generally loosely considered as all the water within 3 m below the ground surface. Considering the differences in terrestrial water adaptation of different plants and the strong link between water availability to grass plants and root depth [12], it is necessary to explore the varying effects of soil water constraints at different soil depths on ecosystem GPP. In addition to soil drought, atmospheric drought has been recently reported as one of the hydraulic processes that affect ecosystem productivity [13]. Plants initially draw water from the soil for photosynthesis and simultaneously dissipate it in the atmosphere through transpiration from plant leaves [14]. Both supply and depletion stresses can significantly reduce terrestrial carbon uptake and crop yields [6] and cause widespread vegetation mortality. However, current research on the primacy of soil moisture (SM) and atmospheric moisture (vapor pressure difference, VPD) on ecosystem productivity response is controversial. [PERSON] et al. (2016) pointed out that VPD has a greater constraining effect on stomatal conductance and evapotranspiration than SM, which is more important for carbon accumulation. Moreover, as atmospheric VPD will continue to rise in the future, the negative effect of VPD on stomatal conductance during the plant growing season will increase significantly, as will its dominance on carbon fixation [15]. [PERSON] et al. (2020) proposed the opposite conclusion and claimed that soil drought is the main stress that threatens GPP accumulation in more than 70% of regional ecosystems worldwide, and by decoupling SM-VPD, found that the impact of VPD on terrestrial ecosystems is much smaller than that of soil moisture [16]. Given the complexity of the coupling between VPD and SM, it is highly desirable to use novel methods (e.g., probabilistic) to compare the dominance of VPD and SM on ecosystem GPP. Chinese grasslands are mainly distributed in arid and semi-arid regions (typical temperate grasslands and montane grasslands) and the Tibetan Plateau (typical alpine grasslands), where the ecological environment is fragile [17; 18], providing a natural barrier to ecological security in Central and Western China.