## 4 Discussions
### The Explanation of Potential Influencing Factors to Vegetation Dynamics Across Global Drylands
Furthermore, SW acts as one of the most crucial water-related factors for vegetation growth; increasing SW constraints directly reduces the available water supply required for normal physiological maintenance of vegetation growth [51; 71]. However, there have always been controversies over the impact of atmospheric dryness and SW deficits on terrestrial ecosystems. To disentangle the relative impacts of VPD, SW, and CO\({}_{2}\) on vegetation growth, this work conducted two sets of controlled experiments based on the PLS method to quantify the absolute sensitivity of these climatic factors. The analysis indicated that vegetation growth was negatively sensitive to increasing VPD in 66.159% of dryland's vegetated areas (Figure 4e) and positively sensitive to CO\({}_{2}\) concentration and SW in most global drylands from 1982 to 2018 (Figure 4a,c). It is worth noting that LAI changes in dryland ecosystems were several times more sensitive to SW than VPD. Previous studies have even confirmed that SW serves as a dominant water stress driver to vegetation productivity [39] and carbon-water fluxes [38], and the dominance of SW has substantially enhanced in Eurasian drylands [40]. Vegetation growth is increasingly sensitive to the spatial dynamics of precipitation and SW availability in many semi-arid and arid regions [19; 70], and vegetation resilience is highly correlated with water availability across the globe [26; 72]. Seen from the mutual feedback relationship between vegetation greening and SW, early vegetation greening led to summer soil drying in most vegetated areas of the northern hemisphere [73]. Surprisingly, greening China's Loess Plateau has been approaching the threshold of sustainable water-carrying capacity [15; 28]. Additionally, this work further found that SW at different depths contributed differently to vegetation growth, and decreasing SW at the second (7-28 cm) and third depths (28-100 cm) produced stronger water limitations to dryland vegetation than that of SW at the surface and fourth soil depths (Figure 7), which may be deeply associated with vegetation type in dryland ecosystems. Dryland vegetation has well-developed root systems with drought-resistant features, which easily obtain the water they need for plant growth [51; 52; 70]. Large offsets exist in the impacts between enhanced atmospheric dryness, increasing SW limitations, and rising CO\({}_{2}\) concentrations on vegetation growth. This work found that atmospheric and SW constraints on dryland ecosystems have greatly enhanced during the past several decades, i.e., rising VPD coexisting with decreasing SW (Figures S3 and S4). Changes in LAI were negatively sensitive to VPD in >66.16% of global drylands but positively sensitive to changes in SW in 86.36% of dryland vegetated areas (Figure 4). Meanwhile, this study found that the rising VPD and decreasing SW-induced negative contributions offset approximately 38.49% of the positive contributions of CO\({}_{2}\) fertilization to vegetation growth across global drylands from 1982 to 2018 (Figure 8). Similar to our conclusions, a recent study reported that the rising atmospheric VPD-induced negative contributions only counteracted a small part of the warming and CO\({}_{2}\) fertilization-induced increases in global GPP, and global vegetation productivity continuously increased despite rising atmospheric dryness since the 21 st century [35; 74]. This finding highlighted the positive importance of the CO\({}_{2}\) fertilization effect on global vegetation primary productivity [3]. This study found that rising CO\({}_{2}\) concentrations produced great positive contributions to dryland vegetation dynamics (Figure 6). Globally, vegetation dynamics greatly benefitted from rising CO\({}_{2}\) concentrations [3; 4]. CO\({}_{2}\) fertilization effect-induced increases in global annual terrestrial photosynthesis reached approximately 13.5 \(\pm\) 3.5% PgC [16].