## 3 Results
### The Dominance of Water Stress to Vegetation Growth in Global Drylands
#### 3.4.1 Offset in the Contribution Among CO\({}_{2}\), SW, and VPD Based on Satellite Observations  
Figure 8 and Table 4 illustrate the spatial distribution of how much the rising VPD and declining SW-induced negative contribution to vegetation changes offset the CO\({}_{2}\)-induced positive contribution in drylands worldwide from 1982 to 2018. The results showed a similar spatial pattern in which the VPD- and SW-induced negative contribution offset the beneficial effects to vegetation greening due to CO\({}_{2}\) fertilization, termed as \"the offsetting contribution\". Specifically, the negative contribution from rising VPD offset 25.36% of the positive contribution due to increasing CO\({}_{2}\) concentration (Figure 8a). In terms of SW, the negative contribution due to decreasing SW offset 37.33% of the CO\({}_{2}\)-induced positive contribution, and the highest offsetting rate occurred in central Asia and western North America (Figure 8b). Judging from the VPD- and SW-induced negative contribution offsetting the CO\({}_{2}\)-induced positive contribution to vegetation growth, the negative contribution due to rising VPD and decreasing SW offset 38.49% of the positive contribution due to increasing CO\({}_{2}\) fertilization, mainly distributed in the middle latitude of the northern hemisphere and low latitude of the southern hemisphere (Figure 8c). Meanwhile, the study investigated the variations in the offsetting contribution among various land cover types. Collectively, these aforementioned findings revealed that enhanced atmospheric and SW constraints (i.e., rising VPD and decreasing SW) offset over one-third of the positive contributions of CO\({}_{2}\) fertilization to vegetation dynamics across global drylands from 1982 to 2018. Furthermore, this study found that enhanced VPD and SW constraints offset the largest beneficial effects of CO\({}_{2}\) fertilization in forestland ecosystems compared to grasslands and croplands ecosystems, as shown in Figure 9. The statistical distribution percentage of the offsetting contribution was primarily concentrated in the value interval ranging from 0 to 20% across different land covers, in which the offsetting scenarios included VPD-induced LAI decreases offsetting the CO\({}_{2}\) fertilization-induced LAI increases, SW-induced LAI decreases offsetting the CO\({}_{2}\) fertilization-induced LAI increases, and VPD \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Area Percentage (\%)**} & \multicolumn{4}{c}{**SW at Different Soil Depths**} \\ \cline{2-5}  & **0–7 cm** & **7-28 cm** & **28–100 cm** & **100–289 cm** \\ \hline Positive \(\triangle\)LAI(SW \(\mid\) VPD) & 35.78 & 35.74 & 37.01 & 44.07 \\ Negative \(\triangle\)LAI(SW \(\mid\) VPD) & 64.22 & 64.26 & 62.99 & 55.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Area percentage of positive and negative contributions of SW to LAI changes at different soil depths in global drylands. Figure 7: Differences in LAI changes caused by SW at (**a**) 0–7 cm soil depth, (**b**) 7–28 cm soil depth, (**c**) 28–100 cm soil depth, and (**d**) 100–289 cm soil depth. and SW-induced LAI decreases offsetting the CO2 fertilization-induced LAI increases. Based on the corresponding median, the contribution of VPD-induced decreases offsetting CO2-induced increases in LAI in the grasslands was, on average, greater than that of forestlands and croplands, suggesting that increasing VPD had more detrimental effects on grasslands than on forestlands and croplands (Figure 9a). The contribution of SW-induced decreases offsetting CO2 fertilization-induced increases of LAI in the forestlands was higher than that of grasslands and croplands, meaning that forestland ecosystems tended to be more severely constrained by decreasing SW (Figure 9b). Comparably, in the forestland ecosystems, increases in LAI caused by CO2 fertilization were offset by decreases in LAI caused by VPD and SW, which were greater than those caused by the other two landcovers.