## 4 Discussions  
### The Explanation of Potential Influencing Factors to Vegetation Dynamics Across Global Drylands  
Water availability is considered the dominant factor that reshapes the spatial pattern of vegetation types, potential species, plant habitation, fundamental properties of terrestrial ecosystems in middle and low-latitude regions, and even tropical forests which are not generally recognized as water-restricted [19; 21]. Groundwater reserves are closely related to surface water content, which, on a large scale, determines the water supply of plant  
Figure 11: Comparison of the contributions of influencing factors on the trend in LAI in global drylands from 1982 to 2018. (**a**) Multi-model averaging (MMA) of 11 ecosystem models from the TRENDY project and (**b**) satellite observations. roots and thus affects the growth of surface vegetation [51; 52]. Surprisingly, there was a strong correlation between increasing VPD and decreasing SW content. This suggests that increased atmospheric dryness contributed to the acceleration of the water cycle by drawing more water from the surface soil [2; 53]. Therefore, the spatial distribution of multi-year averaged groundwater reserves and its interannual trend across global drylands from 1982 to 2018 may help to explain the spatial patterns of the offsetting contribution that enhanced water constraints counteracted the beneficial contribution of CO\({}_{2}\) fertilization to vegetation growth. In this study, the GRACE satellite TWS reconstruction data were used to explore the spatial consistency between groundwater reserves and the extent of enhanced VPD and decreased SW offsetting rising CO\({}_{2}\)-induced positive contributions to vegetation dynamics (Figure S10 in Supplementary Section S5.1). Interestingly, this study discovered that there was a high degree of spatial consistency between the trends of groundwater reserves (Figure S10) and the offsetting proportion that increased water constraints offset the positive contribution of CO\({}_{2}\) fertilization to vegetation growth. In other words, when groundwater reserves dwindled significantly, the offsetting proportion of the CO\({}_{2}\)-induced positive contribution by increasing atmospheric and SW limitations was higher. Spatially, these regions primarily covered southeastern Australia, western India, central Asia, and western North America. It is acknowledged that climatic extreme events such as the El Nino phenomenon and Southern Oscillation (SO) can affect the global atmospheric water cycle, further producing certain impacts on terrestrial ecosystems, accompanied by accelerated evolution or degradation of terrestrial vegetation [2; 22; 54]. Successions of studies revealed the deep interplay of greening and the ENSO phenomenon on biosphere-atmosphere processes [20; 54]. For instance, a study found the increasing arid land vegetation productivity in northwestern South America during the period of an El Nino Southern Oscillation event [55]. The study also revealed that the combination of Arctic Oscillation (AO) and ENSO can explain approximately 36% of the total variance in NPP changes in the Indo-Myanmar region, which is likely due to regional precipitation anomalies through atmospheric circulation affected by the AO/ ENSO events [56]. Additionally, southeastern Australia, one of the significant areas where the enhanced water deficit-induced negative contributions overwhelmingly offset the CO\({}_{2}\) fertilization-induced positive contributions to vegetation greening (Figure 8), frequently suffers from climate extremes such as droughts, heatwaves, and wildfires [57]. Widely occurring EI Nino climate extremes and SO events aggravate meteorological anomalies in the East Coast regions of Oceania and greatly reduce the regional water availability [12; 58; 59; 12]. Southeastern Australia, which is located on the eastern side of the Great Dividing Range and largely covers tropical desert and tropical savanna climates, is prone to periodic atmospheric and soil droughts and water constraints [60].