## 4 Discussions
### The Explanation of Potential Influencing Factors to Vegetation Dynamics Across Global Drylands
Land use change is confirmed to be one of the most potent factors in vegetation dynamics in dryland ecosystems [3; 4]. Afforestation and reforestation can alter the original vegetation types and greatly contribute to vegetation greening in dryland ecosystem; meanwhile, they can improve the growth condition of surrounding vegetation and form dominant vegetation species by regulating SW balance. Furthermore, changes in land cover type, such as desertification, land abandonment, farmland conversion, and deforestation activity, also affect dryland ecosystems to a certain degree [61; 62; 63]. Vegetation is generally greening sustainably after farmland abandonment and conversion, while vegetation browning would occur in deserted and deforestation areas [4; 7; 8; 10]. Under the background of global warming, woody plant encroachment and a northward shift in the forest line are extremely widespread in high-altitude regions and cause significant vegetation greening in these regions [64; 32]. Moreover, arid ecosystems are extremely fragile and easily affected by changes in other ecosystems [12; 13; 14]. Dryland greening is not an independent evolutionary process within its self-ecosystem but the result of the changes in multiple ecosystems. Climate change in humid regions generally propagates into drylands, such as the drying-wetting transition in the dry-wet boundary area, which further affect vegetation growth and even alter vegetation types [65]. This study found an obvious expansion in global drought-sensitive areas induced by climate change [66], and this expansion accelerates the impact propagation from drought-sensitive areas to dryland ecosystems via land-atmosphere coupling [67]. The Potential Impact of Atmospheric Dryness and Soil Drying on Vegetation Growth in Dryland Ecosystems  
To comprehensively understand the interannual variability in global terrestrial carbon sequestration and even determine the global carbon balance between carbon emissions and absorption, it is crucial to study changes in terrestrial ecosystems and their responses to atmospheric and SW constraints in global drylands [20; 22]. Despite there being differences in vegetation dynamics among different satellite-based products (Figure S11 and Table S4 in Supplementary Section S5.2), global drylands have still been greening since the early 1980s. This study considered a few potential cautions when interpreting the results in the paper. In detail, this study realized that uncertainties in the satellite-observed and model-generated VI products may affect the results of this study. This study, therefore, employed the averaged value of LAI, GPP, and SIF from several widely used satellite-observed LAI, GPP, and SIF products to minimize these uncertainties. This study suggested that LAI, GPP, and SIF had significantly increased in global drylands since the early 1980s (Figure 1), which consistently agreed with the previous conclusions among multiple types of VIs [1; 3; 4]. Spatially, substantial increases in LAI, GPP, and SIF are consistently distributed in most global drylands, especially in South Asia, the North China Plain, southern Africa, and the Sahel region (Figure 3), and this work found that the spatial pattern of the interannual trend in these vegetation indices wholly agreed with those of greening trends from previous literature [1; 3; 67]. Meanwhile, this study also noticed that three VIs consistently decreased in central Asia, eastern Brazil, and the Mongolia Plateau, and this finding from the previous literature proved that vegetation browning in these regions is primarily attributed to grassland degradation, desertification, and climate extremes such as droughts and fires [68; 69]. As is well known, enhanced atmospheric dryness has negative impacts on vegetation growth, and increasing atmospheric VPD could accelerate water transportation from plant root to leaf and then transpiration from leaf to the atmosphere [19; 29; 70].