## 4 Discussion  
In recent years, the response of vegetation growth to changes in atmospheric moisture has attracted much attention. Microscale studies have revealed that whereas moderate VPD promotes vegetation growth, excessive VPD reduces stomatal conductance and inhibits vegetation growth, and these conclusions have become widely recognized. Herein, we investigated the different effects of VPD on vegetation growth from long-term series data at the watershed scale, considering the climatic zoning and vegetation types, and obtaining interesting findings. Aligning with the global VPD trend against a climate-warming background [11; 28], the VPD in the Yellow River Basin has significantly increased and radiated outward from its central sites (the arid region). Overall, the VPD has facilitated vegetation growth in the Yellow River Basin and shows a significantly increasing trend in the basin overall. The coefficient of determination of the linearly trending 10-year moving windows correlation reached 0.7983. The effects of VPD on vegetation differed among the vegetation types in different climatic zones. In general, the enhancement relevance of VPD exceeded the inhibitory effect in natural forest grasslands, but the opposite was observed in irrigation areas. Moreover, the VPD exerted a significantly higher promotional effect on natural forest grassland than on irrigation crops. The inhibitory relevance of VPD on grasslands in the Yellow River Basin is concentrated mainly in Inner Mongolia; when the effects of VPD on the gross primary productivity (GPP) of vegetation in China were studied, it was similarly found that the VPD inhibits the GPP mainly in grassland vegetation in Inner Mongolia [29]. In contrast, the VPD promotes the growth of alpine grasslands in the upper reaches of the Yellow River Basin, where the atmospheric water vapor conditions are relatively humid. Because VPD was dominant in regulating water use efficiency variations in alpine natural vegetation  
Figure 12: Temporal trends of significant changes in percentage areas associated with promotion and inhibition using 10-year moving windows (the solid circles greater than 0 represent the proportion of the area of rasters that exhibit a positive correlation trend for a given 10-year sliding average (i.e., the slope is greater than 0) and pass a test of significance; the hollow circles less than 0 represent a negative correlation trend; the solid and dashed lines indicate the linear trend lines for the solid and hollow data, respectively). ecosystems [30], it can effectively promote plant germination and growth, ultimately improving vegetation productivity in cold regions [31]. The VPD usually promotes the growth of irrigation crops in the arid region of the Yellow River Basin but inhibits crop growth in semi-humid regions. This finding probably reflects the different effects of VPD in different climates. At a higher VPD, crops grown in arid regions can access sufficient soil water content to maintain a higher transpiration rate, thus lowering the effects of VPD on leaf stomatal conductance and vegetative photosynthesis [32]. The intensities of these effects showed an increasing trend in arid regions, most obviously in the Ningxia and Hetao irrigation districts, which are important mega-irrigation districts in China. The effects of atmospheric moisture on crops in arid regions are becoming increasingly prominent as such crops are safeguarded by anthropogenic interventions (e.g., irrigation). These findings of heterogeneity reveal that the relevance of VPD to vegetation growth (promotional or inhibitory) is more related to the climatic zoning of the vegetation than to vegetation types. In the climate-warming environment, the global VPD is predicted to continue its present increase. The atmospheric moisture changes will likely cause frequent and persistent droughts, and the most severe changes are expected in arid regions.