## 3 Results
### Differences in the Effects of VPD on Vegetation  
#### 3.3.1 Correlation between VPD and Vegetation  
Vegetation occupies 87% of the area of the Yellow River Basin, including 48% grassland, 25% cropland, and 14% woodland. These vegetation types are distributed through all four climatic zones. The water effect of natural vegetation in humid regions was not analyzed because the proportions of both the humid regions in the Yellow River Basin and the natural vegetation in humid regions are small. Overall, the area facilitated by VPD was 1.4 times larger than the area inhibited by VPD. The effects of VPD differed among the vegetation types (Figure 9). Based on the relative proportions of the Spearman rank correlation coefficients of the three vegetation types (Table 1), the facilitating relevance of VPD was highest for woodland, followed by grassland and cropland. On cropland, the facilitating effect was exceeded by the inhibiting effect. The correlations of the three vegetation types were heterogeneous in different climatic zones. The VPD mainly promoted the growth of grassland vegetation in the upper reaches of the plateau, where the altitude exceeds 3000 m above sea level (belonging to semi-arid and semi-humid regions) and natural grassland is concentrated, and in the grassland zone in the gully area of the Loess Plateau (belonging to semi-humid regions). The highest inhibitory effects of VPD were mainly distributed in the grassland zones of Yinshan Mountain and the Maowuosu sandland (belonging to arid and semi-arid regions), and in the grassland-cultivated land of the Loess Plateau (belonging to semi-arid and semi-humid regions). The promotional relevance of VPD to cropland was mainly apparent in the western part of the Ningxia and Hetao irrigation zones in arid regions, the central part of the irrigation district of the downstream Yellow River (belonging to semi-humid regions). The inhibition effect was distributed in the interspersed distribution zone of cropland and grassland on the Loess Plateau (belonging to semi-arid and semi-humid regions), and on the Fen-wei Plain (belonging to semi-humid regions). The promotional relevance of VPD to woodland, which is more obvious than its relevance to the other vegetation types, is centrally distributed in the semi-humid region of the woodland zone in the gully area of the Loess Plateau and is dispersed in high-altitude woodland zones such as the upper Qilian Mountains. Figure 8: Significant changes in annual average LAI during the growing season in the Yellow River Basin (Significant Decrease (Sign-D), Slight Decrease (Slig-D), Slight Increase (Slig-I), Significant Increase (Sign-I)). The distribution patterns of Spearman rank correlation coefficients between the three vegetation types and VPD were generally consistent in different climatic zones (Figure 10). Positive correlations exceeded negative correlations in semi-arid and semi-humid regions, with the exception of cropland in semi-humid regions. In the arid region, positive correlation distributions of the three vegetation types were outnumbered by negative correlations. Judging from the spatial correlation distribution and the distribution pattern of the box plots, whether VPD promotes or inhibits the growth of vegetation is largely related to the climatic zone of the vegetation. That is, the relevance of VPD to vegetation is more strongly influenced by differences among the climatic zones of the vegetation than by differences among the vegetation types themselves.