## 3 Results  
### Variation Trends of Meteorological Drought Indices and Vegetation GPP  
We used the M-K test method to analyze the annual trend, seasonal trend, and significance of SPEI and VPD trends from 2001 to 2020 (Figure 2). From the perspective of the annual average change trend (Figure 2a), the SPEI, accounting for 68.0% of the total area, showed an upward trend. The areas showing a decreasing trend were mainly distributed in southwest and northern China.  
Figure 2: Spatial distribution of annual (**a**) changing trend, (**b**) \(p\)-value, and seasonal changing trend in (**c**) spring, (**d**) summer, (**e**) autumn, and (**f**) winter across China from 2001 to 2020 using the standardized precipitation evapotranspiration index (SPEI) based on the Mann–Kendall (M–K) trend test.  
The analysis of the seasonal change trend in SPEI showed large differences in the changing trend when different seasons were considered (Figure 2c-f). Among the different seasons, the largest proportion of area (\(\sim\)54.3%) with a decreasing trend according to the SPEI was found in spring, while the smallest proportion (\(\sim\)19.9%) was found in autumn. However, it is worth noting that the regions with decreasing trends were not significant (\(p>0.05\)) during the four seasons. In terms of vegetation types, for the annually averaged SPEI trends, evergreen forests presented the largest proportion (\(\sim\)46.4%) of decreasing trends, while mixed forests showed the smallest (\(\sim\)15.1%) (Table 2).  
Similarly, the spatial distribution of the annual average and seasonal trends of VPD are shown in Figure 3. The trend of dryness and wetness changes, characterized by VPD and SPEI, are generally consistent. The area with an increasing trend of annual average VPD accounted for 53.1% of the total area. Furthermore, the regions with significant trends showed upward tendencies. As with SPEI, the changing trend of VPD in different seasons also varied. Among the different seasons, the largest proportion (\(\sim\)71.5%) of areas with an increasing trend of VPD was found in spring, while the smallest (\(\sim\)41.6%) was found in autumn. In all four seasons, a significant upward trend was observed. As for different vegetation types, grasslands had the largest proportion (\(\sim\)65.1%) of increasing trends, while mixed forests had the smallest (\(\sim\)25.2%) (Table 2).  
Finally, the same results are presented for GPP in Figure 4. Annual GPP showed an overall upward trend, with 90.7% of the regions showing an increasing trend and 9.3% showing a decreasing trend from 2001 to 2020. Except for the decline in GPP in some areas in western China, most of the other regions presented positive tendencies, with highly significant increasing trends (\(p<0.01\)). On the contrary, the values for the areas with a downward trend were not significant (\(p>0.05\)). Among the different seasons, the largest proportion (\(\sim\)87.8%) of areas with an increasing trend of GPP was found in autumn, while the smallest (\(\sim\)65.9%) was found in winter. From the perspective of different vegetation types, deciduous forests had the largest proportion (\(\sim\)97.5%) of increasing trends, while evergreen forests had the smallest (80.1%) (Table 2).