## 4 Discussion
### Effects of Drought on Vegetation GPP  
Few studies have analyzed the relationship between vegetation GPP and water deficit in different seasons at the national level. In China, we observed significant differences in the effects of water deficit on vegetation in different seasons. For different seasons, GPP correlated (in both positive and negative directions) almost identically with VPD and SPEI (Figures 7 and 9). The relationship between SPEI and GPP and between VPD and GPP both indicated that the proportion of areas where water deficit negatively affected vegetation was greater in summer and autumn than in spring and winter. This result is consistent with previous studies in which vegetation activity was found to be more sensitive to drought events during the growing season [65]. In spring, vegetation grows well as long as there is sufficient soil moisture and atmospheric vapor pressure [66]. In addition, related studies have confirmed that spring vegetation greening may exacerbate summer drought [67; 68]. Summer drought limits vegetation transpiration and directly reduces photosynthesis [69]. Similarly, autumn drought will also promote stomatal closure of leaves to decrease transpiration [70]. Moreover, the correlation between meteorological drought and hydrological drought is stronger in summer and autumn compared to spring and winter [71], which may be another crucial reason for the findings of our study. It was found that different vegetation had distinct sensitivity to water deficit in various seasons. All vegetation types were the most negatively affected by water deficit in the fall, except for grasslands, which were the most negatively affected in the summer. Previous studies only considered the relationship between drought index and vegetation in all periods of time, but did not pay attention to the relationship between drought index and GPP only during drought periods. This study found that there were some differences between the two results; in terms of spatial characteristics, drought had negative effects on vegetation GPP in northern China, southwest China, and the Qinghai-Tibet Plateau. It is worth noting that these areas are highly overlapped with areas where drought severity and frequency have been increasing in recent decades.  
The serious adverse effects of water deficit on vegetation GPP were mainly concentrated in Inner Mongolia, of which the main vegetation type is grassland. grasslands are among the most fragile ecosystems, where both climate change and human activities are significantly impacting their productivity [72]. Grasslands consist mainly of herbaceous plants whose roots can store a limited amount of water, resulting in a weak resistance to water deficit [73]. Furthermore, there are differences in the response of various grassland types. This study also investigated savannas and concluded that these were some of the more significantly negatively affected by water deficit among the different vegetation types. Indeed, more studies have focused on the response of forests and grasslands to drought, and relatively few studies have been conducted on the impact of drought on savannas [74; 75]. Severe and persistent drought can significantly reduce savanna productivity, leading to structural and compositional changes and even widespread vegetation mortality [75]. As for croplands, this is the vegetation type that receives the most human intervention (e.g., irrigation), but it is still vulnerable to drought. Additional comparison of the differences in the response of crops, including detailed species, to drought would be better conducted at a regional scale. Environment monitoring by using remote sensing is an import measurement [76; 77; 78; 79; 80; 81; 82].