## 5 Discussion
### Effects of Soil Moisture and Atmospheric Humidity on Vegetation Dynamics  
At the nine meteorological sites in the semi-arid region, there were no statistically significant trends of growing season precipitation (Figure S3). Influenced by both precipitation and air temperature, VPD experienced statistically significant increasing trends at two of the nine stations, the land cover types of which are grassland and barren land; VPD at the other seven stations did not experience statistically significant trends (Figure S4, supplementary materials). It is probably not precipitation that drove the vegetation green-up in this semi-arid region, since none of the nine sites experienced a statistically significant precipitation increase. VPD could not explain the vegetation green-up either, because two of the nine sites experienced statistically significant increasing trends of VPD, and the other seven sites experienced no statistically significant trends. [PERSON] et al. [2] suggested that increased atmospheric VPD reduces global vegetation growth. This is probably not the case in this semi-arid region during the period from 2000 to 2016. Nonetheless, the correlation coefficients between the detrended growing season vegetation greenness and the detrended growing season precipitation as well as VPD at the seven low elevation sites were all greater than 0.5, and statistically significant (Figure 7). This suggests that in the semi-arid regions of Northwest China, water plays an important role in driving the interannual dynamics of vegetation. In general, the interannual dynamics of vegetation greenness in this semi-arid region correlate more with precipitation than with VPD. This suggests that soil moisture affects the growth of vegetation more than VPD does at the annual scale in this semi-arid region, which agrees with the claim that soil moisture dominates water stress on ecosystem production globally [38]. [PERSON] et al. [59] investigated the change in ecosystem water use efficiency in response to climate change, and suggested that the seasonal trends of ecosystem water use efficiency are mainly associated with those of climate and not increasing atmospheric CO2 or nitrogen deposition. Our research shows that an increase in vegetation greenness was accompanied by a change in temperature, and this also implies the increase in ecosystem water use efficiency, because of the higher temperature sensitivity of GPP relative to ET, as suggested in [59]. [PERSON] et al. [21] suggested that increases in VPD rather than changes in precipitation will dominantly influence future anisohydric grassland productivity. This is presently not the case in this semi-arid region--at six of the seven low elevation sites, vegetation greenness was correlated more with precipitation than with VPD, which may be due to the prevalent isohydric ecosystems in this region. [PERSON] et al. [60] found that shrubs dominated by C3 photosynthetic pathways are more sensitive to VPD than grasses dominated by C4 pathways, which agrees with our results. Most of the vegetation in our study region are C3 plants, hence the vegetation greenness interannual variability is more sensitive to precipitation than to VPD. [PERSON] et al. [61] highlighted the vulnerability of dryland subtypes to more frequent and severe climate extremes--this solicits continued monitoring of semi-arid regions. Interannual vegetation dynamics are affected by direct human interference and environmental factors. Temperature, precipitation, and VPD are the three climatic factors closely related to vegetation photosynthesis in semi-arid regions. Hence, our study focused on the relationship between vegetation greenness and these three factors at the interannual scale. Nevertheless, other factors may also be important in the interannual vegetation dynamics, such as solar radiation, nitrogen deposition, as well as ecological conservation and restoration practices in China.