## 4 Discussion
### Contributions of Climate Variability and Vegetation Carryover to Vegetation Greening  
This study revealed that the strong observed greening trend of vegetation in each active season was mainly contributed by the VGC effect (Figure 8). Although the VGC effect was weaker during the early growing season, it increased from early to late growing season. High-quality vegetation growth in the early growing season determines vegetation growth in the subsequent seasons. The current-season climate factor, especially precipitation, is another important contributor, followed by the climate of the preceding season. Concerning the effect of current-season precipitation, vegetation was more sensitive in the early growing season. These phenomena may be attributed to the characteristics of vegetation--a low tolerance for lack of water during the early growth stage [18; 49]. From this perspective, it is most effective to intervene in the vegetation to promote vegetation growth in spring through implementing ecological conservation and improving water conditions. There is still a proportion of unexplained variance (residuals) in vegetation growth in the three active seasons. These unexplained variances reflected the contributions of other drivers of vegetation growth, such as carbon dioxide fertilization and nitrogen deposition [1; 50; 51], human activities, [24; 52; 53], natural disturbances [7; 15; 54], and precipitation pattern shift [55]. The higher growth rate (black solid line) observed in the early parts of the growing season was primarily inherited from the vegetation carryover and residuals. However, this study focused on the impacts of climate factors and VGC effects at a seasonal scale and, therefore, did not quantitatively analyze other factors. As climate change is a continuous process, it may accumulate over time and then affect vegetation growth. Studies have shown that vegetation is significantly correlated with 2-6 months of cumulative precipitation [7], and precipitation on the Loess Plateau has a positive cumulative effect, especially in April and May [52]. This means that early spring precipitation may affect vegetation greening. Thus, the residuals in EGS may be contributed by the cumulative effect of precipitation. In addition, the melting of alpine snow and ice in early spring also provides moisture for vegetation growth in EGS [52]. The large-scale ecological restoration project, Grain to Green Project (GTGP), which was implemented to improve the ecological environment and maintain the ecological balance in the study area, has reduced arable land and increased forest and grassland. A prior study demonstrated that vegetation restoration is faster for lands converted to woodlands and grasslands than land with no conversion under similar dry conditions [38]. This is also the reason why the VGC effect in the study area is stronger than other factors. In the PGS, it showed a higher contribution of vegetation growth carryover effect, which is primarily inherited from the preceding-season vegetation greening (67%) rather than from the contributions of the climate factors (19%). Without considering the vegetation growth carryover effect, numerous studies suggested that climate factors and human activities are the driving factors for vegetation greening [4; 21; 45]. Yet, our study shows that the VGC effect contributes much more to vegetation greening than other factors. Though the positive VGC effect of vegetation growth can override negative climate impacts under the present climate and carry over into the subsequent year, it is still an open question whether this positive vegetation carryover will continue with future climate change.