## 3 Results
### Inter-Seasonal VGC Dominates Seasonal Vegetation Growth  
At the seasonal scale, the relationships among the three vegetation indicators (NDVI, GPP, and LAI) and each driving factor were analyzed from 2000-2019. The results showed that the preceding-season NDVI significantly (\(p<0.05\)) controls the current season vegetation NDVI in the three active seasons (Figure 3). The VGC effect was also positive for seasonal vegetation growth across the majority (91%, 93%, and 95% for EGS, PGS, and LGS, respectively) of the upper and middle reaches of the YRB (Figure 4). The partial correlation coefficients between the concurrent seasonal NDVI and the preceding-season NDVI in EGS, PGS, and LGS were 0.5168, 0.5743, and 0.6146, respectively (Figure 3), indicating that the strength of the VGC effect on vegetation growth increased from early to late growing season. In addition, the positive effect of VGC controlling vegetation growth in the three active seasons was also verified by examining the other vegetation growth and productivity indicators GPP and LAI (Figures S4 and S5). The matrix diagram and spatial distribution diagram of partial correlation coefficients of NDVI, GPP, and LAI showed a strong similarity. The vegetation growth and productivity of the preceding season were the dominant factors predicting vegetation greenness in each active growing season, implying that our results are reliable and the VGC cannot be ignored in the research of vegetation dynamics. \begin{table}
\begin{tabular}{c c c} \hline
**Mean NDVI** & **Level** & **Proportion (\%)** \\ \hline \(<\)0.1 & Low vegetation cover area & 0.88 \\
0.1–0.3 & Medium-low vegetation cover area & 24.50 \\
0.3–0.5 & Medium vegetation cover area & 33.54 \\
0.5–0.7 & Medium-high vegetation cover area & 30.92 \\ \(>\)0.7 & High vegetation cover area & 10.16 \\ \hline \end{tabular}
\end{table} 
Table 3: Vegetation coverage levels in the upper and middle reaches of the YRB during the growing season from 2000–2019. Compared with the positive effect of the VGC, the intensity and direction of climate factors, including the immediate and lagged impacts of climate, determining the inter-annual variation of seasonal NDVI (GPP and LAI) varied greatly between seasons and across regions (Figures 5, 6 and S6-S9). The concurrent precipitation is the primary climate factor determining the inter-annual variation of EGS and PGS NDVI of the upper and middle reaches of the YRB over the 20-year period (partial correlation: 0.3122, \(p<0.05\); 0.2686, \(p<0.05\), respectively) (Figures 3 and 5d-e). The positive impact of concurrent  
Figure 4: Spatial distribution of the partial correlations between NDVI of each season and that of the preceding season. Correlationpatterns are shown for EGS (**a**), PGS (**b**), and LGS (**c**). Figure 3: Partial correlation coefficients between 20-year seasonal NDVI time series and concurrent climate factors (temperature, TEM, precipitation, PRE, and sunshine duration, SSD), and climate factors (TEM\({}_{\text{PS}}\), PRE\({}_{\text{PS}}\), and SSD\({}_{\text{PS}}\)) and NDVI (NDVI\({}_{\text{PS}}\)) of the preceding season. The color block size represents the element value size. Squares with black outlines show statistically significant correlations at the 95% confidence level. precipitation on vegetation growth in EGS and PGS was also consistent in the analyses of other vegetation indicators GPP (Figures S4 and S6d-e) and LAI (Figures S4 and S8d-e). However, the concurrent precipitation had a much weaker impact on LGS vegetation, in comparison to that of EGS and PGS (Figures 3 and 5f). The immediate impact of precipitation diminished over the season (Figures 3 and 5d-f), contrary to the VGC effect. The precipitation of the preceding season had no obvious positive effect on vegetation growth in EGS and PGS, yet the positive effect in LGS increased (Figures 6d-f, S7d-f and S9d-f). The temperature of the current and preceding season on vegetation growth was very weak and statistically insignificant during the three active seasons.