# Dual Roles of Water Availability in Forest Vigor:
## Discussion
### Adjustment Mechanisms of Environmental Climate and Seasonality  
Our study found that the dual roles of water availability in forest vigor were jointly controlled by the environmental \(T\) and the seasonality (i.e., pregrowing season or growing season). In this study, we found that the roles of water availability in forest vigor in China had both positive and negative roles (Figures 4 and 5 and Figure S1), which is consistent with other research [6; 20]. Specifically, the negative role of water availability in forest vigor was mainly found in regions of \(T\leq T_{\text{threshold}}\) during the pregrowing season. The \(T_{\text{threshold}}\) values ranged within 1.3-4.7 \({}^{\circ}\)C and varied according to the forest type, consistent with other phenological research which found that a temperature lower than 5 \({}^{\circ}\)C is too cold for forest growth during the pregrowing season [52; 53; 54]. As a result, forests located in cold regions (\(T\leq T_{\text{threshold}}\)) are not limited by water availability, but by temperature [22]. As a result, there was no difference in the frequency of \(r_{p}\) and \(r_{n}\) in the pregrowing season, which means that nearly 50% of forests showed no significant change when the water availability increased or decreased at a 0.05 significant level (i.e., \(\left\lvert r\right\rvert>0.514\)) (Figure 5). Additionally, given that temperature is usually negatively correlated with water availability [55], the negative correlation is probably an indirectly response of those forests located in cold regions (\(T\leq T_{\text{threshold}}\)) to water availability during the pregrowing season (Figure 13a). With increases in environmental \(T\), the importance of water availability for forest vigor increased, which caused the area percentage of the positive correlations to increase accordingly (Figure 13a,b). As \(T\) further increased and reached a threshold (i.e., \(T>T_{\text{threshold}}\)), the main limiting factor for forests became water availability and thus the positive role of water availability appeared (Figures 6, 7 and 13a,b). Additionally, the dual roles of water availability in forest vigor were driven by the environmental \(WB\) and the seasonality. Previous research indicated that forest vigor during the growing season is commonly controlled by water availability [19; 22], especially for drier regions of \(WB\leq WB_{\text{threshold}}\) (Figure 13b). Therefore, they mainly show a positive response to water availability (Figures 8 and 9). With increases in environmental \(WB\), the importance of water availability for forests decreases, while the impacts caused by other limited factors (e.g., nutrients, diseases, and insects) emerge [23; 37; 56] (Figure 13b,c). As a result, forests located in wet regions (i.e., \(WB>WB_{\text{threshold}}\)) showed not only positive correlations, but also negative correlations, with water availability, which caused nonsignificant differences in the area percentages between the positive and negative correlations (Figure 8b,d). The results are consistent with other relevant research showing a negative correlation in wet regions [6; 20] (Figure 13c). The \(WB_{\text{threshold}}\) is a key parameter for determining whether or not the forest is limited by water availability. In this study, we found \(WB_{\text{threshold}}\) of 129.1-238.8 mm/month, which varied according to the forest type (Figures 8 and 9). In addition, values of \(WB_{\text{threshold}}\) were not sensitive to the selected \(SPEI\) time-scale (Figure 10), implying that these thresholds were invariant and useful for evaluating the effects of drought. However, the roles of water availability in forest vigor were vegetation type-independent. The planted and natural forests showed similar rules due to the fact that temperature and water were the necessary resources for all forests [22; 54]; that is, both environmental climate (i.e., \(T\) and \(WB\)) and seasonality (i.e., pregrowing season or growing season) adjusted the sensitivity of forests to temperature and water.