## 3 Results
### Relationships of WUE Trend with Driving Factor Trends  
The ridge regression coefficient was used to represent the effects of GPP and ET on WUE trends. Among the six major landscape types in the STC, GPP had a positive effect on WUE while ET had a negative effect. The effect of GPP on the trend of WUE was higher than that of ET, and the gap between GPP and ET was largest in cropland (Table 1). The effects of climate change, vegetation growth, and elevated atmospheric CO\({}_{2}\) concentration (i.e., MAT, MAP, Rg, SWC, VPD, Ca, and LAI) on interannual variations in WUE from 1982 to 2018 are shown in Figure 3. In the STC, MAT, LAI, Rg, and MAP had positive direct effects on the WUE trend; among them, the direct effects of MAT (\(\alpha\) = 0.49) and LAI (\(\alpha\) = 0.42) were the strongest, while those of Rg (\(\alpha\) = 0.10) and MAP (\(\alpha\) = 0.02) were the weakest. In contrast, VPD (\(\alpha\) = \(-\)0.20) and Ca (\(\alpha\) = \(-\)0.03) had negative direct effects on WUE. The direct effect of SWC on WUE was also relatively weak (Figure 3a). The spatial distribution of the direct effects of the drivers is shown in Figure S4. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **Whole** & **ECF** & **EBF** & **DBF** & **Shrub** & **Grassland** & **Cropland** \\ \hline GPP & 0.17 & 0.15 & 0.14 & 0.12 & 0.15 & 0.16 & 0.19 \\ ET & \(-\)0.06 & \(-\)0.06 & \(-\)0.07 & \(-\)0.07 & \(-\)0.06 & \(-\)0.06 & \(-\)0.05 \\ \hline \end{tabular}
\end{table} 
Table 1: Ridge regression coefficients of GPP and ET for the annual WUE trends for different landscapes in subtropical China. Figure 2: Spatial patterns of interannual trends in (a) WUE, (b) GPP, and (c) ET in subtropical China. The top insets indicate the frequencies of significances; spatial pattern of significances is shown in Figure S2. The right insets indicate the interannual trends of WUE, GPP, and ET of different landscapes, with whiskers indicating the standard deviation of all grid cell values. The total effect of the driver on the WUE trend is the sum of the direct effect and the cumulative indirect effects. The total effect of LAI on the WUE trend was very strong because the cumulative indirect and direct effects were both positive. LAI, MAT, and Ca had a strong positive correlation, and both LAI and MAT had a strong positive direct effect on the WUE trend; therefore, Ca had a strong positive indirect effect on the WUE trend through LAI and MAT. The opposite direct and indirect effects offset each other and resulted in a total positive effect of Ca. There was a positive correlation between VPD and  
Figure 3: Path diagrams between climate variables (average annual temperature (MAT), annual accumulated precipitation (MAP), and annual solar radiation (Rg), soil water content (SWC), vapor pressure deficit (VPD)), leaf area index (LAI), and elevated atmospheric CO\({}_{2}\) (Ca), and WUE in the study area. Numbers on arrows between each factor and WUE represent direct effects, and numbers in parentheses represent total effects. Numbers on arrows between every two factors represent correlation coefficients. (**a**) Whole of the study area, (**b**) ECF, (**c**) EBF, (**d**) DBF, (**e**) shrub, (**f**) grassland, (**g**) cropland. R\({}^{2}\) values beside the response variables represent the variance explained by the environmental factors and the constructed relationships. (\(\alpha\) indicates direct effect and \(\beta\) indicates total effect.) MAT, Rg, and LAI; the cumulative positive indirect effect resulted in a positive total effect of VPD on the WUE trend. The direct effect of SWC on the WUE trend was weak, and the total effect of SWC on the WUE trend was mainly due to the cumulative indirect effect. There was a positive correlation between MAT and LAI, VPD; however, LAI and VPD had opposite direct effects on the WUE trend. The opposite indirect effects by LAI and VPD offset each other and resulted in a weaker total effect of MAT. The characteristics of MAP were consistent with those of Rg, and the total effect was mainly driven by the direct effect.