## 3 Results
### Attribution of Variation in WUE  
#### 3.3.1 Actual Contribution of Driving Factors  
The spatial patterns of the actual contributions of GPP and ET to the variation in WUE (Figure 4a,b, respectively) were as follows: for the whole region, GPP change contributed to a WUE change of 0.0014 g C/m\({}^{2}\)/mm/year, while ET contributed to a WUE change of \(-0.0020\) g C/m\({}^{2}\)/mm/year. These findings suggest that the increase and decrease in WUE are mainly affected by changes in GPP and ET, respectively. Among the six major landscapes, ET changes negatively contributed to the WUE of all landscapes while GPP changes positively contributed to the WUE of DBF, grassland, and cropland but negatively contributed to the WUE of ECF, EBF, and shrubland (Table 2).  
The actual contribution maps of the climate change, vegetation growth, and elevated atmospheric CO2 concentration (i.e., MAT, MAP, Rg, SWC, VPD, Ca, and LAI) for the WUE trend from 1982 to 2018 are shown in Figure 4. Overall, the changes in LAI and MAT contributed to increases in WUE (Table 2) of 0.0026 g C/m\({}^{2}\)/mm/year and 0.0002 g C/m\({}^{2}\)/mm/year in WUE, respectively. Moreover, changes in Ca, VPD, and MAP caused decreases in WUE of 0.0021 g C/m\({}^{2}\)/mm/year, 0.0012 g C/m\({}^{2}\)/mm/year, and 0.0001 g C/m\({}^{2}\)/mm/year, respectively, while changes in SWC and Rg contributed weakly to the WUE trend (Table 2).  
The actual contributions of climate change, LAI changes, and Ca to the WUE trends of different landscape types in the STC show that LAI changes made the largest contribution to the EBF, DBF, and cropland WUE trends at \(-0.0045\) g C/m\({}^{2}\)/mm/year, 0.0036 g C/m\({}^{2}\)/mm/year, and 0.0078 g C/m\({}^{2}\)/mm/year, respectively. For grasslands, Ca had the largest contribution to the WUE trend at \(-0.0043\) g C/m\({}^{2}\)/mm/year, while for shrubland, VPD contributed the most to the WUE trend at 0.0029 g C/m\({}^{2}\)/mm/year. In ECF, LAI changes, Ca, and VPD contributed equally to the WUE trends. The actual contributions of the drivers in different landscape types to WUE trends are listed in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **Whole** & **ECF** & **EBF** & **DBF** & **Shrub** & **Grassland** & **Cropland** \\ \hline GPP & \(14\) & \(-9\) & \(-10\) & \(5\) & \(-1\) & \(105\) & \(18\) \\ ET & \(-20\) & \(-12\) & \(-26\) & \(-22\) & \(-6\) & \(-105\) & \(-6\) \\ LAI & \(26\) & \(-8\) & \(-45\) & \(36\) & \(-15\) & \(70\) & \(78\) \\ Ca & \(-21\) & \(-5\) & \(15\) & \(-24\) & \(-21\) & \(-43\) & \(-32\) \\ SWC & \(0\) & \(0\) & \(-1\) & \(0\) & \(0\) & \(0\) & \(0\) \\ VPD & \(-12\) & \(-7\) & \(5\) & \(-29\) & \(29\) & \(-20\) & \(-41\) \\ MAT & \(2\) & \(0\) & \(2\) & \(2\) & \(0\) & \(1\) & \(4\) \\ MAP & \(-1\) & \(-2\) & \(-13\) & \(-4\) & \(-1\) & \(-2\) & \(2\) \\ Rg & \(0\) & \(1\) & \(1\) & \(2\) & \(1\) & \(-7\) & \(0\) \\ \hline \end{tabular}
\end{table}
Table 2: Actual contributions of GPP, ET, climate change, LAI, and Ca to annual WUE trends for different landscapes in subtropical China (\(\times 10^{-4}\) g C/m\({}^{2}\)/mm/year).  
Figure 4: Spatial variations in the actual contribution of (**a**) GPP, (**b**) ET (**c**) LAI, (**d**) Ca, (**e**) SWC, (**f**) VPD, (**g**) MAT, (**h**) MAP, and (**i**) Rg to WUE in the subtropical region of China (STC).