## 4 Results
### Attributions of Historical GPP Changes  
(1) Spatial and temporal variation of historical GPP  
The changes in annual GPP in the different sub basins of the Yangtze River Basin are illustrated in Figure 5. In the whole Yangtze River, GPP increases with a slope of 9.18 gCm\({}^{-2}\)year\({}^{-1}\)a\({}^{-1}\), and it shows obvious growth trends in the Jialing River, Wujiang River, upper trunk stream, Dongting Lake and Poyang Lake, with the slopes of 15.74 gCm\({}^{-2}\)year\({}^{-1}\)a\({}^{-1}\), 11.16 gCm\({}^{-2}\)year\({}^{-1}\)a\({}^{-1}\), 14.49 gCm\({}^{-2}\)year\({}^{-1}\)a\({}^{-1}\), and 12.67 gCm\({}^{-2}\)year\({}^{-1}\)a\({}^{-1}\), respectively, while in Taihu Lake, GPP shows less growth trend within the slope of 1.35 gCm\({}^{-2}\)year\({}^{-1}\)a\({}^{-1}\). In general, the GPP in the Yangtze River Basin shows an increasing trend, but the increasing trends are different in different sub basins. The spatial distribution of GPP is shown in Figure 6. In spring and autumn, the GPP in the Yangtze River Basin is mostly concentrated around 150 gCm\({}^{-2}\)mon\({}^{-1}\); in summer, GPP is mostly concentrated around 300 gCm\({}^{-2}\)mon\({}^{-1}\); in winter, the GPP is mostly less than 40 gCm\({}^{-2}\)mon\({}^{-1}\). The regions with high GPP values are concentrated in the southwest and southeast of the Yangtze River Basin, Jialing River, and Hanjiang River. To further identify the historical trends of GPP changes in the Yangtze River Basin during different seasons, the slopes of the monthly mean GPP changes in different seasons are investigated as shown in Figure 7. The results show that the changing trends of GPP in the Yangtze River Basin are most obvious in spring, and the regions with significant increases occupy a large space, among which the growth trend is most significant in the Jialing River, Hanjiang River, and the upper trunk stream at greater than 3 gCm\({}^{-2}\)mon\({}^{-1}\)a\({}^{-1}\); in summer, the regions with significant growth are only concentrated in the Jinsha River, Jialing River, and part of Hanjiang River, while Dongting Lake and Poyang Lake have a certain decreasing trend. In autumn, the regions with significant increases are concentrated in the southwest of the Yangtze River Basin, with a growth slope of 2-3 gCm\({}^{-2}\)mon\({}^{-1}\)a\({}^{-1}\). In winter, the growth trend of GPP in the entire basin is slight and concentrated at 0-1 gCm\({}^{-2}\)mon\({}^{-1}\)a\({}^{-1}\) \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sub Basins** & **R\({}^{2}\)** & **NSE** &
\begin{tabular}{c} **RMSE** \\ **(gCm\({}^{-2}\)mon\({}^{-1}\))** \\ \end{tabular} \\ \hline Yangtze River & 0.97 & 0.97 & 11.03 \\ Jinsha River & 0.98 & 0.98 & 7.38 \\ Mintuo River & 0.93 & 0.90 & 19.35 \\ Jialing River & 0.95 & 0.95 & 18.00 \\ Wujiang River & 0.93 & 0.91 & 21.24 \\ Upper trunk stream & 0.94 & 0.92 & 20.90 \\ Dongting Lake & 0.90 & 0.89 & 19.93 \\ Hanjiang River & 0.96 & 0.95 & 20.15 \\ Poyang Lake & 0.85 & 0.81 & 21.97 \\ Middle trunk stream & 0.93 & 0.92 & 16.61 \\ Lower trunk stream & 0.85 & 0.81 & 21.87 \\ Taihu Lake & 0.85 & 0.76 & 22.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Model performance in monthly GPP simulation by LUE-RE model. Figure 5: Changes of annual gross primary productivity in the Yangtze River Basin. 2. Sensitivities of GPP to different climate factors  
The sensitivities of monthly GPP to different climate factors are shown in Figure 8. Figure 8a shows that GPP increases as CO\({}_{2}\) enrichment in nearly all the months, and GPP is most sensitive to elevated CO\({}_{2}\) in spring (April, May) and October, November, and December in the Yangtze River Basin. The highest sensitivity coefficient ranges between 0.389 (Jinsha River) and 0.651 (Jialing River) in the basin. The sensitivity of GPP to temperature (TEMP) changes is shown in Figure 8b indicating an obvious negative correlation from late spring (May) to autumn (October) and positive correlation from winter (November) to spring (April) in most subregions.