## 5 Discussion  
### Driving Factors on Historical GPP Variations  
At annual scale, GPP shows an increasing trend in the Yangtze River Basin, which can be observed almost globally and is consistent with many previous studies [32; 33]. However, GPP in the Yangtze River Basin does not maintain an increasing trend throughout the year at seasonal scale, and it increases mainly in April and May. Based on the sensitivity and contribution analysis of GPP, it can be seen that the main factor affecting the changes in GPP in spring is elevated CO\({}_{2}\), indicating that elevated CO\({}_{2}\) dominates the GPP changes in the Yangtze River Basin. At the same time, it can be seen that the impact of elevated CO\({}_{2}\) on GPP is mainly concentrated in spring and autumn, and the reason for the smaller increase in GPP in autumn is that the negative effects of precipitation and solar radiation offset a portion of the positive effects of elevated CO\({}_{2}\). From previous studies, it can also be seen that the fertilization effect of carbon dioxide is the most important factor in increasing vegetation productivity [34; 35]. The increase in CO\({}_{2}\) concentration can enhance photosynthesis by controlling the closure of stomata and improving water use efficiency [36]. Figure 16 shows the correlations between the sensitivity of GPP to elevated CO\({}_{2}\) (deCO\({}_{2}\)), temperature (dTEMP), precipitation (dPREC), and solar radiation (dSRAD) with temperature and precipitation in different months. It can be observed that the impact of elevated CO\({}_{2}\) on GPP is highest in spring when temperatures warm up and vegetation begins to grow. This may be due to the fertilization effect of carbon dioxide improving the water use efficiency of vegetation, leading to a significant increase in GPP. It can also be seen that in summer, when there is more precipitation, the correlation between deCO\({}_{2}\) and precipitation shows negative values. The more precipitation, the less obvious the response of GPP to carbon dioxide fertilization in regions, which can also prove that the fertilization effect of CO\({}_{2}\) mainly affects vegetation during periods of water scarcity. At the same time, there is also a certain correlation between deCO\({}_{2}\) and temperature, mainly showing a positive correlation,  
Figure 15: Changes of future vs. historical GPP in different sub basins under different scenarios. which proves that an increase in temperature is beneficial for enhancing the fertilization effect of carbon dioxide. However, a negative correlation in summer also indicates that there is an optimal temperature. If the temperature is too high, it may inhibit the fertilization effect of carbon dioxide. In addition, the prolonged growth season of vegetation due to temperature rise further strengthens the fertilization effect of carbon dioxide. Although changes in elevated CO\({}_{2}\) play a dominant role in vegetation change, the impact of climate variables on vegetation cannot be ignored. Many studies have shown that precipitation, temperature, and radiation are important factors affecting vegetation productivity. This study found that the sensitivity of GPP to precipitation is relatively low, mainly because the Yangtze River Basin has a large amount of precipitation, which is relatively humid, and the limiting effect of water on it is relatively low. From the correlation between dPREC and precipitation, it can be seen that they mainly show a negative correlation, proving that precipitation in areas with lower precipitation has a greater impact on vegetation productivity. It can also be found from other studies that precipitation plays a significant role in water-limited semi-arid ecosystems and is the main cause of vegetation changes [37]. Temperature affects various physiological processes of plants. Before reaching the optimal temperature, an increase in temperature usually increases plant photosynthesis and promotes plant growth.