## 5 Discussion
### GPP Changes in the Future  
In the future, GPP shows a significant upward trend, indicating that the carbon sequestration capacity of the Yangtze River Basin is constantly increasing. It can be clearly observed that under the SSP1-2.6 scenario, the GPP shows a downward trend from rapid growth to slow growth, indicating that the maintenance of stable or even decreasing elevated CO\({}_{2}\) leads to a decrease in the fertilization effect of CO\({}_{2}\), and the decrease in elevated CO\({}_{2}\) also leads to the maintenance of stable temperature, further weakening the negative effects of spring and autumn temperatures on vegetation, and alleviating the pressure of summer high temperatures on GPP. While under other scenarios, GPP increases with the continuous increase in elevated CO\({}_{2}\). From the future trends of GPP changes in the four scenarios, it can be seen that the changes in GPP are almost consistent with the changes in elevated CO\({}_{2}\). Over the past century, the terrestrial biosphere has responded to anthropogenic carbon dioxide emissions with the greatest increase in photosynthetic activity, which is proportional to the increase in atmospheric carbon dioxide concentration [42]. This phenomenon of GPP proportional to the increase in atmospheric elevated CO\({}_{2}\) will continue in the future, and elevated CO\({}_{2}\) will still dominate the changes in GPP. This is mainly because the increase in elevated CO\({}_{2}\) leads to a significant improvement in photosynthesis and water use efficiency at the leaf level of vegetation, and this change may alleviate the impact of future drought stress on vegetation [43]. The future growth of GPP in the Yangtze River Basin will be most significant in spring and autumn, partly due to the fertilization effect of elevated CO\({}_{2}\), and partly due to the positive impact of temperature rise. The increase in temperature will prolong the growth season of vegetation, leading to a larger leaf area in earlier spring and delaying the aging time of vegetation in autumn, resulting in a longer growth cycle of vegetation [44], combined with the fertilization effect of elevated CO\({}_{2}\), resulting in the highest growth trend of GPP in spring and autumn. In other stable climate conditions, vegetation will be within a specific temperature range, below the optimal temperature. Increasing temperature can promote enzyme activity, but beyond this temperature range, excessive heat can lead to stomatal closure and enzyme inactivation [45]. In summer, as the optimal temperature has already been exceeded, future high temperatures may have a significant negative impact on GPP, and the increasing temperature may have an increasing negative impact on vegetation, resulting in a lower growth rate of GPP. However, high concentrations of CO\({}_{2}\) can still bring positive effects to GPP growth in summer, and the positive effect of this CO\({}_{2}\) is higher than the negative impact of temperature rise, resulting in GPP being able to maintain an increasing level during the summer. The growth of GPP may lead to an increase in evapotranspiration, or affect runoff, resulting in a decrease in runoff [46]. Based on the above results, it can be seen that the vegetation productivity in the Yangtze River Basin will continue to increase under the conditions of climate change in the future, which may also lead to changes in hydrological processes. This situation may be more severe in spring, and the increase in vegetation productivity indicates that the water demand of future vegetation may increase, leading to an increase in transpiration, affecting water infiltration, and ultimately leading to a decrease in runoff. At the same time, due to the low precipitation in spring, it may cause the occurrence of drought events. Although there is more precipitation in summer, the risk of drought may also be further exacerbated by future temperature increases and increased vegetation productivity.