## 4 Discussion
### Effects of Carbon Allocation to Leaves on Gross Primary Productivity  
There is a positive correlation between \(\Delta\)LAI and GPP in almost all forest types from April to June (Figure 8), indicating that the development of leaves can directly affect forest productivity. In addition, because of the implementation of various ecological restoration projects from 2001 to 2021, the development status of forest canopy leaves and their photosynthetic carbon sequestration capacity have displayed an upward trend through the years [45], resulting in a positive correlation between \(\Delta\)LAI and GPP. There are two main reasons for this situation. The first is the driving effect of photosynthesis. Leaves are the main site for plant photosynthesis, which is the process of energy conversion and carbon fixation in plants. Therefore, a larger leaf area provides more sites for photosynthesis and more energy and carbon sources for plants, thus promoting the total primary productivity of the plants [46]. The second reason is the absorption of nutrients and water. Better leaf development increases the root surface area available for the absorption of water and nutrients. Therefore, it enables trees to acquire water and nutrients more efficiently to support an increase in their total primary productivity [47].  
The leaf development in a given month has a time requirement for the promotion of GPP. For example, \(\Delta\)LAI in April does not directly increase GPP in April. It has the strongest positive correlation with GPP in May and is also positively correlated with GPP in other months, but the correlation gradually weakens. Similarly, the increase in \(\Delta\)LAI in May and June is more conducive to an increase in GPP in the later part of the growing season. The relationship between \(\Delta\)LAI and GPP is the result of the combined effects of internal physiological regulation of trees and external environmental influences. First, April is still within the transition season, when the weather is cooler, there is less sunlight, and the humidity is high. Therefore, although leaf area increases, there may not be a direct positive correlation with GPP in April [48]. Second, May is usually the month when spring transitions to early summer, which provides sufficient light energy and suitable temperatures, allowing plants to utilize leaf area for photosynthesis more effectively [49]. Finally, as the growing season progresses, during May and June, trees may have fully entered the growing season stage with greater growth and photosynthetic capabilities. At this time, plants can better utilize an increase in leaf area to enhance photosynthesis and carbon fixation capacity, so the increase in \(\Delta\)LAI in May and June is more conducive to an increase in GPP in the later part of the growing season [50].  
However, the impact of \(\Delta\)LAI on the GPP is not always positive, and the opposite phenomenon occurs for individual forest types at specific growth stages. Especially in MF, the increase in \(\Delta\)LAI in April has a positive effect on the GPP from April to June but becomes an inhibitory effect on the GPP in July and subsequent months. This phenomenon is caused by the overdevelopment of the canopy in the early part of the growing season, thereby promoting the initial GPP; however, too many leaves lead to an increase in water transpiration [51]. If the water supply in the environment is limited, excessive leaves may intensify the plant's water transpiration, causing it to face the risk of dehydration and water stress, which are not conducive to a later increase in GPP [52]. In addition, the increase in the number of leaves leads to the requirement of a high quantity of soil moisture and nutrients [53], which is not conducive to carbon and water synthesis in the later stage of photosynthesis [54].