# Relationship between CO2 Fertilization Effects, and Stand Age, Stand Type, and Site Conditions
## 5 Discussion  
Our study found that in Heilongjiang, China, both broad-leaved and coniferous forests exhibited substantial differences in CFE depending on the age of the trees and the quality of the site. Initially, there was a rapid increase in the CFE in young trees, reaching a maximum between 10 and 20 years, with a gradual decline until the stand age reached 100 years, at which point the CFE tended to stabilize. These results indicate that the ability of the same forests to offset elevated CO\({}_{2}\) in the atmosphere at different growth stages is not consistent, and this ability has a significant nonlinear relationship with stand age. Moreover, we found that the CFE differed across stand types. Compared to broad-leaved forests, the CFE in coniferous forests had a longer period of rapid increase and a higher maximum value. We also found that the quality of the site was critically important, with young trees at high-quality sites having CFE values more than twice those at low-quality sites. However, in more mature stands (80-100 years old) the CFE values of high-quality sites were much lower--only slightly higher than those of low-quality sites. In our study, we used different GPP data products to estimate CFE, and we found that the results of different products were relatively consistent, enhancing the authenticity of the reported results. Several factors have been suggested to enhance the CFE. (a) Higher CO\({}_{2}\) concentrations in the external environment directly increase the CO\({}_{2}\) content and the ratio of CO\({}_{2}\) to O\({}_{2}\) in green vegetation leaves, which stimulates carboxylation by the Rubisco enzyme in the leaves to increase the photosynthetic rate of the vegetation [19]. (b) Increased CO\({}_{2}\) concentration reduces the degree of stomatal opening in leaves, improves water use efficiency in vegetation, and stimulates GPP [45]. (c) Increasing the CO\({}_{2}\) concentration may promote an increase in leaf area index (LAI), corresponding to a larger light interception area, thereby increasing the photosynthesis output [46; 47]. The juvenile stage of trees is a critical period for their growth and development. When the carbon storage capacity of trees is low, rapid material accumulation is required, and the cells in the leaves have a strong metabolism and strong photosynthesis potential [48]. Increasing the atmospheric CO\({}_{2}\) concentration can help to promote tree photosynthesis, increase nutrient and water absorption and utilization efficiency [21], and promote their growth and development. Therefore, vegetation has a higher demand for CO\({}_{2}\) and a higher CFE during the juvenile stand stage. The LAI is also relatively low in a juvenile tree stand. With the increase in stand age, the amount of vegetation increases, and the LAI increases rapidly [49], which may be an important reason for the rapid increase in CFE. As the age increases, the forest's carbon absorption and turnover gradually reach equilibrium [17]. Several studies have shown that trees are subjected to more severe nutrient and water restrictions as they age [22; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52], and this restriction may become more severe as the atmospheric CO\({}_{2}\) concentration increases [53; 54; 55]. This limiting effect further inhibits CFE, so the intensity of CO\({}_{2}\) fertilization begins to decline as the tree age increases. The adaptation of trees to gradually increasing atmospheric CO\({}_{2}\) concentrations may also be an important factor affecting the reduced impact of the CFE [56]. Our findings are consistent with observations using radial growth of trees, such as those of [PERSON] et al. [21], that the CFE in juvenile trees is higher than that in old trees. However, some studies suggest that CFE is absent during the tree aging stage [19; 57].