# Relationship between CO2 Fertilization Effects, and Stand Age, Stand Type, and Site Conditions
## 5 Discussion
They suggest that due to environmental and physiological limitations of the older trees in their study area, the CO\({}_{2}\) concentration exceeded the maximum carbon absorption capacity of stands with older-aged trees [58; 59], so they will not react to a further increase in CO\({}_{2}\) concentration. Most of these studies focus on observing tree rings, which differ from the GPP levels we observed. Some studies have also found that the biomass distribution is different in older trees compared to younger trees, characterized by the transfer of excess carbon to the roots and the soil and reduced carbon transfer to the stem, resulting in the inability to reflect the fertilization effect of atmospheric CO\({}_{2}\) in tree rings [60; 61; 62]. Therefore, unlike our studies, studies on tree rings are not able to detect the effects of CFE on the GPP of older trees. In terms of the differences in the CFE among different stand types, our study found that coniferous forests had a lengthier stage of \(\beta_{GPP}\) increase compared to broad-leavedforests. We believe this is mainly due to different growth strategies and physiological adaptations among different stand types. Broad-leaved forests usually grow rapidly during the juvenile tree stage, but with the increase in tree age, the growth rate gradually slows down, entering the mature stage relatively early. Their responses to changes in the external environment are more pronounced. In contrast, juvenile coniferous forests grow relatively slowly, and their growth periods can last longer. We also found that the CFE in coniferous forests had a higher maximum value than in broad-leaved forests. This is because evergreen coniferous forests typically occupy poorer-quality habitats than deciduous broad-leaved forests and have higher nutrient uptake and water efficiency [63; 64; 65; 66]. The leaf N and P content of broad-leaved trees is higher, while the C content is lower than that of coniferous trees [65; 67]. Therefore, under similar N and P nutrient conditions, the leaf formation efficiency of evergreen coniferous forests is significantly higher than that of deciduous broad-leaved forests [68; 69]. As a result, coniferous forests have a higher photosynthetic potential under similar environmental conditions. SCI is a comprehensive evaluation of forest site conditions. Many studies have shown that a forest's growth status and productivity are closely related to site conditions [43; 70]. We found that the CFE under high-quality site conditions was significantly higher than that under medium- and low-quality site conditions, consistent with previous research findings on nutrient and water constraints [20; 21; 22; 52]. These studies suggest that areas poor nutrient and water conditions, such as low soil P and N content and arid environments, strongly limit CFE. In contrast, the limiting effect is weak or non-existent in areas with good nutrient and water conditions. A more suitable climate and terrain under high-quality site conditions can indirectly enhance the CFE. Our research also found that the age constraints are more pronounced due to the lower environmental constraints under high-quality site conditions, so the CFE in high-quality sites decreases faster with age. In addition, our results indicate that at the juvenile forest stage (1-20 years of age), the differences in CFE among different site quality levels were the greatest, and the environmental constraints were the strongest. This is due to the strong photosynthetic capacity of juvenile trees and their high demand for water, nutrition, and other inputs. As a result, site conditions significantly impact CO\({}_{2}\) fertilization. With increased stand age, the difference across site levels gradually narrows, and the difference between high-quality and medium-quality site conditions disappears at the aging forest stage (80-100 years of age). We suggest that this is mainly due to the low photosynthetic potential of aging forests and the small margin for further increase in photosynthesis.