# Relationship between CO2 Fertilization Effects, and Stand Age, Stand Type, and Site Conditions
## 1 Introduction  
Since the Industrial Revolution, fossil fuel combustion has increased the atmospheric CO2 concentration from about 280 ppm [1] to 416 ppm in 2020 [2]. The IPCC Sixth Assessment Report indicates that over the past decade, human activities have emitted an average of 10.9 \(\pm\) 0.9 PgC yr\({}^{-1}\) of CO2, of which 31% (3.4 \(\pm\) 0.9 PgC yr\({}^{-1}\)) is stored in vegetation in terrestrial ecosystems [3]. The rapidly increasing concentration of CO2 in the atmosphere poses a significant challenge to global sustainable development. Globally, there is particular emphasis on the monitoring of CO2. The rapid development of techniques such as satellite-based estimation of atmospheric CO2 concentrations and monitoring pf CO2 emissions using mobile devices has enabled accurate measurements of CO2 concentrations at various scales. This advancement provides robust support for numerous ecological studies [4, 5]. Carbon uptake from terrestrial ecosystems has significantly slowed the rate of climate change. There is considerable evidence that vegetation photosynthesis has gradually increased over the past 60 years, along with the increase in anthropogenic CO2 emissions [6, 7], and terrestrial carbon uptake has continued to increase [8, 9, 10, 11]. This phenomenon is due to the increase in vegetation productivity caused by the increase in CO2 concentration [12, 13], termed the CO2 fertilization effect (CFE) [14]. CFE varies depending on plant architecture, plant physiology, and environmental factors. [15; 16; 17; 18; 19; 20]. Therefore, stand age, stand type, and site conditions may have a key impact on CFE. It has been demonstrated that the CFE is higher in juvenile trees and has a stronger effect than in mature and old trees [17; 19; 21]. However, only the differences between juvenile and old trees have been compared. Thus, a comprehensive evaluation of the relationship between CFE and the overall stand age is lacking. Moreover, thus far, studies have not emphasized the differences between different stand types and site conditions. This has hindered the in-depth understanding of the forest carbon cycle and rational formulation of policies to address climate change. Existing studies typically utilize traditional methods such as indoor CO\({}_{2}\) control experiments, free-air CO\({}_{2}\) enrichment (FACE) experiments, flux tower observations, and tree-ring observations [17; 22; 23; 24]. However, these methods have certain drawbacks. Due to space constraints, indoor experiments can only focus on juvenile trees and cannot evaluate mature trees [25; 26]. Although FACE solves some of the limitations of indoor experiments [27], the increase in CO\({}_{2}\) concentration in the FACE experiment is sudden, which differs from the slow rise in atmospheric CO\({}_{2}\) concentration in nature [21]. Therefore, whether the results of FACE experiments can reflect the changes in tree physiology and growth in the natural environment is debatable. Flux tower and tree-ring observation experiments only indicate the level of the sample point, and their spatial continuity is low, in addition to issues with the short time frames during these experiments. Gross primary production (GPP) denotes the total amount of carbon fixed in the process of photosynthesis by plants in an ecosystem. Quantifying the GPP of terrestrial ecosystems has been an important part of quantifying the global carbon cycle [3]. Moreover, GPP plays an important role in quantifying CFE and investigating the patterns of change in CFE [15]. The carbon cycle process model and remote sensing methods can achieve long-term continuous observation of GPP in a large-scale spatial range, which solves the shortcomings of traditional methods. However, due to the assumptions of the carbon cycle process model, it is difficult to fully parameterize the complex physiological processes of plants, and there can be a great deal of uncertainty in the predicted results [28; 29].