## 4 Discussion
### Seasonally Dependent Responses of Carbon Fluxes to Environmental Factors  
Even over very short timescales, environmental factors have a significant influence on the variability of carbon fluxes. Some correlations between environmental variables and carbon fluxes vary seasonally [59; 60]. The NEP response combined the effects of temperature, precipitation, and other environmental factors on its two components: GPP and RE. Even in forest ecosystems with the same vegetation type, the response of NEP to a given forcing may differ significantly due to the competitive response of GPP and RE to environmental factors [61; 62]. The correlation between NEP and mean annual air temperature was weak in our selected forest ecosystems (Figure 3), which was in line with the findings of [PERSON] et al. [22]. The spring temperature had a greater effect on annual GPP than the mean temperature in other seasons, with a PC of 0.81 (Figure 4a). Warmer weather early in the season stimulated vegetation growth and CO\({}_{2}\) uptake, while temperature-enhanced RE dominated later in the season. This was consistent with the modeled mechanism proposed by [PERSON] et al. [59]. Across a range of biota, spring is the season with the strongest link between carbon fluxes and environmental factors [63]. Temperature is the main factor that directly regulates the growth cycle and potential photosynthetic activity of vegetation, thus directly affecting carbon fluxes [64]. This seasonal disparity in the sensitivity of carbon fluxes to temperature variations underscores the importance of considering seasonal dynamics when assessing carbon fluxes in forest ecosystems [65]. Precipitation is another important factor influencing changes in the trends of carbon fluxes [47], and it can directly determine both GPP and NEP [66]. Annual GPP and NEP increased with autumn precipitation (Figure 4e,f). In autumn, precipitation can also extend the growing season by maintaining favorable soil moisture conditions, which supports higher rates of carbon sequestration and leads to an increase in NEP [67]. A global forest study discovered that GPP became saturated in locations with an annual precipitation of more than 1500 mm [68]. Additionally, NEP tended to increase with summer precipitation, suggesting that environmental changes may have played a role in the increase in NEP in the region. This was consistent with the reported changes in moisture sensitivity in boreal forests [69] and may also be the result of post-infestation forest restoration and active forest management policies [70]. These findings underscore the complex interplay between precipitation and plant physiological responses in regulating carbon fluxes in forest ecosystems. In the summer and autumn seasons, high levels of radiation can suppress both gross primary production (GPP) and ecosystem respiration (RE) due to several interconnected physiological and environmental mechanisms (Figure 4c-f). Furthermore, high radiation often coincides with elevated temperatures, exacerbating water stress in plants. Under these conditions, plants close their stomata to minimize water loss through transpiration. However, this also limits CO\({}_{2}\) intake, which is necessary for photosynthesis, further reducing GPP. This stomatal closure is a protective mechanism but results in a trade-off where the plant's carbon sequestration capacity is compromised [71]. The increased respiratory demand under high temperatures and radiation can lead to a scenario where RE increases more than GPP, resulting in a net reduction in NEP [72]. In the ecological zone where this study was conducted, summer is the main vegetation growing season, and therefore NEP is more susceptible to climate warming and drought during the summer months. VPD is a key determinant of atmospheric dryness and an important control on ecosystem carbon dynamics [73]. The VPD was found to significantly suppress carbon fluxes at annual scales (Figure 3), which was consistent with previous studies [74; 75].