## 4 Discussion  
### Changes in Environmental Factors Lead to Consistent or Decoupled Trends in GPP and NEP  
Since carbon fluxes are an important component of forest ecosystems, understanding the intricate connections between environmental factors and carbon fluxes is crucial for determining the variability of ecosystem carbon fluxes and enhancing future projections. Our study found that the response of NEP to environmental changes was dependent on the combined response of GPP and RE. The combined response of ecosystem GPP and RE to the environment determines NEP, rather than a single physiological process [37]. Temperature, precipitation, and VPD are important controlling factors for GPP and NEP in most areas worldwide. Mechanistically, temperature influences enzymatic activities related to photosynthesis and respiration, thus impacting GPP and NEP. Photosynthetic enzymes have temperature-dependent activity, and higher temperatures generally enhance photosynthetic rates up to an optimal point (Figures 3 and 4), beyond which the rates decline due to thermal inhibition [38; 39]. Precipitation affects soil moisture availability, influencing plant water status and stomatal conductance, which in turn affects photosynthesis and respiration processes. Adequate soil moisture promotes stomatal opening, allowing CO2 uptake for photosynthesis, thereby increasing GPP (Figure 4b). Conversely, soil moisture deficits lead to stomatal closure to conserve water, thus reducing GPP (Figure 4d) [40]. We found a consistent trend in GPP and NEP when temperature and precipitation had significantly opposite trends and when temperature had a significantly positive correlation with VPD in the year, spring, summer, and autumn (Tables 4-6). [PERSON] [41] reported that forests maintain the hydrological cycle through evapotranspiration, which cools the climate through feedback from precipitation, with temperature trending in the opposite direction to precipitation. There is also a close relationship between temperature and VPD. An increase in temperature leads to an increase in the rate of evaporation, which increases the VPD [42]. Vegetation will reduce water loss by closing stomata in response to high temperatures and high VPD conditions [43]. Therefore, GPP and NEP were consistent with each other in the spring, summer, and autumn when the temperature and VPD trends were the same (Tables 4-6), which agreed with the findings of [PERSON] et al. [44]. When SW was synchronized with temperature changes in summer, GPP and NEP changed in tandem (Table 5). [PERSON] et al. [45] found that summer temperatures were increasing in temperate forests in the eastern United States through a study of global climate patterns, and the increasing levels of radiation enhanced the GPP [46]. Several studies have also shown that the spatial variation of GPP is mainly determined by temperature, with GPP increasing as temperature increases [47; 48] and our findings are consistent with that (Figures 4 and 5). At forest sites where GPP and NEP were decoupled, a higher VPD led to higher transpiration rates but decreased stomatal conductance to CO2 and water vapor, which in turn decreased GPP. Under hydrological stress, the VPD limits stomatal conductance and evapotranspiration more than precipitation in a variety of humid and semi-humid biomes (Figure 6) [49]. However, when there is no water stress, the opposite relationship may occur, i.e., a higher VPD promotes GPP [50], which leads to a decoupling of GPP and NEP in this forest ecosystem (Figure 4f). In cold temperate forest ecosystems, summer precipitation contributes even more to the increase in annual NEP [51], which may result from the leaf expansion's long-term impacts and the stresses that occur during early wood production, and our analysis of materiality found conclusions that are consistent with it (Figure 6c). In contrast, a substantial decrease in annual NEP could result from the dryness occurring between germination and complete leaf growth, with non-synchronous changes in GPP and NEP [52].