## 4 Discussion
### The Effect of Climate Change on the GSL, GPPmax, and GPP  
Climate change influenced both the GPPmax and the GSL, essential determinants of the annual GPP [58; 59]. Our results showed that the annual GPP increased significantly due to the large increase in the GPPmax. Specifically, the GPPmax was significantly correlated with the TEM in the summer. This is because the high values in ecosystem fluxes appear in summer in the Northern Hemisphere [49]. Our results were consistent with previous studies, which indicated that the rising annual mean temperature would increase GPP. For example, a one degree increase in the annual mean temperature would lead to an increase in GPP of 43.76 gC/m\({}^{2}\)/yr in natural forests, while planted forests will increase by 69.18 gC/m\({}^{2}\)/yr [26]. A 1.3 \({}^{\circ}\)C increase in temperature stimulated the positive changing trend in the net primary productivity in China during 1961-2010 [60]. However, [PERSON] et al. (2010) reported that climate warming stimulates photosynthesis more than respiration in the spring [31], which means that ecosystems with a higher GPPmax tend to have a larger annual GPP at the same GSL [30]. A vapor pressure deficit was also found to be a dominant control for stomatal conductance and, thus, for limiting photosynthesis [61]. The differences indicated that climate change in different seasons enormously impacted the carbon cycle of deciduous forest ecosystems. Climate warming caused the advance of the leaf unfolding date, affecting the annual GPP, by influencing the GSL. Climate warming usually leads to a longer GSL, stimulating the GPP [62]. The response of the vegetation phenology to temperature was well divided between the advance in the leaf unfolding date and the delay in the leaf falling date. We found a significant relationship between the leaf unfolding dates and TEM in spring in China's deciduous forest ecosystems (2.7 days/\({}^{\circ}\)C). [PERSON] et al. (2011) found that the Northern Hemisphere's leaf unfolding dates advanced 5.2 days during 1982-1999 due to climate warming [63]. [PERSON] et al. (2022) indicated that warmer temperatures during the previous growing season led to an earlier leaf unfolding date in the current year in temperate and boreal forests [64]. However, an earlier leaf unfolding date may result in less available water in the soil, and limited plant growth later [32]. We did not find a significant correlation between the leaf falling date and climatic factors in China's deciduous forest ecosystems. [PERSON] et al. (2014) reported that a warmer temperature led to a later leaf falling date, with a 1 \({}^{\circ}\)C temperature change leading to a 1.8-day change in the leaf falling date [65]. A possible reason was the large uncertainty in estimating the key climate factors for vegetation phenology changes in the autumn [66]. The precipitation and global radiation also play essential roles in temperate and boreal ecosystems, in addition to the temperature. In a forested savannah, the net ecosystem exchange reached up to 2.5 tons C/ha when the precipitation rose to 585 mm/yr [67]. More precipitation could improve the soil water supply, enabling a longer carbon gain in autumn, while higher radiation might enable a greater net carbon gain as the day length becomes shorter [68]. Besides climatic factors, the land carbon cycle might be influenced by the rising CO\({}_{2}\) concentration [5; 6], increasing atmosphere nitrogen deposition [7], changing vegetation cover [4], vegetation recovery [69], and the agricultural Green Revolution [70]. However, it was noteworthy that the primary productivity might be weakened by deforestation [7] and the increasing risks of drought and heat [71]. Most studies have focused on the effect of CO\({}_{2}\) on the GPP. In particular, the elevated atmospheric CO\({}_{2}\) concentration substantially contributes to vegetation productivity. [PERSON] et al. (2020) found an increase of 11.6% in the global GPP with a rise of 100 ppm in the atmospheric CO\({}_{2}\)[72].