## 3 Results
### The Effects of Climatic Factors on the GSL, GPPmax, and GPP  
Climate change affects processes, such as the GSL and GPPmax, ultimately leading to an increased annual GPP. During 2001-2020, the annual GPP was significantly correlated with TEM in natural and planted forests (R = 0.39, R = 0.40; \(p<0.05\), respectively), but not with PRE and RAD (Figure 6). The relationships between the GPP and climatic factors were similar in natural and planted forests. The extension of the GSL is largely determined by the temperature rise. The GSLs of natural and planted forests were highly correlated with temperature during 2001-2020 (natural forests: R = 0.50 (\(p<0.05\)); planted forests: R = 0.60 (\(p<0.05\)); Figure 6). For example, the GSL extended by 2.56 and 4.17 days per degree with the increase in TEM for natural and planted forests during 2001-2020, respectively. The correlation between the  
Figure 5: A conceptual framework for the regulation of the growing season length (GSL) and maximum gross primary productivity (GPPmax) in the long-term trend in the annual gross primary productivity (GPP) in natural forests (**a**), and planted forests (**b**), for the period 2001–2020. The variation in the GSL was affected by the variation in the leaf unfolding date and leaf falling date. Solid lines indicate significant changes, while dotted lines indicate non-significant changes. Figure 6: The relationships between the annual gross primary productivity (GPP), growing season length (GSL), and maximum gross primary productivity (GPPmax) and the annual mean temperature (TEM), annual sum precipitation (PRE), and annual sum global radiation (RAD) across natural and planted forests during 2001–2020. Note: the white font indicates \(p<0.05\). GSL and climatic factors of planted forests was larger than with natural forests, indicating that planted forests were more sensitive to temperature rises. However, the GPPmax for both natural and planted forests was not correlated with three climate variables (Figure 6), because the GPPmax was also influenced by other environmental factors (e.g., rising CO\({}_{2}\) and nitrogen deposition). From 2001 to 2020, the GPP was significantly correlated with TEM in summer (R = 0.52 and 0.51; \(p<0.05\), respectively, in natural and planted forests; Figure 7). The GPP increased by 91.33 and 131.34 (gC/m\({}^{2}\)/yr)/\({}^{\circ}\)C with the increase in TEM in summer for natural and planted forests during 2001-2020, respectively (Table S1). The relationships between the GPP and climatic factors (i.e., TEM, PRE, and RAD) in different seasons were similar in natural and planted forests from 2001 to 2020 (Figure 7). The correlation between the GPPmax and TEM in summer were also significant (R = 0.71 and 0.59; \(p<0.05\), respectively, in natural and planted forests; Figure 7). The GPPmax increased by 16.34 and 18 (gC/m\({}^{2}\)/mon)/\({}^{\circ}\)C with the increase in TEM in summer for natural and planted forests, respectively (Table S2). In addition, the increase in TEM in spring led to the extension of the GSL over 2001-2020 (R = 0.71 and 0.82; \(p<0.05\), respectively, in natural and planted forests; Figure 7). The GSL extended by 2.39 days \({}^{\circ}\)C and 3.59 days/\({}^{\circ}\)C with the increase in TEM in spring for natural and planted forests (Table S2). The increase in RAD in spring and winter also led to an extension in the GSL (Figure 7). Therefore, compared with natural forests, the sensitivity of the GPPmax and GSL to climatic factors (i.e., TEM, PRE, and RAD) in different seasons was larger in planted forests from 2001 to 2020 (Figure 7). Figure 7: The relationships between the annual gross primary productivity (GPP), growing season length (GSL), and maximum gross primary productivity (GPPmax) and the annual mean temperature (TEM), annual sum precipitation (PRE), and annual sum global radiation (RAD) in different seasons across natural and planted forests during 2001–2020. Note: the white font means \(p<0.05\).