## 2 Materials and Methods
### FORCCHN Model Description
#### 2.2.2 The Main Equations of the Model  
The gross primary productivity of an individual tree is given by the equation:  
\[GPP_{i}=\min(GPPM_{i}\times f_{c}\times f_{dry}\times f_{T},R_{c/n}\times S_{n}) \tag{1}\]  
where \(GPP_{i}\) is the daily gross primary productivity of individual trees; \(GPPM_{i}\) is the maximal daily gross primary productivity of individual trees; and \(f_{c}\),\(f_{dry}\),\(f_{T}\), and \(R_{c/n}\times S_{n}\) represent the effects of carbon dioxide, water, temperature, and soil available nitrogen on \(GPP\), respectively. \(S_{n}\) is the soil available nitrogen, and \(R_{c/n}\) is the C/N ratio parameter of the assimilation. \[GPPM_{i}=\frac{2\times Am_{j}\times Dl}{Kl_{j}}\times\ln\frac{1+\sqrt{1+Kl_{j} \times Sl_{j}\times\frac{PAR_{i}}{Am_{j}}}}{1+\sqrt{1+Kl_{j}\times Sl_{j} \times\frac{PAR_{i}\times\exp(-Kl_{j}\times LAl_{j})}{Am_{j}}}} \tag{2}\]  
where \(Dl\) (daylength) is the possible sunshine duration (h), \(PAR_{i}\) is the noon canopy photosynthetically active radiation of the individual tree (Wm\({}^{-2}\)), and \(LAI_{i}\) is the leaf area index of the \(i\)th tree. For the \(i\)th individual in the \(j\)th plant functional type, \(Am_{j}\), \(Kl_{j}\), and \(Sl_{j}\) represent the maximal photosynthesis [kgC/(m\({}^{-2}\cdot\)h\({}^{-1}\))], the extinction coefficient, and the initial slope of light intensity and photosynthesis [kgC/(m\({}^{2}\cdot\)h))/(W\(\cdot\)m\({}^{-2}\))], respectively. \[f_{c}(C_{s})=1+\frac{C_{s}-C_{0}}{C_{s}+2C_{0}} \tag{3}\]  
where \(C_{s}\) is the CO\({}_{2}\) concentration of the simulation year, \(C_{0}\) is the CO\({}_{2}\) reference concentration, and the atmospheric CO\({}_{2}\) concentration was approximately 410 ppm in 2019, according to NASA observations [61; 62]; therefore, \(C_{0}\) was set at 410 ppm. \[f(sw,rh)=\left\{\frac{\min\left[1,\frac{sw}{FC}+\max(rh-0.5,0.1)\right]}{dry} \right\}^{0.5} \tag{4}\]  
where \(sw\) is the soil water content (cm), \(FC\) is the field capacity (cm), \(rh\) is the air relative humidity, and \(dry\) is the individual's capability of enduring drought, which ranges from 0 to 1. \[f_{T}(T)=(\frac{T_{\max}-T}{T_{\max}-T_{opt}})^{\frac{T_{\max}-T_{opt}}{T_{ \max}-T_{\min}}}\times(\frac{T-T_{\min}}{T_{opt}-T_{\min}})^{\frac{T_{opt}-T_{ \min}}{T_{\max}-T_{\min}}} \tag{5}\]  
where \(T_{opt}\) is the photosynthetic optimum temperature (\({}^{\circ}\)C), \(T_{max}\) is the photosynthetic maximum temperature (\({}^{\circ}\)C), \(T_{min}\) is the photosynthetic minimum temperature (\({}^{\circ}\)C), and \(T\) is the average daily temperature of the day (\({}^{\circ}\)C). \begin{table}
\begin{tabular}{c c c c} \hline
**Symbol** & **Unit** & **Carbon Pool** & **Value** \\ \hline S\({}_{1}\) & 1/d & Aboveground metabolic litter pool & 0.080 \\ S\({}_{2}\) & 1/d & Aboveground structural litter pool & 0.021 \\ S\({}_{3}\) & 1/d & Belowground metabolic litter pool & 0.100 \\ S\({}_{4}\) & 1/d & Belowground structural litter pool & 0.027 \\ S\({}_{5}\) & 1/d & Fine woody litter pool & 0.010 \\ S\({}_{6}\) & 1/d & Coarse woody litter pool & 0.002 \\ S\({}_{7}\) & 1/d & Belowground coarse litter pool & 0.002 \\ S\({}_{8}\) & 1/d & Active soil organic matter pool & 0.040 \\ S\({}_{9}\) & 1/d & Slow soil organic matter pool & 0.001 \\ S\({}_{10}\) & 1/d & Inert soil organic matter pool & \(3.5\times 10^{-5}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of soil decomposition rate in the FORCCHN model. The autotrophic respiration of each plant includes maintenance respiration and growth respiration.