## Materials and Methods
### Parameterization of the Biome-BGC Model  
Three files are required to run the Biome-BGC model: initialization file, meteorological data file, ecophysiological constants file. Initialization files are used to control the running process of the model. Firstly, the reading paths of the prepared meteorological and ecophysiological parameters files are specified. Then, the environmental information of each county needs to be described (Table 1), including latitude, altitude, effective depth of soil, soil texture, albedo, etc. Finally, the result variables of model simulation output are defined. The soil characteristic parameters in this study were based on the \"National 1:1 million Digital Soil Map\" developed by Nanjing Soil Research Institute of Chinese Academy of Sciences and Soil Environment Department of the Ministry of Agriculture of China. The meteorological data file is comprised of daily maximum temperature, daily minimum temperature, daily average temperature, daily precipitation, daily water vapor pressure difference, daily short wave radiation, and day length. Measured values from the national ground stations of China Meteorological Administration within the TGRA were used; each district (county) had a located meteorological station (Figures 1 and 2 shown the distribution of the stations), with a total of 34 years (from 1981 to 2014) of observations. The ecophysiological parameter files of the Biome-BGC model are based on seven types: deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), evergreen needle leaf forest (ENF), deciduous needle leaf forest (DNF), shrub forest (SHRUB), C3 grassland, and C4 grassland. The Numerical Terradynamic Simulation Group (NTSG) of the Montana University provided a set of default ecophysiological parameters, containing 42 parameters for each vegetation type. These default values are assigned based on the average value of parameters obtained from the literature research or the value determined by clustering analysis after literature retrieval and statistical analysis of each parameter for each community [35]. According to the forest vegetation types and the main species in the TGAR, the ecophysiological constants of _Pinus massoniana_ and oaks were used for the ENF and DBF, while the default parameters were used for the EBF and SHRUB. For the mixed coniferous broadleaved forest (MIX), the Biome-BGC model does not provide the corresponding ecophysiological parameters, making a direct assessment of these parameters impossible. However, previous studies have shown that we can effectively simulate the productivity of MIX forests by simulating the productivity of both vegetation types respectively and using a subsequent proportion conversion like 50% for each forest type [36; 37]. Some parameters, such as carbon allocation from roots to leaves, carbon allocation from stems to leaves, carbon allocation from thick roots to stems, carbon to nitrogen ratio of leaves, carbon to nitrogen ratio of leaf litter, carbon to nitrogen ratio of roots, and carbon to nitrogen ratio of woody parts, can be locally parameterized by actual measurement or literature findings [22; 23; 26; 38; 39; 40]; for other parameters that are difficult to be obtained, default values can be adopted, which are provided by the model (Table 2). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No** & **Counts** & **Latitude** & **Longitude** & **Altitude (m)** & **Soil Type** &
\begin{tabular}{c} **Annual** \\ **Precipitation (cm)** \\ \end{tabular} \\ \hline
1 & Fengjie & 31\({}^{\circ}\)01\({}^{\prime}\) N & 109\({}^{\circ}\)32\({}^{\prime}\) E & 607.3 & Yellow rendzina & 112.79 \\  
ulong & 29\({}^{\circ}\)19\({}^{\prime}\) N & 107\({}^{\circ}\)45\({}^{\prime}\) E & 277.9 & Yellow soil & 102.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Value of ecophysiological parameters of forests used in Biome-BGC.