## 3 Materials and Methods
### Forest Aboveground Biomass Mapping  
Three kinds of forest AGB results were produced, including field-, Landsat- and ALS-based forest AGBs. Firstly, the field-based forest AGB was produced to build statistical models with the field based-LAI for different forest types. Based on these models and the Landsat-based LAI map, the Landsat-based AGB map of 2008 was created and served as the baseline AGB in the BEPS model to initialize the carbon pools. Finally, both the field- and ALS-based AGB worked as validation data to evaluate the BEPS result. * Field-based forest aboveground biomass  
At the individual tree level, we computed the biomass of leaf (\(W_{\text{L}}\)), stem (\(W_{\text{S}}\)), and branch (\(W_{\text{B}}\)) components of four different tree species (i.e., larch, pine, birch and aspen) based on the one or two factor allometric equations as Equations (3) and (4):  
\[W_{\text{i}}=\text{a}\times\left(D^{2}H\right)^{\text{b}} \tag{3}\]  
\[W_{\text{i}}=\text{a}\times D^{\text{b}} \tag{4}\]  
where \(W_{\text{i}}\) is the biomass (kg/m\({}^{2}\)) for leaf (i = L), stem (i = S), and branch (i = B), respectively; a and b are the coefficients; \(D\) and \(H\) are the DBH (cm) and tree height (m) of the individual tree. We then obtained total aboveground biomass of an individual tree by summing up the biomass of different aboveground components of an individual tree which calculated by 12 different allometric equations collected from existing published literatures [41, 42] (Table 1). At the forest plot level, the total forest AGB was achieved by summing up the AGBs of individual tree within the forest plot. * Landsat-based forest aboveground biomass  
Soil adjusted vegetation index (SAVI), which was proposed by [PERSON] [43] in 1988, was less sensitive to soil influence compared to other vegetation indexes. So, we produced the LAI map of 2012 based on the statistical model between field-based LAI and SAVI [43, 44]. Accordingly, the forest AGB map of 2012 was generated using the statistical relationship between forest field-based LAI and AGB at the plot level [45, 46]. Assuming that the statistical model between field-based LAI and AGB for the same study area with same forest types was identical in different years, the Landsat ETM+ image-based AGB map of 2008 was generated, and further served as the base map to update forest AGBs for each year from 2009 to 2012. * ALS-based forest aboveground biomass  
Based on the ALS data which explicitly contained the 3-D forest canopy structural information, we first filtered non-ground points and generated the normalized canopy surface model (NCSM) with the grid size as 1 m \(\times\) 1 m after removing topographic effect using the digital elevation model (DEM) [47, 48]. Then, the first return point cloud data, which were considered as the appropriate proxy to estimate forest AGB [49], were used to build the statistical model with forest plot AGB. Among each \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Species** & **Stem Biomass** & **Branch Biomass** & **Leaf Biomass** & **Source** \\ \hline
**Larch** & \(W_{\text{s}}=0.0461\times(D^{2}H)^{0.8722}\) & \(W_{\text{B}}=0.0356\times(D^{2}H)^{0.5624}\) & \(W_{\text{L}}=0.0140\times(D^{2}H)^{0.5628}\) & [41] \\
**Pine** & \(W_{\text{s}}=0.3364\times D^{2.0067}\) & \(W_{\text{B}}=0.2983\times D^{1.144}\) & \(W_{\text{L}}=0.2931\times D^{0.8486}\) & [42] \\
**Birch** & \(W_{\text{s}}=0.0494\times(D^{2}H)^{0.9011}\) & \(W_{\text{B}}=0.0142\times(D^{2}H)^{0.7686}\) & \(W_{\text{L}}=0.0110\times(D^{2}H)^{0.6472}\) & [41] \\
**Aspen** & \(W_{\text{s}}=0.2286\times(D^{2}H)^{0.6933}\) & \(W_{\text{B}}=0.0247\times(D^{2}H)^{0.7378}\) & \(W_{\text{L}}=0.0108\times(D^{2}H)^{0.8181}\) & [41] \\ \hline \hline \end{tabular} Note: Here \(D\) is the DBH (cm); H is the tree height (m); \(W_{\text{s}}\) is the stem biomass (kg/m\({}^{2}\)); \(W_{\text{b}}\) is the branch biomass (kg/m\({}^{2}\)); \(W_{\text{L}}\) is the leaf biomass (kg/m\({}^{2}\)).  
\end{table}