## 2 Materials and Methods
### In situ Sample Plot Data Collection  
Based on the historic forest survey data, 10 forest types were chosen for this study. The forest types were first analyzed using the 8 th National Forest Inventory (NFI) data, and then the top seven tree species were selected according to the stock volume. These species included _Cunningham lancelata, Pinus massoniana, Quercus_ sp., _Pinus elliotti, Populus_ sp., _Cinnamoun camphora_, and _Cupressus funebris_. In addition, three mixed forest types were selected, including broad-leaved mixed forests, coniferous and broad-leaved mixed forests, and coniferous mixed forests. These ten types account for over 98% of the total FSV in Hunan (Table 1).  
Figure 1: The study area, Hunan Province, and the distribution of the sampling plots (training data and test data). Note: “Forest” is defined as a natural forest with an area larger than 0.5 ha and forest cover over 10%.  
A total of 15 survey levels were generated in each forest type. Each forest survey type was further divided into five tree height levels and three canopy density degrees (0.2-0.39, 0.4-0.69, and 0.7-1.0). At each surveyed level, four sample plots were designed and investigated. Finally, 459 useful circle sample plots with a diameter of 30 m were obtained from 15 October 2017, to 30 April 2018. The tree heights (measured by a VERTEX LASER VL5 manufactured in Haglof, Sweden) and diameters at breast height (DBHs) were measured for all individual trees (measured by a caliper with a DBH greater than 5 cm) in the circle sample plot. TBSJDPT (Figure 2) was put forward and used to locate the sample plot center and the individual trees [39].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample Plot Type** & **Stock Volume (m\({}^{3}\))** & **Proportion (\%)** \\ \hline Total forest stock volume & 330,992,700 & 100.00 \\ _Cunningham lanccolata_ & 109,035,700 & 32.94 \\ _Pinus massoniana_ & 46,395,900 & 14.02 \\ _Quercus_ sp. & 5,098,800 & 1.54 \\ _Pinus ellotti_ & 3,020,400 & 0.91 \\ _Populus_ sp. & 2,022,900 & 0.61 \\ _Cinnamomum camphora_ & 2,244,100 & 0.68 \\ _Cupressus funebris_ & 1,329,900 & 0.40 \\ Broad-leaved mixed forests & 84,515,000 & 25.53 \\ Coniferous and broad-leaved mixed forests & 36,681,900 & 11.08 \\ Coniferous mixed forests & 34,136,400 & 10.31 \\ Total & 324,481,000 & 98.00 \\ \hline \hline \end{tabular} Note: The Latin name of the tree species is in italic.  
\end{table}
Table 1: The proportion of the FSV from the ten forest types in Hunan Province, China.  
Figure 2: TBSJDPT workflow diagram for a given field sample plot.  
To match the pixels of the Sentinel-2 imagery, the sample plot data were resampled using the ArcGIS 10.2.2 software (Esri, Redlands, CA, USA). By using the center position of each sample plot, the 25 m \(\times\) 25 m area sample plots were generated by using the ArcTools \"Buffer and Feature Envelope To Polygon\". The generated shapefile was then used to extract the trees in the sample plot. Using the formula built by Hunan Forestry Industry Survey and Design Research Institute, we calculated the FSV of each tree, summed it to obtain the FSV of the sample plots, and then scaled it up to obtain the FSV per hectare (Table 2). In addition, we used the \"boxcox\" function in the R software to transform the FSV data to a normal distribution (f(y) = y\({}^{\lambda}\)) [77].