## 4 Results  
### Stem Volume Modeling and Mapping  
Figure 3 shows the trends of the stem volume _vs._ the backscattering intensity (dB) for both HH and HV polarizations. An increasing trend can be observed with the increasing parameter and increasing backscattering intensity, which is consistent with many of the other related studies [19, 20, 21, 22, 23, 30, 31]. According to these results, a second polynomial function from the HV polarization was selected and applied to obtain the stem volume information for the whole of Japan, because it has higher correlation compared to HH (HH: R\({}^{2}\) = 0.2123, HV: R\({}^{2}\) = 0.2198). The equation of the function is described as:  
\[V=1.4192\sigma_{HV}^{0^{2}}+60.472\sigma_{HV}^{0}+773.8 \tag{4}\]  
where \(V\) is the stem volume and \(\sigma_{HV}^{0}\) is the backscattering intensity from the HV polarization. Validation of the backscattering model was assessed by using the remaining observation plot data that were not  
Figure 3: Relationship between Stem Volume (m\({}^{3}\)/ha) and backscattering for HH (**left**) and HV (**right**) polarization at mean 25 m pixel spacing data from 2009 and 2010 (HH: R\({}^{2}\) = 0.2123, HV: R\({}^{2}\) = 0.2198). used in the modeling process. Using the plot data as ground truth information, the root mean square error (RMSE) between the estimated results and the reference data was calculated. The RMSE result is 105.58 m\({}^{3}\)/ha, and R\({}^{2}=0.164\) as shown in Figure 4. This model will be applied to obtain the stem volume information for the forested areas within Japan. Figure 4: Validation of the backscattering model. The X-axis is the estimated volume (m\({}^{3}\)/ha) using the model, and the Y-axis is the reference data from the observation plot. R\({}^{2}=0.164\) Root Mean Square Error (RMSE) = 105.58 m\({}^{3}\)/ha. Figure 5: _Cont._  
Figure 5 shows the interpretation of the original and the corrected imagery of the PALSAR data. In addition to the interpretation, a quantitative evaluation of the performance of the correction was conductedby assessing the non-correlation between \(\cos i\) and the corrected data. Comparing the correlation coefficients, the value shows \(\mathrm{r}=0.4\) and \(\mathrm{r}=0.06\) for non-corrected and corrected images, respectively. From these results, it is presumed that the correction of the PALSAR was performed successfully. The formula (Equation (4)) obtained through the model was applied to the corrected PALSAR imagery to obtain stem volume information for the entire forest area of Japan. By doing this, the spatial distribution of the estimated stand volume for Japanese forests can be shown. However, this model is specific to the coniferous stands in Japan, and the possibility that backscattering characteristics of different forest types would give different trends due to the structure of the stand and leaf size should be  
Figure 5: Visual interpretation of the Phased Array L-band Synthetic Aperture Radar (PALSAR) 50 m at the Kyushu Island region, for (**a**) the original image and (**b**) the slope-corrected image of the HV polarization data. The (**c,d**) images show the zoomed areas from the (a,b) images, respectively. Figure 6: Stem volume map of Japan for the (**a**) coniferous forests and (**b**) deciduous and evergreen broadleaf forests. Forest cover data were overlaid with the stem volume map to extract only the forest types needed for each stem volume map. taken into account. Instead, for the broadleaf forests, a polynomial model developed by [PERSON] [38] from the evergreen trees of Indonesia was applied to fit the situation (Equation (5)). Utilizing the two different models specific to those forest types, the stem volume distribution across Japan was obtained, as seen in Figure 6a,b. Using the model developed from the results of this study, the maximum stand volume of the coniferous forests reaches up to 1069 m\({}^{3}\)/ha; for the broadleaf forests, it reaches up to 414 m\({}^{3}\)/ha.