## 4 Results
### CO\({}_{2}\) Sequestration Estimates and Errors
#### 4.4.1 CO\({}_{2}\) Estimates and Potential Errors
This study used the reference information as a representative growth curve for each tree type, and using the precipitation data, it has been assumed that the other regions of Japan would similarly lie along one of those growth curves. To make sure, additional references were collected  
Figure 15: Triangular relationship of the forest classes and their accuracy and errors. to compare the curve information to other regions of Japan [42,43]. According to the tree growth curve information from the Kagoshima region (Kyushu district) and Aomori region (Tohoku district), there are differences in the growth curves depending on the different site productivity areas; however, comparing the two figures, a large difference between growth curves even in opposite environmental regions (the warmer Kagoshima region, and the cooler Aomori region) cannot be confirmed [42,43]. It is known that there is a most suitable growth curve for each Prefecture in Japan, but this study considered that by making an attempt to normalize the differences of the growth curves by using the precipitation data and setting the threshold. Thus it can be said that the growth curve differences among each region are averaged out and reduced to the least possible error in this aspect. Still, this is one point that could be considered as a potential error in the estimation. Finally, for number 4, the sequestration information was modified from the Tochigi Prefectural Government by taking the mean or weighted mean of each tree type and using that value as representative of each forest type tree age class. The Prefectural data for the CO\({}_{2}\) sequestration is estimated using the guidelines commonly used for estimations in the forests of Japan, based on the following equation:  
\[ \begin{array}{l}\mbox{Annual CO${}_{2}$ $ Sequestration (per ha)}\\ \mbox{ = Annual Growth (m${}^{3}$/ha) $\times$ Biomass Expansion Factor}\\ \mbox{ \times Bulk Density $\times$ Carbon Content $\times$ CO${}_{2}$ Equivalent Factor}\end{array}  \tag{6}\]  
Annual growth is measured by stem volume growth. The biomass expansion factor is used for expanding the stem volume into an estimate for the whole volume of the tree, including the volume of leaves, branches and roots together. This value usually differs among tree types and species. Bulk Density represents the weight per cubic meter of volume. Carbon content commonly uses the value of 0.5, which means that the wood is composed of approximately 50% carbon. Finally, to convert the carbon value to a CO\({}_{2}\) sequestration value, the CO\({}_{2}\) Equivalent Factor of 44/12 is multiplied, where 12 and 44 are the molecular masses of carbon and CO\({}_{2}\), respectively. An error that can occur from this is the sequestration value used in multiplying the total forest area, which would be different if it were actually broken down in further detail for each specific tree type and age. Some trees capture more or less CO\({}_{2}\) compared to other tree types within the same forest type, but those values were averaged for this study. Another possible error source is the minimum and maximum sequestration value for each tree age class. Because it is difficult to discriminate between forests using a one-year age distribution, the tree ages were reclassified into 5-year interval ranges. This study used the mean value within that range, and there are minimum and maximum values for those categories, and those can cause errors in the sequestration estimation as well. Table 6 shows the minimum and maximum sequestration values for each forest type at each tree age class range. As shown, there are some classes that have high sequestration values at the peak age of coniferous trees (10-15 years) at site 1, which is both a little higher than the average value from the evergreen broadleaf forests of Tadaki and Hachiya [15] and also similar to the deciduous broadleaf forests. Looking at all the ages, the biggest differences between maximum and minimum can be observed in the peak aged forest classes. This point is one of the more probable errors.