## 5 Discussion
### Stem Volume Method for CO\({}_{2}\) Sequestration Estimates  
#### 5.3.1 CO\({}_{2}\) Sequestration Values and Trends  
The conventional method [15], when used alone for estimating CO\({}_{2}\) sequestration, results in large errors in the total amount compared to methods that give special attention to tree age. The stem volume method for discriminating tree ages within each forest type clearly shows the importance of this variable for avoiding over- or underestimations, depending on the growth stage of the forests examined. The use of L-band SAR data rather than optical data [5] made this discrimination possible, yielding sequestration rates by tree age for trees up to 75 years of age for coniferous and 50 years of age for deciduous and evergreen broadleaf forests. It is thus clearly established that sequestration rates of 13.5 t-CO\({}_{2}\)/ha/yr [15] are only valid for 10-15-year-old coniferous forests in high growth rate regions. In contrast, deciduous broadleaf forests reach just below the 8.7-t-CO2/ha/yr threshold only in the 15-20-year-old range. For evergreen broadleaf forests, this is a large issue when the conventional method is applied, because it shows approximately 3 times greater value than observed in the results of this study, meaning that there would be a large overestimation of the forest carbon sink capacity if such a value is applied. High sequestration values from evergreen broadleaf forests are difficult to confirm for many reasons; however, the amount of sequestration by evergreen trees in Japan is acknowledged to depend on total solar radiation and air temperature. Gross Primary Product (GPP) results from the total radiation, while the Net Primary Product (NPP) is set by the amount of respiration subtracted from the GPP [47]. In this case, higher air temperature leads to higher respiration and decreases the NPP. On the other hand, when both total radiation and air temperature are low, respiration rates decrease but NPP becomes the same or even greater compared to higher radiation periods [37]. Depending on the season and location of the observation, the average value of evergreen broadleaf forests will differ, and it might well be possible that the large value obtained by Tadaki and Hachiya [15] resulted from such causes. It is very important to note that the averaged sequestration value for the coniferous forest type matches the sequestration value by tree ages obtained here for the 10-15-year-old coniferous trees at Site 1. This is also the category of the second fastest-growing period for coniferous trees at that site. The same is also true for deciduous broadleaf forests for 15-20-year-old trees; it is just a little lower than the average value. This strongly implies that an estimation based on the averaged values alone will round up the values for trees in different growth stages, causing an overall overestimation of the carbon sink capacity [5], both for natural and planted forests. The results emphasize the fact that sequestration differs among tree ages. This reality should be taken into consideration in all cases, otherwise, erroneous values of the forests sequestration capacity will be obtained, generating results that possibly have errors from using less accurate values. The uncertainty assessment gave a clear view of how and why the estimations can fluctuate, where the estimated total CO\({}_{2}\) lies between the minimum 75.80 Mt\(\cdot\)CO\({}_{2}\) and maximum 137.69 Mt\(\cdot\)CO\({}_{2}\). The minimum estimate results from land cover error and the maximum from sequestration value error. After examining the results presented in this paper, it can be assumed that these two parameters are the most important factors when estimating sequestration amounts. Focusing on the results where 111.27 Mt\(\cdot\)CO\({}_{2}\) was shown, the fluctuation of the estimated amount is a difference of approximately 23 to 32%.