Estimation of carbon stock in Fagus crenata secondary forest using X-ray densitometry

Global warming due to increase in the atmospheric carbon dioxide level is a matter of growing concern and the carbon-absorbing function of the forest is now an issue gathering public attention. Under such conditions, it is necessary to calculate carbon stock in tree stems, and understand the details of the bulk density of sterns. The aim of this study is to estimate the carbon stock in the forest with guaranteed accuracy by stem analysis and soft X-ray densitometry, using a Stem Density Analyzer (SDA). Samples of beech (Fagus crenata) were collected at Kaminagawa Experimental Forest, Yamagata University. Ten trees were selected from the standing trees in the investigation area. The wood disks were obtained from the stem at a 1-m interval starting from the height of 0.2m. From each disk, four blocks were cut out in the four directions, and a 1.4-mm-thick strip was prepared from each block using a double-toothed circular saw. Annual ring width and bulk density were measured by soft X-ray densitometry. Each 1-m-long stem segment was regarded as a cone trapezoid, and using the average of annual ring widths in four directions, wood volume for each year was calculated. The calculated value was multiplied by average of bulk density to obtain wood dry weight for each year, and it was converted to carbon stock by multiplying by the rate of carbon. SDA was used for these calculations. The distribution of bulk density in the stem was variable in the same individual, and even the average bulk density of each individual varied with the individual, even though the bulk density was measured for the individuals with similar ages and under similar environmental conditions. These results indicate that the examination of bulk density is important for the calculation of carbon stocks. The diameter at breast height (DBH) of trees was found to have a close correlation with the carbon stock. Therefore, we estimated carbon stock per unit area using the correlation formula between DBH and carbon stock in each tree in the investigation area. The estimated carbon stock per unit area in the stand was 44% greater than that estimated by the method currently proposed by IPCC method. Carbon stock can be estimated with high accuracy using the correlation formula between DBH and carbon stock.