Abstract
Long-term monitoring of radiocesium (137Cs) contaminations in the bark and wood caused by the Fukushima Dai-ichi Nuclear Power Plant accident in 2011 requires partial sampling from a standing tree without felling it. Because 137Cs distributions within the bark and wood are assumed not to be uniform, it is necessary to understand the observational errors in 137Cs concentration caused by partial sampling and to check the validity of this method. The objectives of this study are to examine 1) the circumferential distributions of 137Cs concentration in the bark and wood and 2) the observational errors in 137Cs concentration determined via partial sampling compared with bulk (felling) sampling. The circumferential distributions were investigated by dividing the tree stems collected in 2015 into eight directional segments (four segments for small trees). The relative standard deviations of the 137Cs concentration among the directions (the mean of all trees) were 34% and 13% for the bark and wood, respectively. The patterns of the circumferential distributions were not biased toward a specific direction and were not speciesdependent. Partial sampling was achieved by collecting bark pieces (3 cm × 3 cm) from four directions and wood cores (12 mm in diameter) from 1–2 directions during the period of 2016–2020. The observational errors caused by the partial sampling were estimated to be approximately 38% and 8%–18% for the bark and wood, respectively, and were considered random (unsystematic). These results indicate that, for example, to estimate the mean value of the 137Cs concentration in the bark in a forest stand with the same accuracy as the bulk sampling with n = 3, n for the partial sampling should be increased to 6–8.
