Spatial heterogeneity of radiation emission on a secondary mixed forest floor in northeastern Japan after the Fukushima Daiichi Nuclear Power Plant explosions

Abstract

ABSTRACTThe spatial distribution of radiation air dose was assessed on the floor of a secondary mixed forest located 35 km northwest of the Fukushima Daiichi Nuclear Power Plant. All trees with diameter at breast height ≥5 cm were measured and mapped within two plots. Radiation air doses were measured on the forest floor at each 1-m grid point and at stem bases and were analyzed in relation to bark texture, tree strata position, and leaf phenology. Radiocesium concentration measured from soils sampled in a similar manner (2-m grid point and at stem bases) in one of the two plots showed significant correlation with the air dose rate. Air dose rate showed a highly heterogeneous spatial distribution and tended to increase toward the stem bases. This trend was distinctive in some tree functional groups, with the rate being higher in some smaller deciduous trees with smooth bark (e.g. Fraxinus lanuginosa, Acer rufinerve). The trends were indistinctive in larger canopy trees with thick, coarse bark (Quercus mongolica subsp. crispula): in such trees, air dose rate become lower at stem bases than those in the outer range of the concentric zone in some large evergreen conifer trees (Abies firma). However, radiocesium concentration measured near tree stem bases (1 m away from the base) did not vary in relation to tree functional groups. Bark smoothness, which facilitated immediate vertical runoff of contaminated water to lower positions, may have increased air dose rates at the tree bases but would not have enhanced the heterogeneity of radiocesium concentration in the forest floor soils.