Fig.1　Temporal changes in the ¹³⁷Cs inventory in litter layers

Although the ¹³⁷Cs inventories of these two contrasting forest types were similar, the ¹³⁷Cs inventory in the litter layer of the cedar forests (CF), where the litter mass was large, had remained large since just after the accident. The ecological half-life of ¹³⁷Cs in the litter layer in CF was approximately 2 years, whereas that in deciduous broad-leaf forests (DBF) was approximately 1 year.

Fig.2　Spatial-distribution map of the γ-ray-dose rate at the forest ground surface in a 20 m × 20 m plot

Circles in the panels indicate standing trees, and the diameters of the circles show the diameters at chest height (approximately 1.2 m). Cedar forests (CF) (a), in which more than half of the deposited radioactive Cs remained in the litter, had a higher γ-ray-dose rate on the ground surface than deciduous broad-leaf forests (DBF) (b), in which nearly 80% of the radioactive Cs was transferred to the lower-layer soil. In addition, the γ-ray-dose rate had high spatial heterogeneity in both CF and DBF, and the distributions had no relation with the position or size of the standing trees.

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Reference

1. Koarashi, J., Atarashi-Andoh, M. et al., Forest Type Effects on the Retention of Radiocesium in Organic Layers of Forest Ecosystems Affected by the Fukushima Nuclear Accident, Scientific Reports, vol.6, 2016, p.38591-1-38591-11.