To accurately predict the long-term dynamics of radiocesium (137Cs) in forests contaminated by the nuclear accident, it is important to monitor 137Cs fluxes in forest ecosystems more than ten years after the accident. Among these fluxes, 137Cs transfer via litterfall—a major pathway by which trees return 137Cs to the soil—is one of the key processes to observe. However, in forests more than a decade after the Fukushima accident, observations of litterfall remain limited, and the extent of site-to-site variation has not been evaluated. In this study, we conducted new observations of litterfall over a two-year period from December 1, 2022, to November 28 or 29, 2024, at three Japanese cedar forest sites and three deciduous broad-leaved forest sites in Fukushima Prefecture, and obtained annual 137Cs fluxes via litterfall. Our results revealed that, in Japanese cedar forests, between 0.080% and 0.29% of the initial 137Cs deposition was transferred via litterfall, whereas in deciduous broad-leaved forests, the proportion ranged from 0.22% to 0.37%. Based on the evaluation of site-to-site variation using box plots, including previously reported values, the median proportion of 137Cs flux via litterfall relative to the initial deposition was 0.29% (95% confidence interval: 0.11–0.77%) in Japanese cedar forests and 0.33% (95% confidence interval: 0.27–0.41%) in deciduous broad-leaved forests. The results also indicated that the site-to-site variation tended to be relatively greater in Japanese cedar forests. These findings are expected to support modeling efforts aimed at predicting the distribution of 137Cs in forest ecosystems by providing a basis for evaluating the variability and uncertainty of 137Cs transfer between trees and soil as model parameters.
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