Radiocesium dynamics from canopy to forest floor by main depositional processes in Fukushima forests at quasi-equilibrium state

Accurate measurement of the amounts of radiocesium (¹³⁷Cs) that transfer from the canopy to the forest floor at the recent quasi-equilibrium state is crucial for improving the accuracy of long-term predictions of ¹³⁷Cs distribution in forests. This study is the first to detail the ¹³⁷Cs flux via throughfall, stemflow, and litterfall processes in two Japanese cedar forests (KU1-S and KU2-S) and one deciduous broad-leaved forest (KU1-Q) in the Fukushima Prefecture during the quasi-equilibrium state. From 2020 to 2023, the annual transfer of ¹³⁷Cs from the canopy to the forest floor, relative to the initial deposition, varied among the study plots but never exceeded 2% (0.433–0.564% in KU1-S, 1.06–1.62% in KU2-S, and 0.421–0.461% in KU1-Q). Since the transfer of ¹³⁷Cs from trees to soil is balanced with the transfer from soil to trees at the quasi-equilibrium state, these percentages can be interpreted as the root uptake of ¹³⁷Cs from the soil, which is difficult to observe directly. Analysis of the contributions of litterfall, throughfall, and stemflow to the total transfer of ¹³⁷Cs revealed that litterfall accounted for more than 50% in both Japanese cedar and deciduous broad-leaved forests. We also found that the proportion of ¹³⁷Cs flux via litterfall was higher in Japanese cedar forests compared to the deciduous broad-leaved forest (KU1-S:91–92%; KU2-S:74–77%; KU1-Q:57–60%). These findings regarding the ¹³⁷Cs fluxes at the quasi-equilibrium state will enhance our understanding of the future dynamics of ¹³⁷Cs within forest ecosystems and improve the accuracy of long-term predictions of ¹³⁷Cs activity concentration in forests.