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 (Cs) 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 Cs distribution in forests. This study is the first to detail the Cs 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 Cs 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 Cs 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 Cs from the soil, which is difficult to observe directly. Analysis of the contributions of litterfall, throughfall, and stemflow to the total transfer of Cs revealed that litterfall accounted for more than 50% in both Japanese cedar and deciduous broad-leaved forests. We also found that the proportion of Cs 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 Cs fluxes at the quasi-equilibrium state will enhance our understanding of the future dynamics of Cs within forest ecosystems and improve the accuracy of long-term predictions of Cs activity concentration in forests.