The radiocesium (137Cs) activity concentration in stemflow, which is an element of the 137Cs cycle in the forest, is determined by the supply of 137Cs from the outer bark and leaves. Long-term monitoring data of deciduous broad-leaved trees since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident have shown the 137Cs activity concentration in the outer bark decreasing exponentially, while that in the leaves has remained nearly constant. Therefore, it is expected that the contribution of the outer bark to 137Cs in stemflow eventually becomes smaller than that of leaves over time. We hypothesized that as of 2022-2023 (i.e., more than 10 years after the FDNPP accident), the main source of 137Cs in the stemflow of deciduous broad-leaved trees is mainly leaching from leaves. We tested this hypothesis by conducting two surveys in the autumn of 2022 (September-October) and early summer of 2023 (May-June) in Kawauchi Village, Fukushima Prefecture. Samples consisted of stemflow, leaves, and outer bark from a total of 10 deciduous broad-leaved trees (three konara oaks, three mizunara oaks, two Japanese chestnuts, and two cherry blossoms). Our statistical analyses (correlation, linear and multilinear regression analyses) showed that the 137Cs activity concentrations in stemflow were significantly positively correlated to those in leaves, with no positive correlation detected with the outer bark, suggesting that at the time of the survey, the 137Cs activity concentration in stemflow was mainly influenced by the 137Cs activity concentration in leaves. In addition, we propose a method for estimating the stemflow 137Cs activity concentration in konara oak using data from leaves. Although the method's prediction accuracy is low from 2011 to 2013, it is able to estimate the stemflow 137Cs activity concentration in konara oak. Thus, it can help determine one of the model parameters of 137Cs dynamics within deciduous broad-leaved forests.
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