Temporal patterns of radiocesium decline in current-year branches of coppiced Quercus serrata relative to stand age after the Fukushima nuclear accident.

Forest contamination by radiocesium (137Cs) has hindered the resumption of forestry activities after the Fukushima nuclear accident, particularly the production of coppiced konara oak (Quercus serrata) bed logs for shiitake mushroom (Lentinula edodes) cultivation. Methods are being developed to estimate stem 137Cs activity concentrations using current-year branches as an indirect indicator that can be used to identify available bed logs efficiently. However, the behavior of 137Cs in current-year branches—especially in the juvenile stage—is insufficiently understood, limiting efforts to improve the accuracy of stem concentration estimates. In this study, current-year branches were collected in the winters of 2020–2021 and 2025 from the same konara oak trees in 20 survey plots in Miyakoji, Tamura City, Fukushima Prefecture, where coppice regeneration occurred between 2011 and 2016. Temporal changes were examined using the abovementioned sampling data, along with the previous monitoring data from the winter of 2016–2017. Results showed that in konara oaks aged 1–3 years at the beginning of each 4-year interval, 137Cs activity concentrations in current-year branches decreased over four years by more than that expected from physical decay alone. By contrast, for trees aged 4–9 years at the start of the interval, the 4-year decline in concentrations was approximately consistent with physical decay. Thus, for konara oaks with a stand age of 4–9 years, the influence of stand age can be ignored when applying the developing relationship between current-year branch and stem 137Cs activity concentrations.