Seasonal stability of 137Cs in coppiced Quercus serrata current-year branches: Toward the estimation of trunk 137Cs activity concentrations without felling

Abstract

Radiocesium (137Cs) remains in forests in the Fukushima region due to the Fukushima Daiichi Nuclear Power Plant accident. To resume the use of bed logs in this area for shiitake mushroom (Lentinula edodes) cultivation, estimating 137Cs activity concentrations in the trunk of konara oak (Quercus serrata) using current-year branches has been proposed as an alternative to felling. To establish this method, the seasonal stability of 137Cs in current-year branches is one of the study issues that must be assessed. We monitored current-year branch 137Cs activity concentrations in coppiced konara oaks at six study sites in the Fukushima area from 2018 to 2020. We also investigated relationships between current-year branches and leaves/previous-year branches. Our intensive sampling revealed that the current-year branch 137Cs activity concentration rapidly decreased during the flushing stage (May–June) before becoming relatively stable in July. We confirmed that the current-year branch 137Cs activity concentration was stable during the dormant stage (November–April), indicating it is a suitable representative value for estimating the trunk 137Cs activity concentration. Furthermore, 137Cs activity concentration during the dormant stage could be estimated using current-year branches obtained in the gradual growing stage (July–October) by multiplying the values by 0.75 ± 0.10; however, it could not be accurately estimated in July with measurements above 200 Bq/kg. The 137Cs activity concentrations in the leaves and current-year branches were similar in some periods (e.g., late December to February), and the relationship between current-year and previous-year branch measurements tended to stabilize in August. However, leaves and previous-year branches do not make ideal substitute samples for current-year branches due to the difficulty of sampling (e.g., few withered leaves remain on branches during winter months). Therefore, to estimate the 137Cs activity concentration of the trunk, we recommend the use of current-year branches obtained August–April (correction is required for the period during August–October).