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

Radiocesium (Cs-137) 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 Cs-137 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 Cs-137 in current-year branches is one of the study issues that must be assessed. We monitored current-year branch Cs-137 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 Cs-137 activity concentration rapidly decreased during the flushing stage (May-June) before becoming relatively stable in July. We confirmed that the current-year branch Cs-137 activity concentration was stable during the dormant stage (November-April), indicating it is a suitable representative value for estimating the trunk Cs-137 activity concentration. Furthermore, Cs-137 activity concen-tration 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 Cs-137 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 Cs-137 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).

Automated summary

The study monitored Cs-137 activity concentrations in current-year branches of coppiced konara oaks in Fukushima from 2018 to 2020, revealing rapid decrease during flushing stage and stability in summer months. The method of estimating trunk Cs-137 activity concentration using current-year branches obtained August-April is recommended, with correction needed for values obtained in August-October.