Decadal stability of radiocesium inventories and soil to tree transfer in forests affected by the Fukushima nuclear accident

Understanding the long-term dynamics of radiocesium (¹³⁷Cs) in forests contaminated by the Fukushima nuclear accident requires determining when these dynamics reach a quasi-equilibrium state, in which the flux of ¹³⁷Cs between soil and trees becomes approximately balanced. In this study, we analyzed time-series variations from 2011 to 2020 in the total ¹³⁷Cs inventory and ¹³⁷Cs distribution in aboveground compartments (needles/leaves, branches, bark, sapwood, and heartwood). Time-series analysis using a dynamic linear model indicated that the decay-corrected total ¹³⁷Cs inventory (as of September 1, 2020) and its distribution in the aboveground compartments remained stable from 2017 onward, suggesting that a quasi-equilibrium state had been reached. Given this stability in both inventory and distribution, the aggregated transfer factor (T_ag)-defined as the ¹³⁷Cs activity concentration in aboveground tree tissues divided by the total ¹³⁷Cs inventory in soil-measured approximately 6 years after the Fukushima nuclear accident can be considered representative of the long-term transfer of ¹³⁷Cs from forest soil to trees. These findings on the stability of ¹³⁷Cs in forests provide valuable insights for validating the accuracy of models that predict long-term ¹³⁷Cs activity concentration in stem wood.