Atmospheric Direct Uptake and Long-term Fate of Radiocesium in Trees after the Fukushima Nuclear Accident

Large areas of forests were radioactively contaminated by the Fukushima nuclear accident of 2011, and forest decontamination is now an important problem in Japan. However, whether trees absorb radioactive fallout from soil via the roots or directly from the atmosphere through the bark and leaves is unclear. We measured the uptake of radiocesium by trees in forests heavily contaminated by the Fukushima nuclear accident. The radiocesium concentrations in sapwood of two tree species, the deciduous broadleaved konara (Quercus serrata) and the evergreen coniferous sugi (Cryptomeria japonica), were higher than that in heartwood. The concentration profiles showed anomalous directionality in konara and non-directionality in sugi, indicating that most radiocesium in the tree rings was directly absorbed from the atmosphere via bark and leaves rather than via roots. Numerical modelling shows that the maximum Cs-137 concentration in the xylem of konara will be achieved 28 years after the accident. Conversely, the values for sugi will monotonously decrease because of the small transfer factor in this species. Overall, xylem Cs-137 concentrations will not be affected by root uptake if active root systems occur 10 cm below the soil.