Decadal trends in 137 Cs concentrations in the bark and wood of trees contaminated by the Fukushima nuclear accident

Understanding the actual situation of radiocesium ( 137 Cs) contamination of trees caused by the Fukushima nuclear accident is essential for predicting the future contamination of wood. Particularly important is determining whether the 137 Cs dynamics within forests and trees have reached apparent steady state. We conducted a monitoring survey of four major tree species (Japanese cedar, Japanese cypress, konara oak, and Japanese red pine) at multiple sites. Using a dynamic linear model, we analyzed the temporal trends in 137 Cs activity concentrations in the bark (whole), outer bark, inner bark, wood (whole), sapwood, and heartwood during the 2011–2020 period. The activity concentrations were decay-corrected to September 1, 2020, to exclude the decrease due to the radioactive decay. The 137 Cs concentrations in the whole and outer bark samples showed an exponential decrease in most plots but a flat trend in one plot, where 137 Cs root uptake is considered to be high. The 137 Cs concentration ratio (CR) of inner bark/sapwood showed a flat trend but the CR of heartwood/sapwood increased in many plots, indicating that the 137 Cs dynamics reached apparent steady state within one year in the biologically active parts (inner bark and sapwood) and after several to more than 10 years in the inactive part (heartwood). The 137 Cs concentration in the whole wood showed an increasing trend in six plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend. Overall, the results show that the 137 Cs dynamics within forests and trees have reached apparent steady state in many plots, although the amount of 137 Cs root uptake in some plots is possibly still increasing 10 years after the accident. Clarifying the mechanisms and key factors determining the amount of 137 Cs root uptake will be crucial for predicting wood contamination.