Temporal changes in the radiocesium distribution in forests over the five years after the Fukushima Daiichi Nuclear Power Plant accident.

To elucidate the temporal changes in the radiocesium distribution in forests contaminated by the Fukushima Daiichi Nuclear Power Plant accident, we monitored the (137)Cs concentration and inventory within forests from 2011 to 2015 across nine plots containing variable tree species and different contamination levels. The (137)Cs concentrations in needles and branches decreased exponentially at all coniferous plots, with effective ecological half-lives of 0.45-1.55 yr for needles and 0.83-1.69 yr for branches. By contrast, the (137)Cs concentration in deciduous konara oak leaves did not change over the five years. The concentration of (137)Cs in oak wood increased by 37-75%, whereas that in Japanese red pine decreased by 63% over the five years. In Japanese cedar and hinoki cypress, the (137)Cs concentration in wood showed an increasing trend in half of the plots. The changes in (137)Cs in the organic and mineral soil layers were not strongly related to the tree species or contamination level. Our multi-site, multi-species monitoring results revealed that the pattern of temporal changes in radiocesium in the 9 forest plots was similar overall; however, changes in (137)Cs in needles/leaves and wood differed among tree species.