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Japanese cedar (Cryptomeria japonica) is the main timber species in Japan. The prediction of the temporal changes in the 137 Cs concentration in the stemwood of Japanese cedar after the Fukushima nuclear accident is essential for optimizing forest management in contaminated areas. However, it is difficult to estimate the respective contributions of root and foliar uptake to 137 Cs accumulation in stemwood from simple field measurements, especially in trees that contain the residue of initially-deposited 137 Cs. In this study, we devised a method for estimating the rate of 137 Cs root uptake into stemwood using the 133 Cs content in stemwood and the 137 Cs/ 133 Cs ratio in the exchangeable fraction of soil. As a trial, the method was applied to a cedar stand in Fukushima Prefecture, using available monitoring data from prior studies over 5 years from August 2011 to August 2016. The mean annual rate of 137 Cs root uptake into stemwood over this period was estimated as 53 ± 20 Bq m -2  yr -1 . We note that our method likely provided a maximum estimate, because it is based on the assumptions that 133 Cs in wood is exclusively supplied by root uptake, and that Cs isotopes are taken up by roots in the top 5 cm of mineral soil. Moreover, the mean annual increase of the 137 Cs inventory in stemwood during the study period was measured as 108 Bq m -2  yr -1 , although this value was associated with considerable uncertainty (95% confidence interval from -109 to 324 Bq m -2  yr -1 ). As a result, the maximum estimated rate of 137 Cs root uptake into stemwood accounted for around half of the measured rate of 137 Cs accumulation in stemwood. Our results show that the Cs isotopic approach has potential to distinguish the main pathway of stemwood contamination (i.e., root vs. foliar uptake) following radioactive fallout., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)

This study investigated the transport of 137 Cs within a forest ecosystem by examining temporal changes in the inventory and determining the major pathways of transfer following significant atmospheric deposition. A forested area of eastern Japan was monitored for four years immediately after the Fukushima nuclear power plant accident in March 2011 that released a large amount of radionuclides. The long physical half-life of 137 Cs means that contamination can persist for decades, so it is vital to understand the mechanisms underlying the 137 Cs dynamics in ecosystems. We sampled litterfall, throughfall, and soil, mainly from a cedar stand, over a four-year period, and analyzed the 137 Cs concentrations of each sample to determine the transfer rate and total inventory. After validating our methodology through a comparison with results from an earlier study, we determined the temporal changes in the 137 Cs distribution and in the major transfer pathway. Results showed that most 137 Cs intercepted by canopies was transferred rapidly over the first nine months, and that the major pathway was not litterfall but throughfall. The ecological half-life of the 137 Cs stocked in the canopy was calculated for both the early and later stages of contamination. Although the former is consistent with previous results, the latter ecological half-life is somewhat longer, probably because of dependence on the meteorological and tree physiological conditions at the site. This study presents valuable new data on the post-Fukushima 137 Cs contamination, enhancing our understanding of the associated dynamics in forest ecosystems., (Copyright © 2018 Elsevier B.V. All rights reserved.)