Radiocesium uptake and distribution in Japanese cedar (Cryptomeria japonica D. Don): A review of current knowledge...

The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released large quantities of radiocesium (¹³⁷Cs) into the environment, contaminating vast forested areas. The forest account for nearly 70% of Fukushima Prefecture. With a half-life of 30.2 years, ¹³⁷Cs remains the dominant source of long-term radiological concern in forest ecosystems, where it can contribute to human and wildlife exposure through external irradiation, food chain transfer, and secondary environmental releases. Among affected species, Japanese cedar (Cryptomeria japonica), the most dominant conifer in Japan, has been a focal point of research due to its ecological importance and economic value for timber production. Recent studies have revealed unexpected and species-specific patterns of ¹³⁷Cs distribution in cedar stems. Although initial contamination was dominated by sapwood enrichment immediately after fallout, subsequent monitoring showed progressive translocation into heartwood, leading to heartwood/sapwood concentration ratios exceeding 2-3 by 2020. This anomalous enrichment, linked to heartwood moisture and potassium content, contrasts sharply with Konara oak and Japanese red pine, which remain sapwood-dominated, and cypress and larch, which tend toward equilibrium. Such findings highlight complex transport mechanisms involving xylem and phloem pathways, ray parenchyma activity, and diffusion processes, as well as strong influences of environmental and site-specific factors. Understanding these dynamics is essential for predicting the long-term fate of ¹³⁷Cs in forest ecosystems, managing radiological risks, and guiding forestry practices, including safe timber utilization and effective remediation strategies. This review synthesizes current knowledge on radial and vertical ¹³⁷Cs distribution in Japanese cedar stems, explores mechanistic explanations, and highlights key research priorities, including long-term monitoring of heartwood-sapwood redistribution, improved understanding of cesium transport mechanisms, and the development of predictive models to support sustainable forest management in radiologically contaminated landscapes.