現在の樹木が吸排出する放射性セシウム量を解明～木材のセシウム濃度予測の高度化に向けた観測～

2025-05-27

2025-05-26 森林総合研究所

森林総合研究所は、森林種子の安定供給を実現するため、種子生産技術の高度化に関する研究を進めています。これまで、森林資源の持続的な活用には、種子の安定供給が不可欠であり、そのための技術開発が求められてきました。同研究所では、種子の生産と価格、苗木の生産と価格、供給体制の基本的方向性、採種園における種子の安定生産技術・システムの確立など、多岐にわたる課題に取り組んでいます。これらの研究成果は、森林資源の持続的な活用や再造林の推進に寄与することが期待されます。

図1 セシウム133を用いた樹木と土壌間のセシウム137の吸収量と排出量の釣り合いを評価する手法の概念図
上向き矢印が吸収量、下向き矢印が排出量を示し、青矢印がセシウム133、黄矢印がセシウム137の動きを示しています。

＜関連情報＞

安定セシウムを用いた落葉広葉樹林における放射性セシウム循環平衡の評価：福島事故後のパイロットスタディ Assessment of radiocesium cycle equilibrium in a deciduous broad-leaved forest using stable cesium: A pilot study after the Fukushima accident

Wataru Sakashita, Takuya Manaka, Ren Tsuneoka, Tadashi Sakata, Naohiro Imamura, Junko Nagakura, Yoshiki Shinomiya
Journal of Environmental Radioactivity Available online: 3 March 2025
DOI:https://doi.org/10.1016/j.jenvrad.2025.107653

Highlights

Quasi-equilibrium was assessed in the ¹³⁷Cs cycle of a deciduous broad-leaved forest.
¹³⁷Cs/¹³³Cs ratios were constant in the above-ground component and stemflow.
These ratios enable us to assess the quasi-equilibrium of the Fukushima forest.

Abstract

To accurately predict long-term ¹³⁷Cs activity concentrations in stem wood, it is crucial to assess whether the ¹³⁷Cs cycle in forests has reached quasi-equilibrium between trees and forest soil. A method has been proposed to evaluate this equilibrium based on the concentration ratio of ¹³⁷Cs to stable cesium (¹³³Cs) within the tree compartments. However, this assessment has not yet been conducted in the recent situation following the Fukushima accident. In this study, we investigated the relationship between the concentration ratios of ¹³⁷Cs and ¹³³Cs in tree leaves, inner bark, stem wood, and stemflow in a deciduous broad-leaved forest at Kawauchi Village, the Fukushima Prefecture, as of 2022–2023. Our comparisons between ¹³⁷Cs and ¹³³Cs concentrations revealed that ¹³⁷Cs/¹³³Cs concentration ratios were constant in the above-ground compartment and stemflow, suggesting that the cycle of ¹³⁷Cs in the Fukushima forest has reached quasi-equilibrium 11–12 years after the accident. This concentration ratio-based assessment for quasi-equilibrium may be applicable to forest sites where long-term monitoring is not conducted.

準平衡状態にある福島の森林における主な沈着プロセスによる樹冠から林床への放射性セシウム動態 Radiocesium dynamics from canopy to forest floor by main depositional processes in Fukushima forests at quasi-equilibrium state

Wataru Sakashita, Naohiro Imamura, Tamotsu Sato, Tsutomu Yagihashi, Ren Tsuneoka, Tadashi Sakata, Yoshimi Ohmae, Satoru Miura, Yoshiki Shinomiya
Journal of Environmental Radioactivity Available online: 8 March 2025
DOI:https://doi.org/10.1016/j.jenvrad.2025.107656

Highlights

We observed ¹³⁷Cs flux from canopy to forest floor in Fukushima from 2020 to 2023.
The annual ¹³⁷Cs flux ranged from 0.4% to 1.6% of the initial deposition.
Litterfall was the primary transfer pathway among the depositional processes.
Contribution of each process to the total flux varied depending on tree species.

Abstract

Accurate measurement of the amounts of radiocesium (¹³⁷Cs) that transfer from the canopy to the forest floor at the recent quasi-equilibrium state is crucial for improving the accuracy of long-term predictions of ¹³⁷Cs distribution in forests. This study is the first to detail the ¹³⁷Cs flux via throughfall, stemflow, and litterfall processes in two Japanese cedar forests (KU1-S and KU2-S) and one deciduous broad-leaved forest (KU1-Q) in the Fukushima Prefecture during the quasi-equilibrium state. From 2020 to 2023, the annual transfer of ¹³⁷Cs from the canopy to the forest floor, relative to the initial deposition, varied among the study plots but never exceeded 2% (0.433–0.564% in KU1-S, 1.06–1.62% in KU2-S, and 0.421–0.461% in KU1-Q). Since the transfer of ¹³⁷Cs from trees to soil is balanced with the transfer from soil to trees at the quasi-equilibrium state, these percentages can be interpreted as the root uptake of ¹³⁷Cs from the soil, which is difficult to observe directly. Analysis of the contributions of litterfall, throughfall, and stemflow to the total transfer of ¹³⁷Cs revealed that litterfall accounted for more than 50% in both Japanese cedar and deciduous broad-leaved forests. We also found that the proportion of ¹³⁷Cs flux via litterfall was higher in Japanese cedar forests compared to the deciduous broad-leaved forest (KU1-S:91–92%; KU2-S:74–77%; KU1-Q:57–60%). These findings regarding the ¹³⁷Cs fluxes at the quasi-equilibrium state will enhance our understanding of the future dynamics of ¹³⁷Cs within forest ecosystems and improve the accuracy of long-term predictions of ¹³⁷Cs activity concentration in forests.