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放射性セシウム流出量に及ぼす林床状況の影響; 福島の山林の事例

An Effect of forest floor condition on radiocesium outflow in mountainous forest of Fukushima, Japan

新里 忠史  ; 佐々木 祥人 ; 渡辺 貴善 ; 雨宮 浩樹*

Niizato, Tadafumi; Sasaki, Yoshito; Watanabe, Takayoshi; Amamiya, Hiroki*

福島の山地森林における林床状況とセシウム137($$^{137}$$Cs)流出量の関連を把握するため、除染地,未除染地および林野火災の延焼跡地において3年間の長期観測を実施した。除染や延焼により失われた林床被覆が回復するのに伴い$$^{137}$$Cs流出量は減少し、除染地では除染直後の3.24%から0.61%へ、延焼跡地では延焼直後の2.79%から0.03%へと低下した。林床被覆が60%を超えると未除染地や非延焼地と同程度の流出量となり、林床被覆60%は、観測地における流出影響の閾値と考えられる。延焼跡地では林床被覆の回復に伴い、流出物の主体が土壌粒子からリター片に変化したことも、$$^{137}$$Cs流出量の低下に寄与した。山地森林の林床が本来有する土壌侵食に対する保護機能は、$$^{137}$$Cs流出抑制に効果的である。

A three-year monitoring of $$^{137}$$Cs outflow associated with soil erosion from decontaminated and burnt sites using an experimental plot was conducted in a mountainous forest of Fukushima, Japan. Corresponding with recovery of the forest floor cover, such as undergrowth and litter layer, the $$^{137}$$Cs outflow during the monitoring period in the rainy season of Fukushima decreased from 3.24% to 0.61% and 2.79% to 0.03% in the decontaminated and burnt sites, respectively. The forest floor cover ratio of 60% or more is a threshold for an effect on the $$^{137}$$Cs outflow in the monitoring sites. In the burnt site, a change of dominant outflowed materials from soil particles into litter and its fragments also contributes to the decreasing in the $$^{137}$$Cs outflow. A protective function of forest floor cover against the soil erosion inherent in the natural forest environment is effective for the migration control of the $$^{137}$$Cs outflow.

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