Radiocesium Migration from the Canopy to the Forest Floor in Pine and Deciduous Forests

Abstract

We monitored ¹³⁷Cs in litter fall, through fall and stem flow in red pine and deciduous broad-leaved forests after the Fukushima Daiichi Nuclear Power Plant Accident in Fukushima Prefecture from October 2012 to December 2013. The purpose is to elucidate the migration process of ¹³⁷Cs from canopy to forest floor and to clarify the possible influence of forest type on the process. The migration process of ¹³⁷Cs showed seasonality; leaf-fall and rainy seasons promoted ¹³⁷Cs migration through the increase in litter fall and through fall respectively. Species composition and stand density of forests caused the difference in the amount of litter fall, through fall and stem flow, and their ¹³⁷Cs concentration. The difference then changed the amount of ¹³⁷Cs migration from canopy to forest floor. Total ¹³⁷Cs migration from canopy to forest floor in the year 2013 were 2,457 Bq/m² (stem flow, 41; through fall, 134; litter fall, 2,281) in the pine forest and 4,032 Bq/m² (stem flow, 213; through fall, 600; litter fall, 3,219) in the broad-leaved forest. Litter fall had the largest contribution to the ¹³⁷Cs migration in both forests. We concluded that radiocesium was localized on the forest surface at the initial deposition caused by the incident, then gradually diffused in forests while being incorporated in trees, and was incorporated in the mineral cycle in forests at 1.5 years after the incident.