Factors influencing the changes in 137Cs levels with time in boreal-forest plants in Sweden

The distribution and turnover of radiocesium from the reactor accident at Chernobyl in 1986 has been monitored in boreal forest vegetation in Sweden since 1986. The purpose has been to characterise the distribution of 137Cs in plants and biotopes over time. The study was mainly performed in an experimental forest where the fall-out in 1986 was 23 000 (S.D. 11 000) Bq/m2137Cs. In addition, we included 36 sites in a radius of ∼ 200 km from the experimental forest representing a range of ground deposition levels from 8000 to 80 000 Bq/m2 of 137Cs. Significant differences in 137Cs activity were found between plant species. The highest concentration in 1986 was 48 000 Bq per kg 137Cs dry mass in Nymphaéa cándida, and the lowest was 31 Bq per kg 137Cs dry mass (Epilobium angustifolium). We found a weak, but significant, correlation between ground deposition and cesium concentration in Betula pubescens and Vaccinum myrtillus. A significant decline in 137Cs levels occurred between 1986 and 1990. The cesium level in bilberry declined faster in young mixed forests (5- to 10-year-old clear cuttings) than in mature coniferous forests.