FACTORS INFLUENCING THE CHANGES IN CS-137 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 i n Sweden since 1986. The purpose has been to characterise the distribu tion of Cs-137 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/m2 Cs-137. In addition, we included 36 sites in a radius of approximately 200 km from the experimental forest represent ing a range of ground deposition levels from 8000 to 80 000 Bq/m2 of C s-137. Significant differences in Cs-137 activity were found between p lant species. The highest concentration in 1986 was 48 000 Bq per kg C s-137 dry mass in Nymphaea candida, and the lowest was 31 Bq per kg Cs -137 dry mass (Epilobium angustifolium). We found a weak, but signific ant, correlation between ground deposition and cesium concentration in Betula pubescens and Vaccinum myrtillus. A significant decline in Cs- 137 levels occurred between 1986 and 1990. The cesium level in bilberr y declined faster in young mixed forests (5- to 10-year-old clear cutt ings) than in mature coniferous forests.