THE RADIAL-DISTRIBUTION OF SR-90 AND CS-137 IN TREES

The distributions of Sr-90 and Cs-137 in tr ee trunks were measured fo r eight species. The concentration of Cs-137 was relatively uniform in the wood xylem and did not show any correlation with the temporal rec ord of cumulative deposition of fallout in the northern hemisphere, in dicating that Cs-137 was mobile in the wood xylem. The distributions o f Sr-90 were classified into three groups. All of the gymnosperms - re d spruce, eastern hemlock and white pine - showed a maximum Sr-90 spec ific activity in rings formed in the 1960s and the overall distributio n was closely associated with the temporal record of the cumulative de position of fallout. Three of five angiosperms - hickory, elm and Amer ican beech - showed similar distributions to the gymnosperms except th at the maximum Sr-90 specific activity corresponded to the 1970s. The other two angiosperms - yellow poplar and sugar maple - did not show a ny correlation with the cumulative fallout deposition. The radial dist ribution of Sr-90 in gymnosperms was simulated considering a steady-st ate cycling of Sr-90 in the forest ecosystem. The model suggested that a few per cent of stable Sr are replaced annually in the nutrient poo l of gymnosperms.