¹³⁷Cs content and distribution in understorey spruces

Radiation and Nuclear Safety Authority (STUK), Research and Environmental Surveillance, P.O. Box 14, 00881 Helsinki, Finland

Abstract

Understorey spruces (Picea abies) grown after the Chernobyl accident were studied in Finland in 1998. Ten trees were sampled from three pine forests on unmanaged mineral soil. ¹³⁷Cs contamination of the ground, estimated from soil samples, varied from 24 to 66 kBq m^-2. Contents and distributions of ¹³⁷Cs and dry matter were compared by trees of varying age and size. Stemwood contained 19-36 % of the dry mass but only 4-7 % of the radiocaesium. Needles contained 21-48 % of the dry mass and 37-60 % of the ¹³⁷Cs. Roots contributed to the dry mass more than to the ¹³⁷Cs content of a tree. Activity concentrations in needles and branches were highest in current year growth, and in upper parts of the trees. Also wood and bark had higher ¹³⁷Cs concentration in upper parts of the stems than in lower parts. Concentrations per unit deposition (m² kg^-1 dry weight) were calculated for comparison of uptake by trees from different sites. They varied by the stage of growth in several tree parts. For instance, in current year needles they were on average 0.098 for older trees and 0.13 m² kg^-1 for younger trees.