The distribution of radioactive caesium in boreal forest ecosystems

The behaviour of radioactive caesium (Cs-134 and Cs-137) in boreal forests of the Nordic countries is reviewed with emphasis on its distribution in various time perspectives. The analysis has thus been focused on data of relevance for both early and later phases after fallout over forest areas. Possibilities and limitation in using data from other time periods or regions, than that characterised by fallout over the boreal zones after the Chernobyl event are also discussed. This concerns extrapolations from information pertaining to neighbouring ecological areas - at higher altitudes (alpine, and sub-alpine regions) or below the southern limit (i.e. in the hemiboreal and nemoboreal zones), and to future time with respect to predictions of the behaviour of Cs-137, based on results for OLD (i.e. from atmospheric weapons tests - mainly in the sixties) versus CHERNOBYL caesium. Beside the principal terrestrial constituents of the soil-plant-animal system, the BOREAL FOREST ECOSYSTEM will for the present purpose be considered to comprise the semi-aquatic and aquatic components pertaining to peat, open peat bog, and ground water. This implies that run-off from a catchment constitutes the main link between the terrestrial part considered here and the aquatic ecosystem proper. In boreal forests the humus layer usually retains a major fraction of the fallout of radioactive caesium, evidently even several decades after deposition. This notable feature, as well as a persistent high availability in important food-chains, emerges from the present Nordic radioecological research. Both constitute facets of a singularity conservative - although not at all static - situation prevailing for radioactive caesium in the boreal forest. The implication is that for Cs-137 physical decay will be the major factor of loss from the boreal ecosystem in a long-term perspective, and that runoff, particularly from peat bogs, is expected to be the second in order of importance. (orig./HP)