4.5. The Distribution of Radioactive Caesium in Boreal Forest Ecosystems

Summary Experience from current research in the Nordic countries concerning the behaviour of radioactive caesium (134 Cs and 137 Cs ) in boreal forests 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 limitations 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 137 Cs based on results for OLD (i.e. from atmospheric nuclear 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 runoff 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 singularly conservative - although not at all static - situation prevailing for radiaoctive caesium in the boreal forest. The implication is that for 137 Cs 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 - even in areas subjected to logging during the period. There seems to be a gradual change from these conditions southwards of the boreal zone, where differences in the vertical distribution between nuclear weapons and Chernobyl caesium are manifest, and processes operate, leading to significant decrease in availability. Physical decay and decreasing availability constitute the dominant factors for the loss of radioactive caesium in these areas.