Processes, dynamics and modelling of radiocaesium cycling in a chronosequence of Chernobyl-contaminated Scots pine (Pinus sylvestris L.) plantations

In a large forested area affected by the Chernobyl radioactive fallout, especially in CIS, the lasting recycling of radiocaesium (¹³⁷Cs) by the trees is a source of long-term contamination of woody products. The quantitative description of the ¹³⁷Cs dynamics in contaminated forest is a prerequisite to predictive modelling and further management of such territories. Three even-aged mono-specific Scots pine stands (17, 37 and 57 years old) were selected in a contaminated woodland in southeastern Belarus to constitute an adequate chronosequence. We determined the potassium and radiocaesium annual fluxes involved in the biological cycling in each stand using a well-documented calculation methodology. Qualitatively, ¹³⁷Cs was shown to be rapidly recycled in trees through the same pathways as K and to redistribute similarly between the tree components. Compared to K, a higher fraction of ¹³⁷Cs, corresponding to about the half of the annual uptake, is immobilised in perennial organs. With tree development, trunk wood and bark become prevailing sinks for ¹³⁷Cs since they represent an increasing pool of biomass. In the pine chronosequence, the current root absorption, respectively, mobilizes 0.53, 0.32 and 0.31% year^-1 of the total ¹³⁷Cs pool in soil. Variations in the ¹³⁷Cs uptake do not reflect differences in the ¹³⁷Cs balance between stands. In the two older stands, 51 and 71% of the current tree contamination are related to earlier accumulation subsequent to the initial fallout interception and recycling. The soil is the dominant source of long-term tree contamination. A simple modelling based on the measured ¹³⁷Cs fluxes indicates that, for young stands, radioactive decay-corrected contamination would stabilize after reaching a maximum of 25 years after the ¹³⁷Cs deposition. Stemwood presents a maximum of 15 years after the deposition and decrease afterwards mainly through radioactive decay. In the older stands, the decontamination is constant without local maximum of ¹³⁷Cs level in the wood. The ¹³⁷Cs contamination of tree components is the result of different influential processes like root uptake, internal translocation and immobilisation. For more accurate predictions, the calibration of existing models would be benefited by comparing with the ¹³⁷Cs annual fluxes instead of the simple transfer factor coefficients. In the perspective of other applications, there is a need of such data for other radionuclides as well as for heavy metals.