The true distribution and accumulation of radiocaesium in stem of Scots pine (Pinus sylvestris L.)

The radial and vertical distributions of radiocaesium, potassium and calcium were determined in two Scots pine stands (17 and 58yr old) similarly affected by the Chernobyl fallout. For both age classes, concentrations are always the lowest in the stemwood, highest in the inner bark and intermediary levels were observed for the outer bark. Due to the cumulative character of its biomass, however, stemwood is a long-term major reservoir of ¹³⁷Cs. With tree development, changes in the ¹³⁷Cs radial distribution are well described by variations in the sap ascent pattern and reveal an important transfer between tree rings. It is shown that, both the biomass evolution and knowledge of the evolution of the ¹³⁷Cs radial gradient are important to predicting ¹³⁷Cs accumulation in wood with time. According to the common transfer factor (TF) approach, one would expect a decrease in radiocaesium accumulation with time (from 0.0047±0.0013 to 0.0035±0.0008m²kg^-1 for the 17 and 58yr old trees, respectively). With the wood immobilisation potential (WIP) approach, it was, however, clearly shown that additional annual uptake was highest for the older stand (3.12±0.23Bqcm^-3yr^-1 for the 58-year-old stand compared to 1.99±0.30Bqcm^-3yr^-1 for the younger stand). Following the WIP approach, it was moreover possible to distinguish between the ¹³⁷Cs incorporated via the root uptake process and a possible lasting effect of interception. It is shown that, whereas for the younger stand (5yr old at the time of the accident) root uptake contributed exclusively to the wood contamination, the former process explained only 48% of the measured total ¹³⁷Cs content in the wood of the older tree.