Caesium-137 in mountain flora with emphasis on reindeer's diet - Spatial and temporal trends

The present study summarizes three decades of studies on ¹³⁷Cs transfer to various species of lichens, graminoids, herbs and woody plants across a ~3000 km² area used as mountain pasture for reindeer and other ruminants. The investigation comprised of field studies covering the period 2011-2016, and a compilation of studies and data for the preceding period (1986-2010). Altogether, more than 700 individual vegetation samples were considered. For lichens, relatively fast decrease in contamination levels was observed during the first decade after the Chernobyl fallout (ecological half-time of about 3 years). For later years there seems to be a continuous re-contamination which results in a "steady state" where time-trends are mainly governed by physical decay of ¹³⁷Cs. For green plants, decline in transfer factors (TF) (i.e. the ratio between activity concentration in vegetation and activity density in soil) during the period 1986-2012 was not as pronounced as for lichens: Some species showed significant decrease with time, while others did not. 25-30 years after the Chernobyl accident, ¹³⁷Cs levels in lichens and green plants were significantly dependent on the levels in soil (R² between 0.53 and 0.57), but there were also some significant differences in transfer between sampling sites. Moreover, marked variability in TFs was found between different plant species growing at the same site, whereas such differences were not found for reindeer lichens.