| Summary: | Research Highlights: a forest is an ecosystem that allows for the assessment of radioactive contamination of the environment over several decades. (1) Background and Objectives: measurements of the activity of the <sup>137</sup>Cs isotope in various elements of a forest ecosystem are one of the most important parameters in the assessment of radioisotope contamination. The translocation of <sup>137</sup>Cs in the environment is determined by the activity of the natural <sup>40</sup>K isotope in soil. The activities of <sup>137</sup>Cs and <sup>40</sup>K isotopes were assessed in two stands of <i>Betula pendula</i> and <i>Pinus sylvestris</i>, differing in age (30, 50, and 80 years old); (2) Materials and Methods: the research was conducted in one of the forest districts of eastern Poland. Wood, litter, and soil were collected for the tests from the sampling sites. The activity of <sup>137</sup>Cs and <sup>40</sup>K was determined using the γ-spectrometric method. Based on the activity of these isotopes in wood and soil, the values of translocation factors (TF) were determined; (3) Results: the highest activity of <sup>137</sup>Cs was recorded in the wood of the oldest stands, the growth period of which coincided with the time period of intensive testing with nuclear weapons. With the growing age of the stand, the value of TF <sup>137</sup>Cs increased, while the value of TF <sup>40</sup>K was not dependent on the age of the stands. Birch wood accumulated more <sup>137</sup>Cs and <sup>40</sup>K isotopes than pine wood. (4) Conclusions: the results show a much greater radioactive contamination of the environment in eastern Poland during the testing with nuclear weapons than after the Chernobyl nuclear reactor explosion. The greater accumulation of radiocesium in birch wood than in pine wood predisposes this species to be more useful when assessing the radioactive contamination of the environment from the past.
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