Airborne radiocesium in Scots pine and Norway spruce needles

This paper presents the activity of radiocesium (134Cs and 137Cs) and chemical composition of Scots pine and Norway spruce needles in southern Finland during the years 1987–1991. In 1991, contents and vertical distribution of radiocesium in soil have been determined both under and between canopies. The total 134Cs content per square meter in soil layers varied within the range of 470–9500 Bq/m2 in stands of Norway spruce and 1200–2300 Bq/m2 in stands of Scots pine; the corresponding figures for 137Cs were 5000–88000 and 11 100–28600 Bq/m2. About 80–90% of the radiocesium was located in the humus and in the first 3 cm of mineral soil. There were no statistically significant differences in the contents of 134Cs and 137Cs in the soil under and between the tree crowns. Most of the 137Cs which originated from nuclear weapons fallout, was in the top mineral soil layers. The radiocesium concentrations in the needles varied considerably from stand to stand. In 1987, with the exception of a few forests, the radiocesium concentrations in the 2-year-old (C + 1) needles were higher than those in the current (C) needles. In the other years, the situation was the opposite. The regression between 137Cs concentration in current needles and the year was linear for pine needles and polynomial for spruce needles.