Response of a forest ecotone to ionizing radiation. Progress report, 15 October 1974--14 October 1975

Compositional and structural characteristics of three forest types, including aspen dominated, maple-birch dominated and an intervening ecotone, were studied before and after chronic γ irradiation in northern Wisconsin. Irradiation occurred during the summer of 1972. By the summer of 1973 the density of viable tree seedlings at 10 m from the radiation source was substantially reduced in all three areas relative to the preirradiation densities of 1971. Re-establishment of tree seedlings has, as of June 1975, been prevented by the vigorous development of ground vegetation. At 20 m, the density of seedlings in the three areas increased during the period between 1971 and 1973 in response to the partially opened canopy. The overall density of seedlings of Populus tremuloides and Acer rubrum increased markedly in the three areas and P. tremuloides invaded some areas in which it had not been found prior to irradiation. By 1974 many of the seedlings at 20 m had reached the sapling size-class. Viable trees greater than 2.5 cm, dbh were eliminated at 10 m in all three areas by June 1974 and were reduced in density of 20 m. Leaf litter production was reduced by up to 94 percent at 10 m and 64 percent at 20 m during the period between 1971 and 1973 in the irradiated areas. (U.S.)