The field-irradiator gamma study: Fourteen years of irradiation of the boreal forest

The Field-Irradiator Gamma (FIG) project is a long-term experiment on the response of boreal forest vegetation to chronic ionizing radiation. The forest was irradiated from 1973 to 1986 by a 370 TBq point source of 137Cs placed at a height of 20 m. The forest is now in the recovery phase. The irradiated forest included several different community types, and each of these was affected differently by the radiation stress. New vegetation zones have now been created because of the selective tolerance to radiation along a gradient from background dose rates to a maximum of 65 mGy h-1. One of the easiest measured indicators of the radiation stress has been photographic documentation of changes in forest communities over time. Measured changes in species composition and the decrease in tree canopy cover at dose rates >2 Gy h-1 have also helped quantify radiation effects. Indicators such as trends in annual growth rings have been less satisfactory. Our experiment suggests that there are no visible impacts at chronic dose rates less than 0.1 mGy h-1 and the threshold for effects likely is between 0.1 and 1 mGy h-1. The experimental area has been preserved to allow measurements of long-term recovery of the site