Growth and morphology in balsam fir: effects of gamma radiation

Abstract

Balsam fir (Abies balsamea (L.) Mill.) is one of the most radiosensitive tree species in the mixed boreal forest of southeastern Manitoba. Trees were exposed for 11 years to gamma radiation at dose rates ranging from background to 62 mGy/h. With increasing radiation dose, many trees and saplings were killed. After 11 years of chronic gamma irradiation the total accumulated dose resulting in 50% death (LD50) was 110 Gy while the dose rate required to kill 50% of the trees (LR50) was 1.5 mGy/h and was still decreasing. At dose rates > 1 mGy/h, growth of leaves and branches and the number of lateral and terminal buds were significantly depressed. At dose rates > 2 mGy/h, lateral and terminal buds were often killed. Few obvious morphological effects were seen on the leaves of irradiated firs, although the leaves were often significantly longer at dose rates ranging from 0.5 to 5 mGy/h. Extra lateral and terminal buds resulting in bushier trees were formed on some fir trees receiving long-term, low-level radiation (0.05 to 0.3 mGy/h). Annual branch growth was also significantly greater. In addition, sapling numbers were greater in plots receiving low levels of radiation (0.078 to 0.11 mGy/h) than in control plots. Other environmental factors, such as the amount of soil moisture and soil conditions, had an ameliorating effect on survival of balsam fir saplings exposed to radiaton.