Effects of ionizing radiation on a natural plant community

Abstract

An “island community” of spring and summer annuals transplanted from a granite outcrop to the gamma radiation field on the Emory University campus received ionizing radiation along a gradient ranging from 8,000 to 130,000 r at rates of 5–82 r/hr from March to August 1959. Ecological analyses of experimental and control communities over the subsequent three year period revealed both immediate and long-range effects of ionizing radiation upon individual species and community attributes. Results indicate that ionizing radiation has both stimulatory and inhibitory effects upon plant growth and that the ecological and morphological effects are reflected by species interactions at the community level. Certain post-irradiation changes in the spring flora are interpreted as resulting from a “stimulating” effect of radiation doses of 8,000–30,000 r upon Arenaria brevifolia Nutt. Following irradiation of the parental generation at these levels, the first filial generation of Arenaria was observed to increase in density, distribution, and growth at the expense of a competitive species, Diamorpha cymosa (Nutt.) Britton. These “stimulatory” effects, apparent only in the first generation, made it possible for this species to continue as the dominant species in the succeeding two generations studied. Other changes in the spring flora resulted from lethal and inhibitory effects of radiation upon plant growth. Changes in the summer flora are interpreted as resulting from the selective elimination of radiation-sensitive species and the subsequent positive selection pressure toward radiation-resistant species. Results of autecological studies indicate that the physiological tolerances of all species usually varied inversely with the level of radiation received. However, under certain conditions, plants which received radiation levels of 8,500–25,000 r exhibited environmental tolerances which surpassed those of non-irradiated controls. Whenever physiological stimulatory effects were observed they coincided with similar ecological and morphological stimulatory effects observed in the same species at radiation levels comparable to those used during community analysis. Under all experimental conditions species tolerances varied independently of dose rate. Results of autecological analyses of the post-irradiation tolerances of spring and summer annuals to a variety of environmental stresses correlate with and confirm results obtained from the community analysis. Under all environmental conditions studied, species which became dominant following the irradiation period consistently exhibited greater physiological tolerances to radiation than did the normally dominant species which they replaced.