Tolerance of Pinus rigida trees to a ten-year exposure to chronic gamma irradiation from cobalt-60

Abstract

The Brookhaven Gamma Field was established primarily to expose cultivated plants to chronic gamma irradiation from cobalt-60. However, after about 7 growing seasons, injury to the native Pinus rigida trees bordering the cultivated field became apparent at exposure rates as low as 5 r/20 hr day. Detailed observations of these trees were started in 1958 and cumulative exposures and average daily exposure rates (obtained by dividing the cumulative exposure by the number of exposure days) were calculated for trees in 10-meter arcs around the source. Cumulative and daily exposures varied from year to year since periods of exposure and the size of the source were not constant. Source size varied from 145 to 3950 effective curies. Radiation injury was expressed in terms of mortality, growth inhibition and reduced seed set. Tree mortality was recorded in May of 1960 and subsequently in each August through 1963 and survival curves plotted for a population of about 500 trees. By the end of 1960 (8 years of exposure) 50 per cent of the trees had been killed by a cumulative exposure of 5.8 kr (3.1 r/day) but a few trees survived exposure rates as high as 10 r/day. Further radiation injury accumulated after 1960 so that by August of 1962 about 20 per cent of the trees had been killed by as little as 5.0 kr at about 2.5 r/day and at an average exposure rate of 3 r/day, totalling about 7.4 kr, 50 per cent mortality occurred. No trees survived cumulative exposures above 13 kr. These lethal exposures are much higher than those reported in other species of pine after acute irradiation. The effect of long-term radiation on tree growth was measured as a reduction in radial enlargement of the trunk and as reduction in needle length. Xylem increments, measured after several years' exposure at less than 4 r/day, were reduced in the upper part of the tree and in some cases completely missing at the base of the tree. The irradiated cambial cells which had produced no xylem increment during several years of poor growth retained the capacity for producing new xylem in favorable years. Reduced growth of needles proved to be a good sublethal index of radiation damage. Slight growth inhibition could be observed at average rates as low as 1.5 r/day on trees which had received a cumulative ten-year exposure of about 3.6 kr and 50 per cent growth inhibition occurred at about 3 r/day. Reproductive processes were affected at low exposure rates also. The number of mature seeds per cone was reduced to about 10 per cent of the control value after an average exposure of about 3.5 r/day or a cumulative exposure to the tree of 7.4 kr for a nine-year period. No seed-bearing cones were formed above about 7.4 kr cumulative exposure. Unpublished data of the authors on interphase chromosome volumes of gymnosperms suggest that this species of pine should be fairly typical of the genus with respect to radiation sensitivity. Other genera of gymnosperms are also expected to have relatively high sensitivities but none is expected to exceed this genus by more than a factor of about 2.