Effects of chronic gamma radiation on the composition of the herb community of an oak-pine forest

A complete inventory of the herb populations of the inner 50 m of the Brookhaven irradiated forest was made in 1968 and 1973. The data are compared with those of earlier inventories, giving information on changes through a ten-year period in a community exposed to chronic radiation stress.

Per cent cover of Carex pensyluanica, which dominates the zone 20–40 m from the source (125-25 R/day in 1973), increased rapidly at first and then more slowly through 1968, then declined somewhat by 1973. There was no shift in its distribution over ten years. Sharp increase in Rubus spp. cover in this zone contributed to the most recent Carex decline.

A greater than eight-fold rise in total density of species other than Carex occurred since 1968, mostly due to a large increase in density of important species, while only those of infrequent occurrence declined or disappeared by 1973. Rate of change in herb populations was slowed by high exposures close to the source and by competition with Carex and Rubus in the sedge zone. The distribution of Rumex acetosella changed in a manner suggesting possible genetic alteration of radiosensitivity. There was a net increase of four species between 1968 and 1973.

Species diversity increased with distance from the source, increased with time from 1968 to 1973 in the outer 40–50 m zone, and changed relatively slowly with time in the inner zones of higher exposure rates. The reasons for the comparatively slow rates of successional change are discussed.

We predict that with future reduction in radiation stress due to source decay and with continuing expansion of Rubus spp. clones, Carex dominance will continue to decline as species diversity and herb population densities increase slowly.