PROLONGED INFLUENCE OF WILDFIRES ON THE GEOCHEMISTRY OF FOREST SOILS, ISLE ROYALE NATIONAL PARK, MICHIGAN AND VOYAGEURS NATIONAL PARK, MINNESOTA

Soil studies in Isle Royale and Voyageurs National Parks show geochemical patterns that correlate with severe historic wildfires that occurred in 1936. In both parks A-horizon soils in areas of the 1936 fires have reduced concentrations of C, Hg, Se, and S compared to adjacent unburned areas. Similar depletion is not present in underlying C-horizon soils suggesting that the A-horizon chemistry is not controlled by underlying bedrock or glacial deposits. In addition, the bedrock of the two parks is very different. Voyageurs Park is underlain by pelitic schist, migmatite, and granite, whereas Isle Royale Park is underlain by basalt and sandstone. In spite of a contrast in bedrock compositions between the two parks, A-horizon soil compositions in both parks show similar correlations with the extent of the 1936 wildfires, further indicating that the geochemical patterns in A-horizon soils are largely independent of geologic influences. Severe forest fires, which consume a high percentage of forest-floor organic material, appear to have a long-lasting effect on soil compositions. Soil carbon content is perturbed for decades because of initial loss of organic material by combustion and by longer-lasting effects of reduced litter input because of destruction of mature forests by the fires. Carbon in A-horizon soils varies from a mean of 17.7% in unburned areas to 8.7% in burned areas in Voyageurs Park and from 10.2% to 7.5 % in Isle Royale Park. Hg, Se, and S are highly correlated with carbon content of soils suggesting that these elements are bound to organic carbon compounds and thus their concentrations are controlled by temporal variations in soil organic matter. Soil concentrations of such easily volatilized elements appear to be strongly influenced by forest fire history and, like carbon, vary cyclically between stand-initiating fires as the mass of forest floor organic material is reduced during and after fires and gradually replenished during forest regeneration.