EFFECTS OF FIRE FREQUENCY ON TREE CANOPY COVER AT ALLISON-SAVANNA, EASTCENTRAL MINNESOTA, USA

Allison Savanna (bur oak-northern pin oak barrens) in eastcentral Minn esota has been managed since 1962 using prescribed burns conducted at different intervals in eight units. To evaluate fire management succes s, preburn tree canopy cover was digitized from aerial photos in 1938 and 1960 (prior to burning) and again in 1987 (following 25 years of b urns.) The fire interval ranged from 1.6 to 5.0 years. Change in tree canopy cover was compared between 1938 and 1960, and between 1960 and 1987, to determine the relationship between fire interval an canopy co ver. In all units, canopy increased between 1938 and 1960, prior to pr escribed burning. In 1987, after 25 years of periodic burning, change in canopy cover showed a significant negative relationship with the nu mber of burns. Percent of wetland in each burn unit did not have a sig nificant effect on changes in canopy cover. Three vegetation plots est ablished in 1990 in unburned, low-, and high-burn frequency units show ed that, with increased burns, fires limited oak recruitment. Mature b ur oaks (Quercus macrocarpa) were more abundant than northern pin oak (Q. ellipsoidalis) in the high-burn unit. Age of bur oak stems ranged from 20 to 200+ years, whereas northern pin oak was usually < 30 years . Results of these small-scale burns should be interpreted cautiously at larger spatial and temporal scales because of the longevity of oaks and the interactions of fire with climatic conditions and topographic features.