The Impact of Forest Fire on the Nutrient Influxes to Small Lakes in Northeastern Minnesota

Abstract

The Little Sioux fire of May 1971 burned most of the mixed coniferous—deciduous forest on the watersheds of Meander and Lamb lakes, two small, low conductivity lakes located in the Boundary Waters Canoe Area (BWCA) of northeastern Minnesota, USA. During 1972, hydrologic and chemical (Ca, Mg, K, Na, and P) budgets were determined for the terrestrial watersheds of Meander and Lamb lakes and for the lakes themselves. Budgets were also measured for Dogfish Lake, a lake physically and chemically similar to Meander Lake but whose watershed was not burned in the Little Sioux fire. These budgets show that the atmosphere supplies a significant fraction of the cations and phosphorus (Ca, 90%; Mg, 35%; K, 95%; Na, 55%; P, 95%) to the BWCA watersheds, with the remainder coming from chemical weathering. The budgets are similar to those reported for other Canadian Shield lakes and watersheds. The impact of the Little Sioux fire on nutrient fluxes was evaluated by comparing the budgets measured at Dogfish Lake with those measured at Meander Lake. This comparison showed that as a result of the burning of Meander Lake watershed, runoff increased 60% and the K and P exports increased 265% and 93%, respectively. The exports of Ca, Mg, and Na did not change significantly. The increase in runoff, probably due to the reduction in vegetative transpiration, is comparable to that measured after the 1970 Entiat fire (western Washington) and to that resulting form clear—felling experiments at Hubbard Brook (New Hampshire) and Coweeta (North Carolina). The increased nutrient losses after fire are less than those observed at Hubbard Brook and Coweeta. There was no indication of a drastic increase in nitrate export such as occurred after the Entiat fire and at Hubbard Brook. The increase in phosphorus loading of Meander Lake due to the fire was 38% (to 25 mg/m2°yr), a value that probably falls within the natural year—to—year variation in supply, and it did not represent a major impact on Meander Lake. Fire is a natural part of the BWCA forest ecosystems. The results of this study indicate that nutrient losses after the Little Sioux fire were minimal, perhaps because it was a spring fire. Fall fires may cause larger nutrient losses because more of the forest—floor material is likely to be consumed and revegetation does not begin until the following spring.