Impacts of wild fire severity and salvage harvesting on the nutrient balance of jack pine and black spruce boreal stands

In August of 1995, wildfires burnt over 50 000 ha of boreal forest in northwestern Quebec. A balance sheet approach was used in order to assess the long term effects of fire and subsequent salvage harvesting operations on nutrient site capital. Following a validation of burn severity indices and maps, we conducted an evaluation of soil nutrient pools in (1) lightly to moderately, (2) severely burned, and (3) unburned stands with similar biophysical characteristics. Above-ground biomass values for unburned stands, precipitation and N biological fixation inputs were drawn from the literature. Weathering rates were drawn from previous work and estimated with the PROFILE model. Fire significantly reduced forest floor dry weight by 41% in the light/moderate class and by 60% in the severe class while forest floor total Ca concentrations increased following both types of burn. Forest floor exchangeable Ca and total Mg concentrations increased following a light/moderate burn. Fire increased exchangeable K concentrations in the 0–10 cm mineral layer but had no other effects on mineral soil concentrations or characteristics. Forest floor nutrient content was significantly reduced on severely burned areas only. Kjeldahl N content was reduced by 44%, exchangeable Mg by 53% and exchangeable K and total K by 60 and 51%, respectively. Reduction of K soil content was important enough that inputs through weathering and precipitation would take 278 years to compensate for soil losses following a severe fire. The projected effects of salvage harvesting on severely burned sites indicated that Ca, Mg and K would not return to their pre-burn level in the course of a 110-year rotation.