Prescribed Burning of Thinning Slash in Regrowth Stands of Karri (Eucalyptus diversicolor) .2. Nitrogen Budgets in Pre- and Post-Burn Fuel

Abstract

Changes in nitrogen content of fuel were investigated following eight low to moderate intensity experimental fires conducted under a range of fire weather conditions in a recently thinned 22-year-old regrowth stand of karri (Eucalyptus diversicolor F. Muell) in Western Australia. The average amount of dead fuel < 100 mm in diameter present before burning was 76 t/ha (range 50 to 107 t/ha). The amount of live fuel was small, with a mean of 4 t/ha. Forest floor litter, consisting of fresh and partly decomposed dead leaves and fine twigs (< 6 mm diameter) contributed about 30% of total fuel weight, and wood fractions > 6 mm in diameter contributed about 60%. Remaining fuel was made up of small twigs, bark and leaves added from the thinning operation. Distribution of nitrogen in fuel fractions differed markedly from distribution of fuel weights with more than 60% of fuel-nitrogen in the litter and only about 25% of fuel-nitrogen in wood greater than 6 mm diameter. Following fire, the average amounts of nitrogen in all fuel fractions was reduced. The majority of nitrogen in wood fractions and in bark and leaves was volatilized during the fires (range 55% to 99%), while on average only about 38% of the nitrogen in litter was volatilized. The amount of nitrogen lost from dead fuel differed between the experimental fires (range 50 to 180 kg/ha) and was significantly related to the total amount of fuel consumed (r² = 0.92). Fuel consumption and nitrogen volatilization increased as the Soil Dryness Index increased and as litter moisture decreased. Nitrogen losses due to burning were small relative to total stores of nitrogen in soil (about 6000 kg/ha) but for the most intense fires were significant in relation to amounts in growing vegetation and surface soil. Burning when moisture content of the litter profile exceeds 90% will reduce combustion of the litter layer. Burning under these conditions allows effective reduction in the flash fuel components located in the upper parts of the fuel bed while retaining much of the nutrient-rich lower strata of fuel. Burns of this type provide effective fire hazard reduction while favouring conservation of nitrogen stored in the litter layer.