Prescribed Burning of Thinning Slash in Regrowth Stands of Karri (Eucalyptus diversicolor) .1. Fire Characteristics, Fuel Consumption and Tree Damage

Abstract

'Fire characteristics, fuel consumption and fire-caused tree damage were investigated for ten low to moderate intensity fires in thinning slash fuel in 22-year-old karri (Eucalyptus diversicolor F. Muell) regrowth in Western Australia. Eight fires were burnt in spring and two in autumn. Most fires consumed greater than 80 per cent of dead fuel < 25 mm in diameter and between 40 per cent and 80 per cent of the litter layer. The total amount of litter and dead fuel < 100 mm in diameter consumed during burning ranged from 24 to 79 t ha^-1 (31-89 per cent). Actual and percentage consumption of fuel < 100 mm diameter were inversely related to the moisture content of the litter profile. Fireline intensities determined from flame length estimates ranged from 90 kW m^-1 to 2820 kW m^-1. Damage to retained trees included stem wounding, crown death and complete death of the stem. Burning in spring when the litter profile was moist resulted in damage to the lower stems of up to 80 trees ha^-1, with damage affecting an average of 3 m² ha^-1 of the retained basal area of 19.5 m² ha^-1. The incidence of stem damage following spring burning was considerably greater when the litter layer was dry. Autumn burning resulted in extensive stem damage and tree mortality and is not therefore recommended. Log debris within 1 m of trees significantly increased the probability of fire-caused damage, and wounds tended to be larger on trees that had debris nearby. The incidence of damage from burning was directly related to the quantity of fuel consumed and heat release per unit area, but was not significantly correlated with fireline intensity. A prescription for burning thinning slash during spring conditions is presented.