Effects of particle fracturing and moisture content on fire behaviour in masticated fuelbeds burned in a laboratory

Mechanical mastication is a fuels treatment that converts shrubs and small trees into dense fuelbeds composed of fractured woody particles. Although compaction is thought to reduce fireline intensity, the added particle surface area due to fracturing could also influence fire behaviour. We evaluated effects of particle fracturing and moisture content (ranging from 2.5 to 13%) on fire behaviour in fuelbeds composed of masticated Arctostaphylos manzanita Parry and Ceanothus velutinus Dougl. shrubs in the laboratory. Fuelbeds composed of fractured particles did not burn with greater intensity than fuelbeds composed of intact particles, as hypothesised. Flame heights ranged from 54 to 95 cm and fireline intensity from 50 to 140 kJ s–1 m–1, approximating values observed in field experiments. Masticated fuelbeds burned with shorter flame heights and longer flaming duration under higher fuel moistures, but duration of lethal heating (>60°C) above fuelbeds did not differ across the range of fuel moistures, averaging 12 min over a 0.1-m2 area. Our results suggest that expected fire behaviour increases due to particle fracturing may be overwhelmed by fuelbed bulk density. The long-duration heating of burning masticated fuels may require managers to mitigate effects to trees and soils when fuel loads are high.