Fuelbed ignition potential and bark morphology explain the notoriety of the eucalypt messmate 'stringybark' for intense spotting

Abstract

In south-eastern Australia, bark of the ‘stringybark’ eucalypt group, and notably that of messmate (Eucalyptus obliqua L’Hér.), is notorious for intense spotting and it is likely that firebrands of this type contribute significantly to loss of life and property during major wildfires. The fuelbed ignition potential by glowing samples was laboratory tested on excised sections of Pinus radiata D. Don litterbed at moisture contents between 2 and 8% and for ‘no-wind’ or for ‘wind’ of 1 m s–1. Prepared samples of outer bark between 0.5 and 1.6 g were combusted until they attained ~20% of initial mass before placement. For ‘no-wind’, flaming ignition did not occur and the probability of glowing ignition appeared to depend on the chance of contact with needles. For ‘wind’, the probability of flaming ignition was a function of sample glowing mass and fuelbed moisture content. Its ignition probability, weight-for-weight, appears to exceed that reported for other forest firebrands, including bark of E. globulus Labill. However, it is likely that the notoriety of messmate bark is also due, in part, to its morphology and in-flight behaviour. It is argued that firebrand laboratory tests to date may poorly reflect the ignition potential of similar samples after flight.