Changes in fuel hazard following thinning operations in mixed-species forests in East Gippsland, Victoria

Abstract

SummaryForests in East Gippsland pose a high inherent fire risk to both internal forest values and surrounding settlements. Thinning operations have been perceived to potentially heighten this risk. Part of this risk relates to the level of fuel hazard, which is influenced by the occurrence of shrub and bark fuel. This study examined the changes in the overall fuel hazard (OFH) (McCarthy et al. 1999) and an equivalent OFH (derived for thinnings fuels) over a 15-year period after thinning. Contrary to the belief of many Gippsland fire managers, the equivalent OFH at thinned sites was on average lower than the OFH in adjacent unthinned sites, primarily due to the reduction of elevated fine fuel (bark and shrubs especially). Fine fuel from slash had largely decomposed by 4 years after thinning. Woody debris larger than 100 mm in diameter remained for 15 years or longer. Thinning, by reducing the OFH, may reduce the likely suppression difficulty by substantially reducing the potential for vertical development of fire at the flaming fire front. However, by increasing the amount of coarse woody material on the ground, thinning may increase the complete extinguishment (blacking-out) difficulty. Opportunistic observation of comparative wildfire severity in adjacent thinned and unthinned forest supported these findings. Thinned coupes may not present the fire hazard that has been assumed—they may in fact be areas of lower fire risk for at least 15 years. Once alight, however, longer burn-out times in the coarse material may prolong complete extinguishment efforts.