Effect of prescribed burning on wildfire severity: a landscape-scale case study from the 2003 fires in Victoria

This study examined the effect of previous fuel reduction burning (FRB) on the severity of the >1 million ha 2003 Alpine Fire in eastern Victoria. Sixty-five paired observations (130 total) of fire severity were completed across the broad fire area using GIS (Geographic Information System) analysis. Despite the broad scatter in this large data set, a number of strong trends were evident. A Fire Severity Index (FSI) was calculated from the correlated fire, weather and topographic variables, with a three-factor model using the forest fire danger index (FFDI), fire or FRB age and percentage of north-western aspect, best explaining the variation in measured fire-severity data. The most important finding was that the reduction in fire severity and suppression assistance effects of previous fuel-reduction burning started to decline substantially when the FFDI exceeded 50. Above FFDI 50, landscape-scale fires became ‘weather-dominated’ and variation in fuel and topography became less important to continued fire spread. The greatest effects of previous FRB in reducing wildfire severity and in assisting fire suppression occurred when (1) the FFDI fell to 25 or less (late in the evening and overnight); (2) the age of the FRB was less than 3 years (i.e. when all three components of fuel—surface, bark and elevated material—were still substantially reduced). Some fire-severity reduction effects were still evident for FRBs up to 10 years old, but there was almost no evidence of FRBs older than 10 years having any effect on fire severity. FRBs up to 10 years old also had measurable effects on increasing burnt area patchiness and decreasing canopy loss, both of which have ecological implications. This study provides fire managers planning rotational landscape FRB with important information on likely effects of the burning on fire severity.