The effectiveness of fuel reduction burning for wildfire mitigation in sclerophyll forests

The wildfires in south-eastern Australia in the 2019-20 fire season were some of the worst in recent memory. The effectiveness of fuel-reduction burning as a risk mitigation strategy is, once again, being scrutinised. Some argue that more fuel-reduction burning is needed, while others suggest that it is of limited use in such extreme fires. In this study, we tested the effectiveness of fuel-reduction burning at a landscape scale in terms of its ability to reduce the severity of subsequent wildfire. To achieve this, we selected all the recent (2015-2019) fuel-reduction burns undertaken in New South Wales and Victoria that intersected with the extent of the 2019-20 wildfires and evaluated whether the fire severity was significantly different in the recently treated areas to that of similar untreated areas in the vicinity. To determine fire severity, Sentinel 2 satellite imagery and the change in normalised burn ratio (a common metric used for rapid and broadscale fire severity mapping) was used. Our results showed that 48% of the 307 recent fuel-reduction burns resulted in statistically significant decreased fire severity. Our results also indicated that more recent fuel-reduction burns had a greater impact, with 66% of burns undertaken in 2019 significantly reducing severity, compared with 42% from 2015. We also analysed each fuel-reduction burn in the context of a range of metrics, including location, elevation, slope, aspect and forest heterogeneity, to assess whether these factors influenced the likelihood that a burn would be effective. Location and, to a lesser degree, forest heterogeneity were found to be significant factors. Our results support the view that recent fuel-reduction burns reduce fire severity. It is unclear, however, whether the differences would be operationally significant under extreme conditions, when wildfires are driven largely by weather, irrespective of fuel loads.