Fuel reduction burning reduces wildfire severity during extreme fire events in south-eastern Australia

Extreme fire events have increased across south-eastern Australia owing to warmer and drier conditions driven by anthropogenic climate change. Fuel reduction burning is widely applied to reduce the occurrence and severity of wildfires; however, targeted assessment of the effectiveness of this practice is limited, especially under extreme climatic conditions. Our study utilises fire severity atlases for fuel reduction burns and wildfires to examine: (i) patterns in the extent of fuel treatment within planned burns (i.e., burn coverage) across different fire management zones, and; (ii) the effect of fuel reduction burning on the severity of wildfires under extreme climatic conditions. We assessed the effect of fuel reduction burning on wildfire severity across temporal and spatial scales (i.e., point and local landscape), while accounting for burn coverage and fire weather. Fuel reduction burn coverage was substantially lower (similar to 20-30%) than desired targets in fuel management zones focused on asset protection, but within the desired range in zones that focus on ecological objectives. At the point scale, wildfire severity was moderated in treated areas for at least 2-3 years after fuel treatment in shrubland and 3-5 years in forests, relative to areas that did not receive fuel reduction treatments (i.e., unburnt patches). Fuel availability strongly limited fire occurrence and severity within the first 18 months of fuel reduction burning, irrespective of fire weather. Fire weather was the dominant driver of high severity canopy defoliating fire by similar to 3-5 years after fuel treatment. At the local landscape scale (i.e., 250 ha), the extent of high canopy scorch decreased marginally as the extent of recently (<5 years) treated fuels increased, though there was a high level of uncertainty around the effect of recent fuel treatment. Our findings demonstrate that during extreme fire events, very recent (i.e., <3 years) fuel reduction burning can aid wildfire suppression locally (i.e., near assets) but will have a highly variable effect on the extent and severity of wildfires at larger scales. The patchy coverage of fuel reduction burns in the wildland-urban interface indicates that considerable residual fuel hazard will often be present within the bounds of fuel reduction burns.

Automated summary

Extreme fire events have increased in south-eastern Australia due to human-induced climate change. Fuel reduction burning is commonly used to reduce wildfires, but its effectiveness under extreme climatic conditions is limited. This study examines the extent of fuel treatment in planned burns across different fire management zones and the impact of fuel reduction burning on wildfire severity. The results show that fuel reduction burn coverage is often lower than desired targets in zones focused on asset protection, but within the desired range in zones targeting ecological objectives. At the point scale, fuel reduction burning moderates wildfire severity for several years in shrubland and forests. Fuel availability plays a significant role in limiting fire occurrence and severity in the first 18 months after fuel reduction burning, while fire weather becomes the dominant driver of high severity fires after 3-5 years. At a local landscape scale, the extent of high canopy scorch decreases as the extent of recently treated fuels increases. However, there is a high level of uncertainty regarding the effect of recent fuel treatment. Overall, recent fuel reduction burning can aid in wildfire suppression near assets, but its impact at larger scales is highly variable.