Are fire refugia less predictable due to climate change?

Fire refugia - unburnt habitat within a wildfire's perimeter - play a key role in wildlife persistence and recovery. While studies have shown that the location of refugia is influenced by local topographic factors, growing evidence points to extreme fire weather becoming the dominant factor driving high-severity wildfires that result in the location of fire refugia being less predictable. Between September 2019 and February 2020, a series of mega-fires in eastern Australia burned largely in broadleaf forest. We assessed burned and unburned areas of forest in eastern Australia using Sentinel-2 satellite data, aggregated monthly over the fire season to calculate a fire severity layer at a 20 m pixel resolution. We found that fires burned 5.7 × 106 ha-1 of forest and woodland. The total percentage area of unburned forest within the wildfire footprint was approximately 10%. The majority (94%) of the unburnt forest and woodland patches within the fire perimeter occurred as patches <1 ha (n = 842 622 and 111 707 ha) with far fewer large unburnt patches (>100 ha) (n = 575 and 286 080 ha). Boosted regression tree analyses of the relationships between fire severity and potential explanatory variables revealed that 63%-78% of the variable importance in the models were climatic and weather-related factors. Fire weather index was the single most important variable for analyses, accounting for 40%-52% of modelled results. Our results reinforce mounting evidence that a shift is underway in the balance between deterministic and contingent factors in the occurrence of fire refugia with local topographic controls being increasingly overridden by severe fire weather conditions, and declining topographic effects as fire severity increases. Further studies are needed over a longer time frame, inclusive of prior forest management impacts, to confirm that the ability to predict fire refugia is permanently declining.