Determinants of the occurrence of unburnt forest patches: Potential biotic refuges within a large, intense wildfire in south-eastern Australia

Abstract

Large, intense fires are generators of heterogeneity in many ecosystems. An important component of this heterogeneity is the occurrence of unburnt patches within the fire boundary: these fulfil a number of ecological functions including serving as refuges for fire-sensitive organisms. An important issue for land managers is the degree to which potential refuges occur ‘naturally’ versus occurring as a result of interventions such as planned burning. Here, we examine the factors contributing to the occurrence of unburnt patches within the Kilmore–Murrindindi fire complex, a severe wildfire that encompassed ∼250,000ha and resulted in 159 human fatalities in Victoria, Australia, in 2009. Though an extreme event, this fire is representative of large, intense fires that periodically occur in south-eastern Australia. Unburnt patches ⩾1ha occupied less than 1% of the area within the fire boundary, with mean size of 27.1ha (range 1–306ha). Overall, the probability of points within the fire boundary remaining unburnt was most strongly influenced by topographic position, vegetation type and fire intensity in the surrounding area. In dry eucalypt forest, time since fire (a surrogate for fuel structure) was also important. The influence of all factors was strongly contingent on prevailing weather during the fire, with no unburnt patches occurring in extreme fire conditions. In this fire, most unburnt patches arose ‘naturally’ due to microclimatic effects of topography and vegetation. While fuel reduction burning has the potential to create unburnt patches in some circumstances, these are likely to exhibit simpler vegetation structure than those arising due to microclimate. Potential refuges in the form of unburnt patches are more likely to arise in landscapes with greater topographic and vegetational variation. Conversely, in more uniform environments, fuel load and/or continuity are likely to have a greater influence on the occurrence of unburnt patches. Understanding these relationships will assist fire managers in directing resources at producing ecological outcomes that are less likely to arise without intervention.