How an unprecedented wildfire shaped tree hollow occurrence and abundance—implications for arboreal fauna

Tree hollows are an important habitat resource used by arboreal fauna for nesting and denning. Hollows form when trees mature and are exposed to decay and physical damage. In the absence of excavating fauna, hollow formation can take up to 200 years in Australian temperate Eucalyptus forests, making tree hollows a critical but slow forming habitat feature. The increasing frequency and severity of wildfires due to climate change has led to increased concern about the landscape-scale loss of nesting space for arboreal fauna, including endangered species such as the folivorous southern greater glider (Petauroides volans). To understand patterns of nesting resource availability, we assessed drivers of hollow occurrence in southeastern Australian mixed-species Eucalyptus forests and quantified the effects of an unprecedented large-scale wildfire, the 2019/2020 Black Summer bushfires, on hollow occurrence and abundance. Tree size and shape, as well as site productivity and topography, were important predictors for hollow occurrence both before and after the fires. The occurrence of the southern greater glider was strongly dependent on high proportions of hollow-bearing trees. While high fire severities had a negative impact on southern greater glider occurrence, the number of hollow-dependent arboreal species was not affected. While the wildfires significantly reduced hollow abundance, we did not find significant effects on hollow occurrence. Fires altered the relationship between tree size and hollow occurrence expressed as a change in the probability of hollow occurrence, with a higher likelihood at smaller tree sizes after the fires. Our findings suggest that post-fire nesting space may be reduced at the tree-scale, while at the stand-scale, hollow-bearing trees persist as biological legacies. These persisting trees can support the recovery of hollow-dependent arboreal fauna, such as the endangered southern greater glider by providing denning and nesting space. Hollow-bearing trees that survived the fires have the potential to form new hollows faster compared to undisturbed mature trees.