The impact of a severe wildfire on canopy structure and composition in a lowland mixed-eucalypt forest in southeastern Australia

Abstract Background Forest fires of unprecedented scale and intensity have become a more frequent occurrence in many parts of the world. In southeastern Australia, the Black Summer fires of 2019–2020 impacted nearly 20 million hectares of forests. After more than a century of human impacts and other fires, there is a risk of shifts in species composition and forest structure. We studied lowland mixed-eucalypt forest in eastern Victoria to determine how variability in fire intensity influenced the structure and composition of the forest canopy. Results We found that resistance to low-intensity fire, as measured by avoidance of crown loss, increased with increasing tree size and varied among species. Resistance to moderate- to high-intensity fire was low, but all species showed significant resilience, with 95% of trees able to recover all or a portion of their crowns through epicormic resprouting. Resilience increased with increasing tree size and varied among species. Eucalyptus sieberi was the least resilient and E. baxteri was the most highlighting an inverse relationship between resistance and resilience . Mortality (i.e., crown loss with no recovery) disproportionally impacted large E. sieberi trees subjected to high-intensity fire. Conclusions As fire intensity increased, the relative proportions of the various species in the upper canopy shifted. Some species increased, while others decreased. Low-intensity fire however resulted in very little change to the structure and composition of the forest canopy. While areas of high-intensity fire may lead to shifts in relative abundance and dominance of eucalypt species, the general resilience of the eucalypt species to fire suggests a substantial inertia in the species composition in these forested landscapes. However, changes in canopy structure due to crown mortality in E. sieberi promoted increased openness which could promote this species regeneration and create a positive feedback loop which facilitates a shift in species composition in gaps created by crown and tree mortality.