Bark attributes determine variation in fire resistance in resprouting tree species

Predicting the impact of wildfires on ecosystem services and habitat values requires quantifying rates of post-fire tree mortality and topkill. For those species that resprout epicormically (i.e. from above-ground buds), rates of post-fire topkill (death of aboveground biomass) can vary considerably. Laboratory studies indicate that bark attributes are key determinants of post-fire topkill in these resprouting species. Specifically, bark thickness and bark density influence the capacity of bark to insulate the cambium from the lethal temperatures generated during wildfires. Field studies are generally consistent with these laboratory studies and demonstrate that smaller trees, with thinner bark, are more vulnerable to post-fire topkill. However, comparatively few studies model topkill explicitly as a function of bark thickness, and fewer still model topkill as a function of bark density. In this study we measured post-fire mortality and topkill across eight tree species with varying bark types. We also estimated pre-fire bark thickness (from relationships between stem diameter and bark thickness derived from unburnt forest) and measured bark density. We undertook our study at two dry sclerophyll eucalypt forests located in eastern Australia. The two study areas were subject to wildfire 18 months prior to measurements, with one site characterised by a semi-arid climate, and the second site (located 400 km south-east) characterised by a humid climate. We found that species with thick bark and a low bark density were most resistant to topkill. We defined vulnerability to topkill as the stem diameter associated with a 50% probability of topkill, estimated from logistic regressions. Multiple linear regression indicated that bark thickness and density accounted for 65% of the variation in vulnerability to topkill among species. This regression excluded one species; Eucalyptus crebra, which was identified as an outlier. This species was the most vulnerable to topkill and was located at the semi-arid study site. This study site had been subject to a more severe pre-fire drought than the mesic site, suggesting that drought may also have influenced post-fire topkill. However, it is not possible to exclude other species-specific factors or site factors such as climate or fire intensity, which may also have impacted the probability of topkill. Our results demonstrate that bark thickness and density are critically important in developing predictive models of post-fire topkill in resprouting forests.