Eucalypt bark thickness and density are explained more by landscape aridity and species than by fire frequency

Context Bark thickness and density are key traits protecting tree cambium from lethal fire temperatures, and may become increasingly important for tree survival as fire frequency and landscape aridity shift under climate change. However, it is unknown how these traits vary with fire frequency and aridity, or whether such relationships differ among bark types. Aims We examined whether bark thickness and density varied with fire frequency and landscape aridity, and whether these relationships differed among smooth, stringy and rough bark types. Methods Data were collected from 3258 trees belonging to 55 eucalypt species in dry sclerophyll and wet sclerophyll forests across eastern Australia, with bark density measured in a subset of 942 trees. We fitted generalised additive models relating bark thickness and bark density to fire frequency and a topographically-downscaled landscape aridity index, separately for smooth, stringy and rough bark types, with species and site being included as random effects. Key results Fire frequency had no detectable effect on either bark thickness or bark density in any bark type. Landscape aridity, by contrast, was the dominant driver of bark thickness, with a consistent U-shaped relationship across all three bark types, being thinnest at intermediate aridity and thicker towards both the wettest and driest ends of the gradient, an effect driven largely by a small number of species at each moisture extreme. Bark density showed no equivalent pattern and was instead far more strongly determined by species identity than by aridity or fire frequency. Conclusions Fire frequency alone is a poor predictor of bark trait variation and both landscape aridity and species distributions need to be properly characterised to understand how eucalypt bark traits vary across the landscape. Because bark type is phylogenetically conserved and unlikely to change as quickly as climate, shifts in aridity may pose risks to bark-related fire resistance that are at least as important as changes in fire regime. Implications These findings highlight the importance of accounting for fine-scale aridity and species, rather than fire frequency alone, when assessing the vulnerability of bark-related fire resistance to future climate and fire regime change.