Bark traits and fire vulnerability in three California foothill oak species

Abstract Background Bark serves an important function in protecting trees from heat damage in fire-prone ecosystems. As fire increases in Mediterranean-climate systems, so does the need to understand fire adaptations of associated tree species, especially in important but understudied groups such as oaks ( Quercus spp.). To determine what bark traits contribute to fire resistance in two widespread California white oak species ( Q. douglasii and Q. lobata ) and one red oak species ( Q. agrifolia ), we measured bark thickness, moisture content, and density, and related these variables to Time to Cambium Kill (τ), a metric that quantifies vulnerability to fire-induced trunk tissue death. We conducted heating experiments on the bark of live trees to record τ. We assessed τ as a function of tree trunk diameter and bark thickness, determining “safe” size thresholds for each species. We also compared our field measurements to predictions from two frequently applied τ models. We searched for evidence of species’ fire-adaptedness by comparing bark thickness:size allometry patterns at 50 cm and 137 cm heights. Results Bark thickness was a much stronger predictor of τ than moisture content or density. Q. agrifolia had a higher bark thickness:stem diameter ratio, lower safe diameter threshold (17.2 cm), and higher safe bark thickness threshold (2 cm) than Q. douglasii and Q. lobata . Existing models that predict τ as a function of bark properties had modest skill, and our fitted models that accounted for species differences performed better. Q. agrifolia most strongly manifested bark allometry indicative of fire-adaptedness. None of the species showed a fire-adapted pattern of bark tapering with height. Conclusion Bark traits and growth patterns can confer fire adaptation in tree species. Q. agrifolia showed stronger evidence of fire-adapted bark traits than Q. douglasii or Q. lobata . More work is needed to develop reliable general models relating time to trunk tissue kill to bark traits for oaks in fire-prone ecosystems.