COS 175-6 - Bark traits and expected fire-adaptedness of three California foothill oak species

Abstract: Fire is increasing in frequency and intensity in many California ecosystems, including foothill oak woodlands. While oaks are generally considered to be fire resistant, we have a limited understanding of the relationship between tree size and fire resistance. Bark is one of the most important mechanisms for a tree to protect itself from fire-induced damage. However, bark traits of California oak species have been little studied. To quantify the “fire adaptedness” of California oak species, bark thickness growth patterns were evaluated in three species that are widespread in California foothill environments: valley oak (Quercus lobata), blue oak (Q. douglasii), and coast live oak (Q. agrifolia). Bole diameter and bark thickness were measured at 0.5 m and 1.37 m above the ground surface. A mixed effects model was used to parse differences in allometric growth patterns by size, and a paired t-test was used to evaluate growth patterns by height. At the higher measurement height, a negative size-dependent allometric growth pattern was observed for all three species, with coast live oak showing the most negative pattern (indicating fire-adaptedness). Bark thickness also differed significantly at the two measurement heights, showing a pattern of bark tapering (indicating fire-adaptedness) for coast live oak only. Future work will relate bark thickness and stem diameter to time to cambium kill, an important metric of a tree’s vulnerability to heat-induced damage. Bark moisture content and density will also be measured, and the relative effects of all bark traits on time to cambium kill will be evaluated to better understand what traits play a key role in fire resistance. This information can be useful toward estimating the vulnerability of a stand based on species and individual tree sizes that are present in the stand. This could aid site preparation efforts prior to prescribed fire implementation, such as fuel reduction efforts (i.e. brush clearing, thinning) around individuals that are expected to be more vulnerable. Altogether, this work contributes toward expanding knowledge of the fire ecology of California oak species.