OOS 36-4 - Mechanisms of fire scar formation on tree stems

Fire scars result from necrosis of phloem and vascular cambium tissue, and often occur predominantly on the leeward side of tree stems due to uneven heating of the stem surface. This uneven heating has been hypothesized to originate from increased residence time and stretching of flames in the wake of the stem, which enhances heat transfer at the stem surface. However, our understanding of the physical mechanisms of this phenomenon has been limited by our ability to collect precise, high-resolution data of stem heating by fire. To address these limitations, we used the NIST Fire Dynamics Simulator to simulate fire spread past a tree stem at wind speeds ranging from 0.84 to 7.55 m s-1 (Re ≈ 3.1 x104 to 2.8 x 105). Increasing wind speed caused flames to become relatively higher and longer lasting on the leeward side of the stem, thus resulting in an increase of local Nusselt number on the leeward side (convective heat flux). Our results shed new light on the mechanisms of fire scar formation, and will inform post-fire tree mortality models, dendrochronological fire history studies, and fire investigations.