How fire scars are formed: coupling a disturbance process to its ecological effect

Using results from the field and laboratory, we give a detailed explanation of an important and commonly observed ecological phenomenon, fire scarring of trees. Four key observations associated with the formation of fires scars are explained using fluid dynamics and heat transfer processes. When a fire passes by a tree, its height increases on the tree's leeward side because of the occurrence of two leeward vortices. The flame height increases in the vortices because the turbulent mixing of fuel and air is suppressed. The flow of gaseous fuel in the vortices becomes greater than the rate of mixing with the air, and hence there is an increased height along which combustion can occur. Fire scars are found only on the leeward side of trees because the vortices increase the residence time of the flame on the leeward side of the tree compared with the residence time of the flame on the tree's windward side. This leads to differential heating around the tree bole. Small trees rarely have fire scars because their cambium is usually completely killed by a passing fire and (or) their foliage is killed by crown scorch. Fire scars are usually triangular in shape; wider at the base and decreasing in width with height. The triangular shape is a result of the temperature isotherms within the standing leeward flame, which are triangular in shape.