Crown Fires

Crown fires are defined as fires that advance through canopy fuels more or less independently of surface fires (DeBano et al. 1998). Crown fires are extremely dangerous, very difficult to fight and often require the use of indirect suppression methods (Fieldhouse and Dickinson 2003; Pastor et al. 2003).

Van Wagner (1977) grouped crown fires into three broad classes: dependent, active, and independent based on the degree of dependence on the surface fire. Dependent crown fires are initiated and maintained by the heat produced by the consumption of surface fuels and may consume individual tree crowns or small clumps of trees. They do not spread from crown to crown except for adjacent trees in a clump. An active crown fire runs through both the surface and aerial strata at the same time, while an independent crown fire burns through crowns independent of the surface fire. Rothermel (1991) further classified fully developed crown fires into two categories: ‘wind driven’ (strong surface winds) or ‘plume-dominated’ (massive convection column development creates strong local winds, firewhirls and firebrand ignitions outside the fire perimeter). The transition from a surface fire to a crown fire is dramatic.

While dependent crown fires are common in the Southeast, active and independent crown fires are uncommon in the region. Particular exceptions are the stand-replacing fires common in sand pine scrub forests in central Florida, in stand-replacing fires in non-indigenous melaleuca forests in south Florida, and limitedly in Table mountain pine forests in the southern Appalachians. General exceptions to this pattern occur in fires with extreme fire behavior (caused by low moisture levels, erratic winds, or high fuel loadings).

Beighley and Bishop (1990) list ideal conditions for the development of crown fires as: dry fuel, low humidity with high temperatures, heavy accumulations of dead and downed litter, conifer regeneration and other ladder fuels, steep slopes, strong winds, unstable atmosphere, and a continuous cover of coniferous trees. In the South, dense shrub understories are often the main surface fuel that contributes to initiating and maintaining crown fires in dense pine stands. Propagation of a crown fire after ignition from below depends on the spread rate in, and mass density of, the foliar fuel in the crown layer (Van Wagner 1977). Therefore, if the crown fire does not spread fast enough it will burn itself out. For example, during extreme weather conditions when the wind speed is high and sustained, a crown fire may continue and spread for many hours, covering large areas and crossing barriers which would impede the spread of a fire under normal conditions (Pyne et al. 1996).