Description

The conditions necessary for the combustion of canopy fuels are not well known but are assumed to be highly influenced by the volume through which the canopy fuels are dispersed, known as canopy bulk density (CBD). Propagating crown fire is defined as a continuous wall of flame from the bottom to the top of the canopy, implying crown fire propagation is actually independent of the vertical fuel distribution. We hypothesize that all foliar canopy fuel is available for the propagation of canopy fire. Therefore, we focus our effort on simplifying Van Wagner's (1977) canopy fire propagation model to accept a canopy fuel metric that uses only foliar biomass per unit area (FBA) rather than the more complex and commonly used CBD. The multiplication of leaf area index (LAI) and the specific leaf area (SLA) of a given tree species results in FBA, making FBA easily related back to ecologically meaningful terms at a range of spatial scales. A variety of instruments can be used to estimate LAI with high accuracy, and SLA values have been published for many species found in fire prone forests of the West. Alternatively, allometric equations can be used to compute FBA using individual tree-level measurements. Using Van Wagner's (1977) data we modify his propagation model and successfully match his published results. In addition, we use Forest Inventory and Analysis data from northern Idaho to compare the critical rate of spread (cROS) predicted by the modified model (using FBA) with the predictions of the original model (using CBD). We find that the two models are statistically equivalent (α = 0.10).