Crown fire initiation is the result of surface fire reaching a fireline intensity sufficient to ignite the fuels in the canopy above. Research indicates that the factors that affect fireline intensity are many and interact in a complex manner. However, the receptivity of the canopy fuels to crown fire is based primarily on three factors: canopy base height, canopy bulk density, and, to a lesser degree, foliar moisture content.
An analysis of Forest Inventory Assessment (FIA) plots and Continuous Vegetation Survey (CVS) plots has been undertaken. The Fire and Fuels Extension of the Forest Vegetation Simulator (FFE-FVS) has been used to obtain canopy bulk density (CBD) and canopy base height (CBH) data for these vegetation plots. The CBH and CBD values are being calculated in nearly 8500 FIA/CVS plots located throughout Montana, northern Idaho, eastern Washington, and eastern Oregon. The FFE-FVS analysis produces the desired variables based on the conditions at the time of plot sampling. These variables are then incorporated into Van Wagner's (1977) crown fire initiation equation and critical rate of spread (mass flow rate of fuel) equation.
The calculated median critical fireline intensities are being compared to the crown fire indicators utilized on the Fire Characteristics Chart. Preliminary analysis of data from Montana (collected between 1989-1999) suggests that the Fire Characteristics Chart overestimates the critical intensities that the canopy fuels of current forests require for surface fire to initiate a crown fire in more than 50% of the stands analyzed. A more in-depth analysis of the data is ongoing and will focus on finding relationships between crown fire susceptibility and measurable stand attributes.