5th Symposium on Fire and Forest Meteorology and the 2nd International Wildland Fire Ecology and Fire Management Congress

Monday, 17 November 2003
The Susceptibility of Forested Stands to Extreme Fire Behavior; an analysis in the northwest United States
Paul Fieldhouse, Stevensville Ranger Station, Stevensville, MT; and J. Dickinson
The extent of extreme fire behavior in the last several fire seasons has highlighted the susceptibility of current forested vegetation stands to crown fire, bringing it to the forefront of national attention. Though much attention has been given towards the wildland urban interface problem and the associated fuels reduction treatments, little work has been focused towards the implications this has for the safety of fire personnel who operate in close proximity to flames. Frequently, suppression tactics and nearly all prescribed fire plans are most effective when personnel can be safely located near the flames. Crown fire often poses significant risk to fire workers and almost always limits control tactics to indirect methods. The inability of forces to safely engage in activities near the fire in both the suppression and prescribed fire roles has prompted this project to focus on the primary goal of identifying the most significant correlations between forest stand characteristics and the susceptibility of transition from surface fire to crown fire behavior.

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.

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