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Interface Forestry

forest stewardship in the wildland-urban interface

Notes and Research

Urban Interface Wildfire Fuel Assessments

Stand and Forest Wildfire Fuels Assessments

Stand level fuel assessments provide important information for use in stand and forest level planning. These assessments provide key information for determining vulnerability to wildfire in the wildland-urban interface. Forest and stand fuel assessments involve the two distinct tasks of assessing ground fuels and assessing canopy fuels:

  • To assess ground fuels, we sample plots on a grid for fuel model characteristics, using one of two methods. We can combine either method with a standard stand exam.
    • The first approach is the fastest of the two and perhaps the most cost-effective. This technique involves visually interpreting and classifying plot conditions based on experience and familiarity with the relevant fuel types.
    • The second approach, which is much more time-consuming, involves comparing plot conditions to photo images of research plots that approximate the models in fuel loading. Each photo in the "photo series" represents a specific variation of one of the general fuel models. The photos accompany a tabulated breakdown of fuel into types and quantities. As with the fuel models, each photo in the series represents a set of measured conditions that have been empirically demonstrated to affect fire behavior in measurable ways in vitro.
  • Canopy fuels assessment requires a stand exam. We then enter the tree data into a computer-based fuels model to determine canopy density, canopy weight, and other variables that can influence crown fire behavior (or lack thereof).

With data from our ground fuels and canopy fuels assessment, we can model fire behavior and assess a stand's vulnerability to tree mortality, crown torching, and crown carried fire, given certain environmental variables (fuel moisture, wind, temperature, and others).

For all fuels and fire behavior modeling, we rely upon Fuels Management Analyst Plus, 3.0.

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Wildfire Hazards and Home Ignition Zone Assessments

Wildfire Hazards and Home Ignition Zone (HIZ) Assessments determine the availability of fuels and ignitable surfaces in the immediate environment of a building. We use the standard Firewise Communities methodology for Assessing Wildfire Hazards in the Home Ignition Zone.

The urban-interface residential owner should be aware of firewise practices, because the unusual circumstances surrounding a wildfire incident present a great risk of personal danger and extreme value loss. And, fire is inevitable in most Montana forest types. Due to 100 years of fire suppression, most forestlands in Montana are grossly overstocked with fuel and present a formidable wildfire hazard.

During a wildfire incident, especially when the fire is growing with little or no containment, labor and firefighting resources are always too scarce to protect all the structures in an incident area. Faced with decisions in the field, a constrained firefighting team will opt for defending the most defensible, firewise building as a matter of personal safety.

The area immediately around a residence (or other structure) -- within 100-200 feet -- is the home ignition zone (H.I.Z.). The H.I.Z. should be properly managed and completely free of standing fuel, such as stacked fire wood, pine needles, bushes and trees, and flammable chemicals. In this state, the H.I.Z. is defensible space, meaning that a fire fighting crew can defend it.

In addition, any building constructed in forest areas should be built from fire-resistant materials and should have no exposed wood. Homes that burn during forest fires usually are not ignited directly from burning trees or crown fires, but rather, from fire-brands (floating embers) or fuels that ignite next to the houses, exposing them to direct flame.

Flammable buildings -- any building with trim, a wooden porch, or any wood showing at all, will require people to defend against firebrands. This need for a human presence necessitates an adequate distance between the flame-front and the firefighters. In addition, flammable buildings must be far enough from radiant heat to avoid combustion, though the threat from firebrands is more likely to be a problem. Taking the needs of firefighters into consideration, some researchers suggest a minimum of 200 feet distance from the building to any conceivable flame front. Others recommend clearing all shrubs and flammable plants within 30 feet of the structure and removing any tree that is within a distance twice the height of the tree. Thus, a 40-foot tall tree should be further than 80 feet from a combustible structure.

Firewise Communities recommends a graduated approach, retaining a few more trees per acre in proportion to distance (with adequate distance between crowns to avoid a crown fire). This approach is more flexible, but certainly depends upon a house prepared to withstand a wildfire.

Furthermore, safe and accessible exit and entrance routes for emergency vehicles and residents should be available at all times. When pressed for manpower, fire-fighters will often skip houses with inadequate access and defensibility, as poor defensibility means increased danger to firefighters!

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