Sep. 20, 2000 MISSOULA, MT – Forest fires in the western states continue to rage, threatening more than just national park land. So far, more than 400 homes have been damaged or destroyed by the fires, and now scientists are looking at why houses are sometimes more at risk that the trees themselves.
"A house can be totally destroyed amid a forest of unconsumed and living trees," says Dr. Jack Cohen, a research physical scientist at the Fire Sciences Laboratory in Missoula, Montana. He is studying why homes in wildland areas burn, and hopes to create a computer model, called the Structure Ignition Assessment Model, that he says "will help characterize a home's ignition potential during wildfires."
Part of the reason houses can be more at risk than the forests themselves is because large trees, like ponderosa pines, are protected from a fire that is burning on the ground. "Ponderosa pine have thick bark that insulates their living cambium [tissue] from heat and elevated crowns that separate their needles and buds from a surface fire," Cohen says. Don Latham, also at the fire sciences lab, adds that trees like the ponderosa and redwood are part of a fire dependent ecosystem. "They have grown up to be fire resistant," he says. So while a stand of trees may be able to tolerate some fire on the ground, "a highly ignitable home," Cohen points out, "will be at greater risk of destruction than the surrounding forest."
Cohen points out that homes are at risk from two types of wildfires; surface fires, which burn the dead, dry material on the forest floor, and crown fires, which consume whole trees, often sending burning embers raining down on not yet burning parts of the forest. According to Cohen, "An intense crown fire can loft large numbers of burning embers out of the fire that can land downwind, sometimes a mile or more, to ignite other fuels including homes." In this case, a burning ember may ignite the home, but not the trees surrounding it.
For home owners, Cohen warns that the danger from crown fires lies in homes with roofs made of flammable material, or with a large collection of dry pine needles collected in the gutters. "These fine fuels can then ignite the flammable materials of a home's exterior." With surface fires, pine needles, dead leaves, flammable shrubs and wood piles that are near or touching homes can bring the fire right up to the structure, where wood siding and other home building materials can ignite.
But there is hope. Cohen recently studied homes that did and did not burn during the Los Alamos, New Mexico Wildfires in May. He found that some homes were more ignition resistant than others. In several cases Cohen found that a scratch line (a narrow strip of land where surface materials are cleared away) around a home kept the building from igniting. There was also a case where pine needles burned on a roof made of gravel and composition shingle, but the house did not catch fire. Cohen believes this is because the house did not have gutters collecting pine needles, which could help ignite the eave edge of the house.
Cohen hopes that his Structure Ignition Assessment Model will give the larger picture of a home's ignition danger. Cohen also suggests that homeowners in wildland areas check out www.firewise.com for educational materials and tips on protecting homes from wildfires.
For more information:
Inside Science News Service
Research Physical Scientist, Fire Behavior Project Fire Sciences Laboratory
Don Latham, PhD
Project Leader, Fire Behavior Project Fire Sciences Laboratory
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