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Prescription for a fire

Sidebar by Chas S. Clifton

Wildfire – July 2000 – Colorado Central Magazine

When a planned 600-acre prescribed burn at Bandelier National Monument exploded into a huge wildfire in May 2000 that burned homes and other buildings in Los Alamos, N.M., many people probably wondered how a fire is “prescribed.”

Typically, fires are started to “reduce fuel load” — in other words, to clear overgrown trees and brush before they burn on their own — or to improve wildlife habitat.

Large animals such as deer and elk in particular do not find much to eat in thick forest but prefer a mixture of meadows, brush, and forest. If they can meet their habitat needs on public land, they are less likely to eat ranchers’ haystacks or wander into subdivisions.

But a certain gentle deception creeps into the public justification for prescribed burns at times. While the fire might be desirable according to the new rules of forest management, whether it is described as benefiting wildlife may depend on whether the agency (Forest Service or BLM) is getting money from the Colorado Division of Wildlife or a private conservation group such as the Rocky Mountain Elk Foundation, both of which have helped finance burns to improve big-game habitat.

Whether a fire is set deliberately depends on the moisture level in the trees and brush, air temperature, relative humidity, and wind speed and direction. These factors can change hour by hour, and the wrong change leads to disasters like the Los Alamos (“Cerro Grande”) fire and the bad publicity that comes with it.

Even when the National Weather Service forecasts upper and lower-level winds, mountainous terrain causes local shifts and eddies. And if a fire gets too large, it creates its own “weather,” sucking up air, creating updrafts and downdrafts, and driving the moisture out of trees.

For the Red Creek prescribed burn in the Wet Mountains on April 10, 2000, fire boss Toni Toelle wanted these conditions:

Air temperature: 66-77° F.

Relative humidity: Less than 25 percent, with 10-20 percent being ideal.

Wind speed: From 1-10 mph, with 3-5 mph being ideal. A light wind helps disperse the smoke, but too much can push the flames across fire lines.

Live fuel moisture: 75-90 percent, with the Gambel oak being 60-70 percent; in other words, dormant.

San Carlos District assistant ranger Mike Smith estimated the Middle Red Creek burn cost $80/acre: about $30/acre for planning and environmental assessments and $50/acre for the actual on-the-ground costs. By comparison, he said, the Forest Service spends $800-$1,000/acre to control a wildfire. –C.S.C.