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Prescribed Burning

A prescribed burn
A prescribed burn

After decades of fire suppression, the natural cycle of low-intensity fires burning through open forest grasslands no longer exists in most ponderosa forests. Prescribed fire--"fire applied in accordance with fuel management plan or for ecocsystem management purposes"--may not only diminish fire risk by thinning the forest and reducing fuel loads, but can also stimulate nitrogen availability and increase herbaceous productivity.

Ignition is usually begun at mid-morning following the break-up of the night time temperature inversion and the establishment of the day time wind pattern. Ignition is usually completed by early afternoon, with burn blocks generally kept small to achieve this objective. Follow-up thinning and maintenance burns are scheduled as necessary to ensure long-term reduction of the risk of destructive fire. Typically, thinning is rescheduled every 10-15 years, while broadcast burns are on a 3-7 year cycle.

Treatment to restore natural conditions must usually also include mechanical thinning and manual fuel removal, especially raking of duff away from the base of high-risk trees. Even then, unintended consequences may result. Benign-looking surface fires now tend to burrow beneath the thick duff layer--the accumulation of undecomposed needles--that has formed a dense mat over the soil. An insulating ash cap can then form, forcing heat into the soil and burning hot enough to kill small roots near the surface. In one study, 40 percent of a stand of otherwise healthy trees died within three years of a prescribed burn (Harrington and Sackett, 1992).

In the wildland-urban interface, prescribed burns are often constrained not only by weather conditions, fuel and soil moisture, and other demands, but also by the impacts of smoke. Neighbors generally tolerate smoke for a day, but after 2-3 days, patience wears thin.

J.G.

References

Greater Flagstaff Forests Partnership. "Flagstaff Interface Treatment Prescription: Broadcast Burning." http://www.gffp.org/pine/fit_rx/fit6.htm#broadcast 4/10/03

Harrington, Michael G.; Sackett, Stephen S. 1992. Past and present fire influences on Southwestern ponderosa pine old growth. In: Kaufmann, Merrill R.; Moir, W. H.; Bassett, Richard L., technical coordinators. Old-growth forests in the Southwest and Rocky Mountain regions: Proceedings of a symposium; 1992 March 9-13; Portal, AZ. Gen. Tech. Rep. RM-213. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 44-50.

National Interagency Fire Center. "Prescribed Fire Statistics." http://www.nifc.gov/stats/prescribedfirestats.html 4/10/03.

Schumann, Martha. 2001. Annotated bibliography: Fuel treatments and fire behavior. Southwest Region Working Paper 3. National Community Forestry Center. 33 p. Available online at http://theforesttrust.org/images/swcenter/pdf/WorkingPaper3.pdf 5/28/03.

Southwest Forest Alliance. "Why the Flagstaff Presettlement Restoration Model Should Not Be Applied to Public Forest Lands." http://www.swfa.org/restoration_booklet_files/index.html 4/10/03.

U.S. Forest Service Fire and Aviation Management. "Prescribed Fire." http://www.fs.fed.us/fire/fireuse/rxfire/rx_index.html 4/10/03.

U.S. Forest Service Region 3. "Santa Fe Forest Forum on Wild and Prescribed Fire: Summary and Conclusions." http://www.fs.fed.us/r3/sfe/watershed/forum.html 4/10/03.

Last edited June 25, 2003

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