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Fuel loads

"When [the new Flagstaff fire chief] arrived in town he asked some people in the fire department at that point to please tell him about the fuel management program… They quickly responded that he didn't need to worry, because they didn't use a lot of diesel in their trucks."

- Paul Summerfelt, Fuel Management Officer, City of Flagstaff

A century of general fire absence in western ponderosa pine forests has led to large accumulations of highly flammable fuels. The problem is exacerbated in the drier Southwestern climate by very slow decomposition rates (Kallander 1969). Sapling thickets generate particularly large amounts of woody fuels, creating ideal conditions for laddering and thereby increasing the probability for crown fires.

Click for larger image
Heavy fuel loads can lead to an unintentionally destructive prescribed burn. Click for larger image.

Under the former regime of frequent, low-intensity fire, rapid buildups of highly flammable fuel in the form of resprouting grasses and needle cast were regularly consumed. Large woody fuels rarely accumulated over extensive areas. Mortality of large trees from surface fires was unlikely, and crown fires exceptionally rare (Biswell et al. 1973).

By 1979, Sackett reported average fuel loads at 22 tons per acre (ranging from 8 to 48 tons per acre) for 62 Southwestern pine stands. Harrington (1982) verified the heavy fuel loading, finding an average of 34 tons per acre in southeastern Arizona. Formerly uncommon large, woody fuels averaged about 8 tons per acre.

Extreme fuel loads make prescribed burning not only risky in terms of potential for escape, but often unintentionally destructive. Consumption of large amounts of fuel generates large amounts of heat. As thick mats of duff smolder for up to 72 hours, ash is formed from the top down, creating an insulating cover. The insulation prevents heat from escaping causing it to penetrate the mineral soil. Burning for long time periods can result in temperatures exceeding 140 degrees F causing instant cambium or root death. Lower temperatures for longer periods can also kill plant tissues.

Studies at Fort Valley and Long Valley Experiment Forests in Arizona measured lethal temperatures deeper than 8 inches in mineral soil on some sites. The first burn in 100 years at the Chimney Spring Prescribed Fire Research Area at Fort Valley killed almost 40 percent of the old-growth ponderosa pines that had survived numerous presettlement fires. Mortality did not appear until several years after the burns and continued to be greater than on unburned sites (Sackett and Haase 1992).

Costly and labor intensive removal of duff, woody material, and ladder fuels decreases potential fire intensity, total heat release, and resistance to control. However, the fuel hazard is only temporarily reduced as up to 2 tons per acre of fine fuels are normally cast annually. Even more can accumulate from fire-injured trees. Repeated burning is essential to remove these fire-created fuels and generally maintain a low fuel hazard.

References

Biswell, H.H., H.R. Kallander, R. Komerek, R.J. Vogl, and H. Weaver. 1973. Ponderosa pine fire management: a task force evaluation of controlled burning of ponderosa pine forests in central Arizona. Tall Timbers Research Station Miscellaneous Publication 2, Tallahassee, Florida, 49 p.

Fire Area Simulator (FARSITE) http://farsite.org/ 4/15/03. A two-dimensional fire growth computer simulation model that uses spatial information on topography, fuel loads and other variables to simulate the spread of wildfires.

Fire Behavior Mapping and Analysis (FlamMap) http://fire.org/perl/nav.cgi?pages=flammap&mode=1 4/15/03. A fire behavior mapping and analysis program that computes potential fire behavior characteristics over an entire FARSITE landscape for constant weather and fuel moisture conditions.

Fire Sciences Lab-Fire Effects Project. "Southern Utah Fuel Management Demonstration Project" http://www.firelab.org/fep/research/sufm/home.htm 4/15/03. A project to support the planning and implementation of a landscape level fuel management program.

Harrington, Michael G. 1982. Stand, fuel, and potential behavior characteristics in an irregular southeastern Arizona ponderosa pine stand. Res. Note RM-418. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mtn. Forest and Range Experiment Station. 6 p.

Harrington, Michael G.; Sackett, Stephen S. 1992. Past and present fire influences on Southwestern ponderosa pine old growth. In: Kaufmann, Merrill R., W.H. Moir, R.L. Bassett, 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. One of the best summaries available on the state of Western ponderosa pine forests and the need for restoration treatments.

Kallander, H. R. 1969. Controlled burning on the Fort Apache Indian Reservation, Arizona. Tall Timbers Fire Ecology Conference 9: 241-250.

Sackett Stephen S. and S.M. Haase. 1992. Measuring soil and tree temperatures during prescribed fires with thermocouple probes. General Technical Report PSW-GTR-131. Albany, CA: Pacific Southwest Research Station, Forest Service. U. S. Department of Agriculture: 15 p.

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.

Zimmerman, G. Thomas. 2003. Fuels and fire behavior. Pages 126-143 in Friederici, Peter, ed. Ecological Restoration of Southwestern Ponderosa Pine Forests: A Sourcebook for Research and Application. Washington, D.C. Island Press. Different forest conditions exhibit different fire behavior. Returning fire to the maintenance role it once played can often not be done safely and effectively without first thinning dense stands.

Last edited June 25, 2003

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