Fuels Planning: Science Synthesis and

Integration

Forest Structure and Fire Hazard Fact Sheet: 2

Fire Hazard

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS-RN-22-2-WWW

September 2004

Fire Hazard

Fire hazard reflects the potential fire behavior and magnitude of effects as a function

of fuel conditions. Understanding the structure of

fuelbeds and their role in the

initiation and propagation of

fire is key to developing effective fuel management strategies. Fuels have been traditionally characterized as

crown fuels (live and dead

Rocky Mountain Region Archives, USDA Forest Service,

www.forestryimages.org

material in the canopy of trees), surface fuels (grass, shrubs, litter, and wood in

contact with the ground surface), and ground fuels (organic soil horizons, or

duff, and buried wood). A more refined classification separates fuelbeds into six

strata, or layers: tree canopy; shrubs/small trees; low vegetation; woody fuels;

moss, lichens, and litter; and ground fuels (duff). Modification of any fuel stratum has implications for fire behavior, fire suppression, and fire effects.

The tree canopy is the primary stratum involved in independent crown fires, and

the spatial continuity and density of tree canopies combine with fuel moisture

and wind to determine rate of fire spread and severity. The shrub/small tree

stratum is also involved in crown fires by increasing surface fireline intensity and

serving as ìladder fuelsî that provide continuity from the surface fuels to canopy

fuels, thereby potentially facilitating active crown fires.

Three fuelbed strata contribute to the initiation and spread of surface fires. Low

vegetation, consisting of grasses and herbs, can carry surface fires when that

vegetation is dead or has low moisture content. Woody fuel includes sound logs,

rotten logs, stumps, and wood piles from either natural causes or management

activities. Wood can greatly increase energy release from surface fires and can in

some cases increase flame lengths sufficiently to ignite ladder fuels and canopy

fuels. Moss, lichens, and litter on the forest floor can also increase energy release

in surface fuels. Because of the potential for surface fires to propagate into crown

firesóeven if tree density and crowns have been greatly reducedótreatment of

surface fuels must be considered when planning treatments for ladder fuels and

crown fuels.

Three fuelbed strata contribute to the initiation and slow spread of smoldering

fires. Ground fuel, primarily soil organic horizons (or duff), contributes most of

the fuel and can burn slowly for days to months. Moss, lichens, and litter have

high surface area and when very dry can facilitate both the spread of smoldering

fires and a transition to surface (flaming) fire. Woody fuels (sound logs, rotten

Forest Structure and Fire Hazard

Team Leader

David L. Peterson

USDA Forest Service

PNW Research Station

400 N. 34th Street, Suite 201

Seattle, WA 98103

Fuels planning: Science synthesis and

integration, an interagency research/

management partnership to support

the Ten-Year Fire Plan, led by Russell

T. Graham, RMRS, and Sarah M.

McCaffrey, NCRS.

Synthesizing

Scientific Information

for Fire and Fuels

Project Managers

Pacific Northwest

Research Station

The information for this

fact sheet was provided by

Kelly OíBrian, Fire and

Mountain Ecology Lab,

University of Washington.
The Fuels Planning fact sheets are based on preliminary findings. Information from fact sheets will be synthesized in an upcoming publication.

logs, stumps, and wood piles) are often underestimated

as a component of smoldering fire but can sustain low

intensity burning for weeks to months, with potential

flaming combustion under dry, windy conditions. Combustion of woody fuel also can contribute significantly

to smoke production and soil impacts (such as loss of

organic matter, erosion).

Quantifying Fire Hazard and Fire Potential

Accurate quantification of fuels in the canopy and

shrub/small tree strata is necessary to understand the

combustion environment of crown fire. The most effective techniques for reducing crown fire occurrence

and severity have been:

• Increasing canopy base height.

• Reducing canopy bulk density.

• Reducing forest canopy continuity.

• Reducing surface fuels.

Objective and quantifiable fuel-treatment criteria will

assist fire managers and silviculturists in achieving desired conditions for canopy base height, canopy bulk

density, and canopy continuity.

To avoid torching and crown fire initiation, canopy base

height (the average height from the ground to the lowest living foliage) may need to be considerably higher

than the height of expected flame lengths for a specified fuelbed. For most dry forests, this value will be

greater than 20 feet (6 meters). Target values of canopy

base height can be inferred from canopy fuel descriptions for various forest types.

To keep an active crown fire from being sustainable,

canopy bulk densityóthe foliage (mass of available fuel)

contained per unit crown volume (kg m-3) of a forest

standómay need to be maintained below a critical threshold (a function of fire weather and fire rate of spread).

This threshold is not well defined; the required reduction in stem density and basal area will vary between

stands, depending on initial stem density and canopy

structure. Target values of canopy bulk density can be

inferred from canopy fuel descriptions for various forest types.

Canopy continuityóthe physical contact of tree canopiesóis a subjective fuel treatment target and is often

difficult to quantify. If the objective is to reduce physical contact of tree canopies, an example of a field-based

rule is that the distance between adjacent tree crowns

should be half the average diameter of the crown of

co-dominant trees in the stand.

Forest Structure and Fire Hazard Fact

Sheets

Look for other fact sheet topics from the Forest Structure and Fire

Hazard Team with information about fire hazard, visualization,

silviculture, uncertainty, and larger scale treatments.

Fuels Planning: Synthesis and Integration

This fact sheet is one in a series being produced as part of a larger

project supported by the USDA Forest Service to synthesize new

knowledge and information relevant to fire and fuels management.

Fact sheets address topics related to stand structure, environmental

impacts, economics, and human responses to these factors. Information in the fact sheets is targeted for the dry forests of the Inland

West, but is often applicable across broad regions of the country.

For more information, please visit our Web site at:

www.fs.fed.us/fire/tech_transfer/synthesis/synthesis_index

Fuelbed Strata

Canopy

Shrub

Low vegetation

Woody fuel

moss, lichens, litter

Ground fuel

Smoldering, residual effects

Surface fuel

Crown fire

Combustion

Environment