- Fire, other disturbances, physical setting, weather, and climate shape the structure and function of
forests throughout the Western United States. More than 80 years of fire research have shown that
physical setting, fuels, and weather combine to determine wildfire intensity (the rate at which it consumes fuel) and severity (the effect fire has on vegetation, soils, buildings, watersheds, and so forth). Millions of acres of forestlands (mainly in dry forests dominated by ponderosa pine and/or Douglas-fir) contain a high accumulation of flammable fuels compared to conditions prior to the 20th century. Forests with high stem density and fuel loading combined with extreme fire weather conditions have led to severe and large wildfires (such as those seen in the summers of 2000, 2002, and 2003) that have put a number of important values at risk. Although homes in the path of a wildfire are perhaps the most immediately recognized value, these wildfires also put numerous other human and ecological values at risk, such as power grids, drinking water supplies, firefighter safety, critical habitat, soil productivity, and air quality.
For a given set of weather conditions, fire behavior is strongly influenced by stand and fuel structure.
Crown fires in the dry forest types represent an increasing challenge for fire management as well as a
general threat to the ecology of these forests and the closely associated human values. Crown fires are dependent on the sequence of available fuels starting from the ground surface to the canopy. Limiting crown fire in these forests can be accomplished by actions that manage in concert the surface, ladder, and crown fuels. Reducing crown fire and wildland fire growth across landscapes decreases the chances of developing large wildfires that affect human values adjacent to forested areas. However, a narrow focus
on minimizing crown fire potential will not necessarily reduce the damage to homes and ecosystems when
fires do occur. Homes are often ignited by embers flying far from the fire front, and by surface fires. Fire
effects on ecosystems can also occur during surface fires where surface and understory fuels and deep
organic layers are sufficient to generate high temperatures for long periods.
Fuel treatments can help produce forest structures and fuel characteristics that then reduce the
likelihood that wildfires will cause large, rapid changes in biophysical conditions. Fuel treatments can also
help modify fire behavior sufficiently so that some wildfires can be suppressed more easily. Subsequent,
sustained fuel treatments can maintain these conditions. Different fuel reduction methods target different
components of the fuel bed. Thinning mainly affects standing vegetation, and other types of fuel treatments
such as prescribed fire and pile burning woody fuels are needed to modify the combustion environment
of surface fuels. In forests that have not experienced fire for many decades, multiple fuel treatments—that
is, thinning and surface fuel reduction—may be required to significantly affect crown fire and surface fire
hazard. Fuel treatments cannot guarantee benign fire behavior but can reduce the probability that extreme
fire behavior will occur. Fuel treatments can be designed to restore forest conditions to a more resilient and
resistant condition than now exists in many forests, and subsequent management could maintain these
conditions, particularly in dry forests (ponderosa pine and Douglas-fir) where crown fires were historically
infrequent. The degree of risk reduction will depend to some degree on the level of investment, social and
economic acceptability of treatments, and concurrent consideration of other resource values (for example,
This report describes the kinds, quality, amount, and gaps of scientific knowledge for making informed
decisions on fuel treatments used to modify wildfire behavior and effects in dry forests of the interior
Western United States (especially forests dominated by ponderosa pine and Douglas-fir). A review of
scientific principles and applications relevant to fuel treatment primarily for the dry forests is provided for
the following topics: fuels, fire hazard, fire behavior, fire effects, forest structure, treatment effects and
longevity, landscape fuel patterns, and scientific tools useful for management and planning.
- Graham, Russell T.; McCaffrey, Sarah; Jain, Theresa B. (tech. eds.) 2004. Science basis for changing forest structure to modify wildfire behavior and severity. Gen. Tech. Rep. RMRS-GTR-120. FortCollins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 43 p.