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Authors

David L. Peterson is a research biologist, Morris C. Johnson is an ecologist, 

and Donald McKenzie is a research quantitative ecologist, U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Research Station, Pacific Wildland 

Fire Sciences Laboratory, 400 N 34th Street, Suite 201, Seattle, WA 98103; James 

K. Agee is a professor, College of Forest Resources, University of Washington, Box 

352100, Seattle, WA 98195; Theresa B. Jain is a research forester, U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station, 1221 S Main Street, 

Moscow, ID 83843; and Elizabeth D. Reinhardt is a research forester, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula 

Fire Sciences Laboratory, 5775 W Hwy. 10, Missoula, MT 59802.

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Abstract

Peterson, David L.; Johnson, Morris C.; Agee, James K.; Jain, Theresa B.; 

McKenzie, Donald; Reinhardt, Elizabeth D. 2005. Forest structure and fire 

hazard in dry forests of the Western United States. Gen. Tech. Rep. PNW-GTR-628. 

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Research Station. 30 p.

Fire, in conjunction with landforms and climate, shapes the structure and function of 

forests throughout the Western United States, where millions of acres of forest lands 

contain accumulations of flammable fuel that are much higher than historical conditions owing to various forms of fire exclusion. The Healthy Forests Restoration Act 

mandates that public land managers assertively address this situation through active 

management of fuel and vegetation. This document synthesizes the relevant scientific 

knowledge that can assist fuel-treatment projects on national forests and other public 

lands and contribute to National Environmental Policy Act (NEPA) analyses and other 

assessments. It is intended to support science-based decisionmaking for fuel management in dry forests of the Western United States at the scale of forest stands (about 1 

to 200 acres). It highlights ecological principles that need to be considered when managing forest fuel and vegetation for specific conditions related to forest structure and 

fire hazard. It also provides quantitative and qualitative guidelines for planning and 

implementing fuel treatments through various silvicultural prescriptions and surfacefuel treatments. Effective fuel treatments in forest stands with high fuel accumulations 

will typically require thinning to increase canopy base height, reduce canopy bulk 

density, reduce canopy continuity, and require a substantial reduction in surface fuel 

through prescribed fire or mechanical treatment or both. Long-term maintenance of 

desired fuel loadings and consideration of broader landscape patterns may improve 

the effectiveness of fuel treatments.

Keywords: Crown fire, fire hazard, forest structure, fuel treatments, prescribed burning, silviculture, thinning.
Preface

This document is part of the Fuels Planning: 

Science Synthesis and Integration Project, a pilot 

project initiated by the U.S. Forest Service to respond to the need for tools and information useful 

for planning site-specific fuel (vegetation) treatment 

projects. The information primarily addresses fuel 

and forest conditions of the dry inland forests of 

the Western United States: those dominated by 

ponderosa pine, Douglas-fir, dry grand fir/white fir, 

and dry lodgepole pine potential vegetation types. 

Information, other than social science research, 

was developed for application at the stand level 

and is intended to be useful within this forest type 

regardless of ownership. Portions of the information also will be directly applicable to the pinyon 

pine/juniper potential vegetation types. Many of 

the concepts and tools developed by the project 

may be useful for planning fuel projects in other 

forest types. In particular, many of the social science findings would have direct applicability to 

fuel planning activities for forests throughout the 

United States. As is the case in the use of all models 

and information developed for specific purposes, 

our tools should be used with a full understanding 

of their limitations and applicability. 

The science team, although organized functionally, worked hard at integrating the approaches, 

analyses, and tools. It is the collective effort of the 

team members that provides the depth and understanding of the work. The science team leadership 

included Deputy Science Team Leader Sarah McCaffrey (USDA FS, North Central Research Station); 

forest structure and fire behavior—Dave Peterson 

and Morris Johnson (USDA FS, Pacific Northwest 

Research Station); environmental consequences—

Elaine Kennedy-Sutherland and Anne Black (USDA 

FS, Rocky Mountain Research Station); economic 

uses of materials—Jamie Barbour and Roger Fight 

(USDA FS, Pacific Northwest Research Station); 

public attitudes and beliefs—Pam Jakes and Sue 

Barro (USDA FS, North Central Research Station); 

and technology transfer—John Szymoniak, (USDA 

FS, Pacific Southwest Research Station). 

This project would not have been possible were it 

not for the vision and financial support of Washington Office Fire and Aviation Management individuals, Janet Anderson Tyler and Leslie Sekavec. 

Russell T. Graham 

USDA FS, Rocky Mountain Research Station

Science Team Leader
Summary

The structure and fuel loading of many dry forests 

in the Western United States have changed considerably during the past century. Fire exclusion, 

forest harvest, and various land use practices have 

reduced the frequency of fires, especially in lowseverity fire regimes, resulting in high accumulations of canopy and surface fuel. When fires occur 

in these stands now, the potential for crown fires 

and large fires is greater than in presettlement 

times. Public policy asserts that managers of national forests and other public lands need to accelerate the treatment of hazardous fuel and reduce 

crown-fire hazard, while maintaining sustainable 

forest ecosystems that provide desired resources 

and values.

This document synthesizes the relevant scientific 

knowledge that can assist fuel-treatment projects 

on national forests and other public lands at the 

scale of forest stands (about 1 to 200 acres). It supports science-based decisionmaking for fuel management and ecological restoration in dry forests, 

and it can contribute to National Environmental 

Policy Act (NEPA) analyses and other assessments 

that need to consider the effects of fuel treatments 

on fire hazard and natural resources. To date, 

there have been few guidelines that explicitly 

link alteration of stand structure and canopy fuel 

with potential fire behavior and fire effects. This 

document provides the scientific basis for applying 

quantitative and qualitative guidelines to modify 

stand density, canopy base height, canopy density, 

and surface-fuel loadings.

The scientific basis for using thinning and surfacefuel treatments (prescribed burning, manual and 

mechanical changes in fuel) to reduce crown-fire 

hazard is explored, as well as evidence for fueltreatment effectiveness in large fires, and the role 

of extreme weather in fire landscapes. In forest 

stands that have not experienced fire or thinning 

for several decades, heavy thinning combined with 

(often multiple) prescribed-fire or other surfacefuel treatments, or both, is necessary to effectively 

reduce potential fire behavior and crown-fire 

hazard. Prescriptions for treating individual stands 

should be developed in the context of fuel conditions across the broader landscape, so that effective 

spatial patterns of reduced fuel can be maintained 

over decades. Until more empirical data on the 

effectiveness of fuel treatments in reducing fire 

behavior and fire effects in large fires are available, 

the following scientific principles can be used to 

guide decisionmaking: (1) reduce surface fuel, (2) 

increase canopy base height, (3) reduce canopy 

density, and (4) retain larger trees.

1

Objective

This document is a synthesis of scientific knowl-

edge that can assist fuel-treatment projects on 

national forests and other public lands. It is 

intended to support science-based decisionmaking 

for fuel and vegetation management in dry forests 

of the Western United States at the scale of forest 

stands (about 1 to 200 acres). By synthesizing key 

scientific information related to the effects of forest 

structure on fire hazard and potential fire behav

-

ior, it highlights ecological principles that need 

to be considered when managing forest fuel and 

vegetation for specific conditions related to fire 

hazard. It also provides quantitative and qualita

-

tive guidelines for planning and implementing fuel 

treatments. This scientific information is needed 

by resource managers and planners for inclusion 

in National Environmental Policy Act (NEPA) anal

-

yses and other documents that need to consider a 

range of alternative fuel treatments.

There are many related topics that must be consid

-

ered when making decisions about fuel treatments, 

such as economic analysis, social implications, 

risks and tradeoffs, and effects of various fuel 

treatments on sensitive species, wildlife, soil, and 

hydrology. These topics are, for the most part, not 

considered here but are addressed in other scien

-

tific literature.

Background

Millions of acres of forest lands in the Western 

United States contain accumulations of flam

-

mable fuel that are much higher than historical 

conditions. Forest fuel conditions have increased 

fire hazard over several decades, the result of fire 

suppression (Covington and Moore 1994), live

-

stock grazing (Savage and Swetnam 1990), timber 

harvest, and farm abandonment (Arno et al. 1997). 

The large wildfires in the summers of 2000 and 

2002 have sharpened our focus on fuel accumula

-

tion on national forests and other public lands. 

During the summer of 2000, 122,827 wildfires 

burned 8.4 million acres in the United States, and 

during the summer of 2002, 88,458 fires burned 

6.9 million acres (USDI BLM 2004).

Managers and policymakers seek a strategy 

capable of reducing the size and severity (dam

-

age to the forest overstory and associated changes 

in resource value) of wildfires in the future. The 

Healthy Forests Restoration Act of 2003 (HFRA 

2003), National Fire Plan (USDA USDI 2001), and 

the 10-Year Comprehensive Strategy Implementa

-

tion Plan (USDA USDI 2002) highlight the need 

for fuel reduction in Western forests. Although the 

scope of work and the available tools are described 

in these documents, little guidance on specific 

stand and landscape target conditions is provided. 

The Joint Fire Science Program and the research 

component of the National Fire Plan focus on 

scientific principles and tools to support decision

-

making and policy about fuel treatments.

Fire, in conjunction with landforms and climate, 

shapes the structure and function of forests 

throughout the Western United States (Agee 1998, 

Schmoldt et al. 1999), from wet coastal forests to 

cold subalpine forests to arid interior forests. 

Climatic patterns, especially magnitude and 

distribution of precipitation, influence the spatial 

and temporal distribution of wildfires (Hessl et al. 

2004, Heyerdahl et al. 2001). Alteration of fire 

regimes by fire exclusion has likely been greatest 

in arid to semiarid forests of the Western United 

States, primarily forests dominated by ponderosa 

pine (Pinus ponderosa Dougl. ex Laws.), Douglas-fir 
2

(Pseudotsuga menziesii [Mirb.] Franco) or both, 

which formerly had more frequent fires than today 

(e.g., Everett et al. 2000). 

Prior to the 20th century, low-intensity fires 

burned regularly in many arid to semiarid forest 

ecosystems, with ignitions caused by lightning 

and humans (e.g., Allen et al. 2002, Baisan and 

Swetnam 1997). Low-intensity fires controlled 

regeneration of fire-sensitive species (e.g., grand 

fir [Abies grandis (Dougl. ex D. Don) Lindl.]) (Arno 

and Allison-Bunnell 2002), promoted fire-tolerant 

species (e.g., ponderosa pine, Douglas-fir, western 

larch [Larix occidentalis Nutt.]), and maintained a 

variety of forest structures including a higher proportion of low-density stands than currently exists 

(Swetnam et al. 1999). These fires reduced fuel 

loading and maintained wildlife habitat for species 

that require open stand structure. Lower density 

stands likely had higher general vigor and lesser 

effects from insects (Fulé et al. 1997, Kalabokidis 

et al. 2002). In many areas, fire exclusion has 

caused the accumulation of understory vegetation and fuel, greater continuity in vertical and 

horizontal stand structure, and increased potential for crown fires (Agee 1993, Arno and Brown 

1991, Dodge 1972, van Wagner 1977). Across any 

particular landscape, there were probably a variety 

of stand structures, depending on local climate, 

topography, slope, aspect, and elevation.

Most fire history data and much of our understanding of fire in the West are from forests with 

low-severity (high-frequency) fire regimes. These 

forests are the ones whose (increased) fuels and 

(decreased) fire frequency have changed the most 

during the past century. The concept of Fire 

Regime Condition Class (sensu Schmidt et al. 

2002) uses current fuel conditions to represent 

the degree of departure from historical fire 

regimes and fuel conditions at a broad spatial 

scale. As a result, many arid and semiarid forests 

that historically were in Condition Class 1 are now 

in Condition Class 2 or 3: fires were frequent and 

low severity in the past, but they now have greater 

potential to be both large and severe. Extreme fire 

weather is associated with large fires in subalpine 

forests of the southern Canadian Rocky Mountains 

(Bessie and Johnson 1995) and the American 

Rockies (Romme and Despain 1989), and these 

types of forests with high-severity (low-frequency) 

fire regimes have probably not changed much. Forests with mixed-severity (moderate-frequency) fire 

regimes may have changed somewhat, but not as 

much as forests with low-severity regimes. 

Approximately 59 percent of Fire Regime 1 forests 

in the Western United States have higher fuel accumulations (currently Condition Classes 2 and 

3) than they would have historically, and about 

43 percent of Fire Regime 2 forests have higher 

fuel accumulations (currently Condition Class 3) 

than they would have historically (Schmidt et al. 

2002). For example, in the inland Northwestern 

United States, forests that would currently burn 

with high severity compose 50 percent of the 

forest landscape compared to only 20 percent 

historically (Quigley et al. 1996) (fig. 1). If these 

general relationships are applied to regional and 

local situations, then they need to be refined with 

site-specific information.

Changes in Forest Structure

Vertical arrangement and horizontal continuity 

of many arid and semiarid low-elevation forests 

in the Western United States differ from historical stand structures (Carey and Schumann 2003, 
3

Figure 1—The proportion of low-severity, mixed-severity, and high-severity fire regimes in the pre-1900 

(historical) period and in recent times in the inland Northwestern United States. Note the increasing 

proportion of high-severity fire regime. (From Quigley et al. 1996)

Figure 2—Representation of changes in vertical arrangement and horizontal continuity in forest stand structure. Today’s forests tend to have more 

fuel strata, higher densities of fire-sensitive species and suppressed trees, and greater continuity between surface and crown fuel.

Mutch et al. 1993) (fig. 2). Current forests have 

denser canopies, a higher proportion of fire-intolerant species, and fewer large trees (Bonnicksen 

and Stone 1982, Parsons and DeBenedetti 1979). 

These conditions increase the probability of 

surface fires developing into crown fires, because 

understory ladder fuels lower the effective canopy 

base height (lowest height above ground at which 

there is significant canopy fuel to propagate fire 

vertically through the canopy [Scott and Reinhardt 

2001]) of the stand (Laudenslayer et al. 1989, 

MacCleery 1995). This departure from historical 

conditions is common in high-frequency, lowto moderate-severity fire regimes (Agee 1991, 

1993, 1994; Arno 1980; Skinner and Chang 

1996; Taylor and Skinner 1998). Historical 

observers (e.g., Weaver 1943) described Western 

forest structures as open with minimum understory vegetation, a condition largely maintained 

by frequent, low-intensity surface fires.
4

Fuel, Fire Behavior, and Fire Effects

Fuel, topography (or physical setting), and 

weather interact to create a particular fire inten

-

sity (energy release, flame length, rate of spread) 

and severity. Although much of this document 

focuses on the effects of forest structure on fire 

hazard and behavior, decisions about fuel treat

-

ment must also consider topography and weather, 

which influence fire at different spatial scales. 

The forest structure needed to achieve a specific 

fuel condition for a particular location will differ 

depending on slope, aspect, and elevation, and on 

temperature, humidity, and windspeed.

Stand structure and wildfire behavior are clearly 

linked (Biswell 1960, Cooper 1960, Dodge 1972, 

McLean 1993, Rothermel 1991, van Wagner 1977), 

so fuel-reduction treatments are a logical approach 

to reducing extreme fire behavior. The principal 

goal of fuel-reduction treatments is to reduce 

fireline intensities (heat release per unit distance 

per unit time), reduce the potential for crown fires, 

improve opportunities for successful fire suppres

-

sion, and improve the ability of forest stands to 

survive wildfire (Agee 2002a). Prescribed fire can 

be implemented under benign weather to reduce 

surface fuel and fireline intensity. Silvicultural 

treatments that target canopy bulk density (the 

foliage mass contained per unit crown volume), 

canopy base height, and canopy closure have the 

potential to reduce the development of all types 

of crown fires (Cruz et al. 2002, Rothermel 1991, 

Scott and Reinhardt 2001, van Wagner 1977) if 

surface fuels are relatively low or are concurrently 

treated.

Fire hazard for any particular forest stand or land

-

scape is the potential magnitude of fire behavior 

and effects as a function of fuel conditions. Understanding the structure of fuelbeds and their role in 

the initiation and propagation of fire is the key to 

developing effective fuel management strategies. 

Fuels have been traditionally characterized 

as crown fuels (live and dead 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 classifica

-

tion separates fuelbeds into six strata: (1) forest 

canopy; (2) shrubs/small trees; (3) low vegetation; 

(4) woody fuel; (5) moss, lichens, and litter; and 

(6) ground fuel (duff) (Sandberg et al. 2001) (fig. 

3). Each of these strata can be divided into sepa

-

rate categories based on physiognomic characteristics and relative abundance. Modification of any 

fuel stratum has implications for fire behavior, fire 

suppression, and fire effects (fig. 4).

Crown fires are generally considered the primary 

threat to ecological and human values and are the 

primary challenge for fire management. The tree 

canopy is the primary stratum involved in 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 (Ro

-

thermel 1983). The shrub/small tree stratum is 

also involved in crown fires by increasing surface 

fireline intensity and serving as “ladder fuel” 

that provides continuity from the surface fuel to 

canopy fuel, thereby potentially facilitating active 

crown fires. 

Passive crown fires (torching) kill individual 

trees or small groups of trees. Active crown fires

(continuous crown fire) burn the entire canopy 

fuel complex but depend on heat from surface fuel 

combustion for continued spread. Independent 
5

Figure 3—Six horizontal fuelbed strata represent unique combustion environments. Each fuelbed category is described by 

morphological, chemical, and physical features and by relative abundance. (From Sandberg et al. 2001)

Figure 4—Fuelbed strata affect the combustion environment, fire propagation and spread, and fire effects. Note that woody 

surface fuel can also contribute to crown fires.
6

crown fires, which are much less common than 

passive or active crown fires, burn canopy fuel 

independently of heat from surface fire because 

the net horizontal heat flux and mass flow rate (a 

product of rate of spread and canopy bulk density) 

in the crown are sufficient to perpetuate fire 

spread.

Crown fires occur when surface fires release 

enough energy to preheat and combust fuel well 

above the surface (Agee 2002b). Crown fire begins 

with torching, or movement of fire into the crown, 

followed by active crown-fire spread in which fire 

moves from tree crown to tree crown through 

the canopy (Agee et al. 2000, van Wagner 1977). 

Torching occurs when the surface flame length 

exceeds a critical threshold defined by moisture 

content of fuel in the canopy and by canopy base 

height (Scott and Reinhardt 2001, van Wagner 

1977). Foliage moisture varies within a tree, with 

newer foliage generally having higher moisture 

than older foliage, and varies greatly during the 

course of a year, depending on the local climatic 

regime. 

The canopy base height, defined as the lowest 

height above which at least 30 lb·ac-1·ft-1 of 

available canopy fuel is present (Scott and 

Reinhardt 2001), determines how critical the moisture factor can become. For example, if foliage 

moisture averages 100 percent in late summer, a 

canopy base height of 7 ft means any surface fire 

with a flame length exceeding 4.5 ft would likely 

produce torching. If the bottom of the crown is 

lifted to 20 ft, the predicted critical flame length 

would be 9 ft, so a much more intense surface fire 

would be needed to initiate a crown fire (Scott 

and Reinhardt 2001).

Active crown-fire spread begins with torching but 

is sustained by the density of the overstory canopy 

and fire rate of spread. Crown fire is unlikely 

below a specific rate of canopy fuel consumption. 

This rate is defined as a function of crown-fire rate 

of spread and canopy bulk density (van Wagner 

1977). Where empirical rates of spread from 

observed crown fires (Rothermel 1991) are used, 

crown-fire hazard can effectively be represented by 

canopy bulk density. Below a critical threshold of 

canopy bulk density (a function of fire weather 

and fire rate of spread) a crown fire can make a 

transition back to a surface fire (Agee 2002b).

To reduce the probability of crown fire, fueltreatment planners should consider how canopy 

base height, canopy bulk density, and continuity 

of tree canopies affect the initiation and propagation of crown fire. As noted above, canopy base 

height is important because it affects crown-fire 

initiation. It is difficult to assess in the field 

because of the subjectivity of its location in a given 

forest stand, making it difficult for even experienced fire managers to quantify it with precision. 

This parameter has been accurately quantified in 

only a few forest stands in the United States and is 

difficult to assess because of the intensive nature 

of data collection required to quantify it. Continuity of canopy can encompass different properties 

related to the adjacency of tree crowns, but clearly, 

horizontal patchiness of the canopy will reduce 

the spread of fire within the canopy stratum.

Our understanding of crown fire, especially under 

severe fire weather conditions, is relatively poor 

owing to the complexity of interactions between 

fuel, topography, and local weather. Although 

many of the principles of crown-fire spread might 
7

appear to be similar across forest types, few experimental data on crown-fire spread for dry Western 

forests exist. For example, foliage moisture and 

foliage energy content affect crown-fire characteristics but are rarely quantified or included in fire 

behavior models (Williamson and Agee 2002). 

Even the basic measurement of canopy base height 

differs between different sources (e.g., Cruz et al. 

2003, Scott and Reinhardt 2001) and is difficult 

to measure on the ground. In addition, understory 

shrubs are typically not included in calculation of 

canopy bulk density, although they are included in 

fire behavior simulation models such as BEHAVE 

(Andrews 1986). As a result, predictions of torch

-

ing and crown-fire spread should be considered 

general estimates until we have a better empirical 

and conceptual basis for quantifying and modeling 

crown fire.

Surface fires were much more common in West

-

ern arid to semiarid forests prior to the 20th cen

-

tury (e.g., Everett et al. 2000). Three fuelbed strata 

contribute to the initiation and spread of surface 

fires (fig. 3). Low vegetation, consisting of grasses 

and herbs, can carry surface fires when that 

vegetation is dead or has low moisture content. 

The contribution of low vegetation to the combus

-

tion environment differs greatly between forest 

systems. Woody fuel can consist of sound logs, 

rotten logs, stumps, and wood piles from either 

natural causes or management activities. Wood 

can greatly increase the energy release component 

from surface fires and can in some cases increase 

flame lengths sufficiently to ignite ladder fuel and 

canopy fuel. Moss, lichens, and litter on the forest floor can also increase energy release in surface 

fuel. These fuel categories differ greatly among 

forest systems; e.g., dead needle litter is important 

in Southwestern ponderosa pine forests, whereas 

large accumulations of moss (live and dead) are 

important in Alaskan boreal forests. Because of the 

potential for surface fires to propagate into crown 

fires—even if tree density and crowns have been 

greatly reduced—treatment of surface fuel must be 

planned in conjunction with treatment of ladder 

fuel and crown fuel.

Surface fires are highly variable depending on 

surface-fuel packing, bulk densities, and size-class 

distributions. Surface fires burn in both flaming 

and postfrontal phases. Energy release rate is high 

during the short flaming phase in which fine fuels 

are consumed, and low during longer glowing and 

smoldering periods that consume larger fuels. Fine 

fuels such as grass typically have shorter flaming 

residence times than large woody materials such 

as logging slash.

Smoldering fires, also referred to as ground fires 

or residual smoldering, are an important but often 

overlooked component of most fires. Three fuelbed 

strata contribute to the initiation and slow spread 

of smoldering fires (fig. 3). Ground fuel, consist

-

ing principally of soil organic horizons (or duff) 

contributes most of the fuel, and can burn slowly 

for days to months if fuel moisture is sufficiently 

low. Deep layers of continuous ground fuel are often found in forests that have not experienced fire 

for several decades, with large additional accumu

-

lations near the bases of large trees. 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) fires. 

Woody fuel (sound logs, rotten logs, stumps, 

and wood piles) is often underestimated as a 

component of smoldering fire, but can sustain 

low-intensity burning for weeks to months, with 
8

potential flaming combustion under dry, windy 

conditions. Woody fuel also can contribute signifi

-

cantly to smoke production and soil impacts.

Each fuelbed and combustion environment is 

associated with different fire effects. Crown fires

remove much or all of the tree canopy in a particu

-

lar area, essentially resetting the successional and 

growth processes of a stand. These fires typically, 

but not always, kill or temporarily reduce the 

abundance of understory shrubs and trees. Crown 

fires have the largest immediate and long-term 

ecological effects and the greatest potential to 

threaten human settlements near wildland areas. 

Surface fires have the important effect of reducing 

low vegetation; woody; and moss, lichens, and lit

-

ter strata. This temporarily reduces the likelihood 

of future surface fires propagating into crown fires. 

Smoldering fires that consume large amounts 

of woody fuel and the organic soil horizon can 

produce disproportionately large amounts of 

smoke. Ground fires reduce the accumulation of 

organic matter and carbon storage, and contribute 

to smoke production during active fires and long 

after flaming combustion has ended. Smoldering 

fires can also damage and kill large trees by killing 

their roots and the lower stem cambium. Because 

smoldering fires are often of long duration, they 

may result in greater soil heating than surface or 

crown fires, and have the potential for reducing 

organic matter, volatilizing nitrogen, and creating 

a hydrophobic layer that contributes to erosion.

Topography influences fire behavior at different 

spatial scales (Albini 1976, Chandler et al. 1983). 

Rate of spread doubles from 0- to 30-percent 

slope, and doubles again from 30- to 60-percent 

slope. Rate of spread on a 70-percent slope can be 

up to 10 times the rate on level ground. A narrow 

v-shaped canyon can radiate heat to adjacent 

slopes, drying fuels ahead of the active fire and 

leading to faster rate of spread. Local discontinui

-

ties such as ridges can create turbulence that 

affects rate of spread and energy release. Topogra

-

phy in conjunction with the general direction of 

fire spread and wind also affects fireline intensity 

and effects. Head fires (in the same direction as 

the main active fire, generally upslope) usually 

have high flame lengths and significant potential 

for crown injury and tree mortality. Backing fires

(opposite the direction of the main active fire, 

generally downslope) typically spread more slowly 

than head fires, result in more complete combus

-

tion of fuel, and cause less damage to trees unless 

ground fuel burns hot enough to kill tree roots and 

the lower cambium. Flanking fires (tangential to 

the direction of the main active fire, generally 

across slope) are intermediate in spread rate and 

effects. All of these fire characteristics are modified 

by weather conditions, discussed in a subsequent 

section.

Fuel Treatments: Thinning and 

Prescribed Fire

Where and when should fuel treatments be 

conducted? Although this is partially a policy 

question, it is also relevant to the science and 

management of fuel and fire. Schmidt et al. (2002) 

provided a classification that suggests where fuel 

treatments might be prioritized (especially Condi

-

tion Class 3) at a coarse spatial scale if the objec

-

tive is to return fuel to historical conditions and 

reduce fire hazard. This is a restoration and fire 

management objective for many low-elevation dry 

forests in the Western United States. This objective 

is more difficult to justify for high-elevation, cold, 

and wet forests.
9

A collateral objective of fuel treatment is often to 

create conditions that are defensible by fire suppression if wildfire should occur. This is typically 

in areas that do not have steep slopes and are 

accessible by firefighting equipment and personnel. Accessibility often limits the possibility of fuel 

treatment in wilderness and other unroaded areas. 

Areas with steep slopes are also difficult to treat 

in terms of the logistics of removing downed logs 

and fuel by thinning, as well as the safety of using 

prescribed fire. The presence of threatened and 

endangered species may also preclude fuel treatments. As noted later in this document, conducting isolated or random fuel treatments without 

considering the fuel and fire hazard across the 

broader landscape may be ineffective.

Fuel treatments typically target crown, ladder, 

and surface fuels with silvicultural operations and 

prescribed burning to modify vegetation in each 

stratum (Peterson et al. 2003). Canopy and ladder 

fuels are modified by forest thinning operations 

that target crown classes, stand basal area, and 

canopy bulk density. Surface fuel, particularly 

woody fuel and litter, can be modified by prescribed fire and a variety of treatments that remove 

and reduce fuel (e.g., pile-and-burn, and crushing and chopping [mastication]). Silvicultural 

treatments and prescribed fire can also modify 

vegetation dynamics in the short and long terms. 

Opening forest canopies increases light to the 

forest floor, with the potential for increased grass 

and shrub fuel, altering fuel structure and in some 

cases successional pathways for vegetation.

Fuel treatment must consider (1) how a forest 

stand is accessed and mechanically treated, (2) 

what material is removed, and (3) what material 

remains on site in terms of species, sizes, and fuel 

composition (e.g., sound vs. rotten wood) (Kalabokidis and Omi 1998, Peterson et al. 2003). 

Management of thinning residues affects the postthinning combustion environment, with an almost 

certain increase in fine fuel if stems and foliage are 

left on site (Carey and Schumann 2003). Groundbased equipment (e.g., a feller buncher) typically 

changes the spatial distribution of fuel. Equipment 

that removes large stems from the stand prior to 

further processing typically increases the fuel load 

less than felling and processing within the stand. 

Helicopter yarding and cable-based systems increase surface fuel unless treated, because logs are 

removed but slash from tree crowns is left behind. 

Lop-and-scatter, cutting residual fuel into pieces 

and scattering them on the forest floor, has often 

been used for the unmerchantable portion of 

thinning. Unless this material is broken and 

compressed into the ground fuel, it can increase 

fireline intensity and flame length. Prescribed 

burning of material left on site can effectively 

reduce residual surface fuel in some situations. Pile-and-burn generally is more effective 

at removing thinning material from the forest 

floor, particularly from the base of living trees. If 

burning is deemed unacceptable because of smoke 

production or carbon release, the material can be 

removed from the forest or chipped and left on 

site. A collateral negative impact of ground-based 

thinning is that roads and skid trails can cause soil 

compaction and damage low vegetation. 

Silvicultural thinning is implemented with the 

principal objective of reducing fuel loads and 

ultimately modifying fire behavior. However, 

breakage, handling of slash, and disruption of the 

forest floor can increase fine-fuel loading (Agee 

1996, Fitzgerald 2002, Weatherspoon 1996). 
10

Rate of spread and fireline intensity in thinned 

stands are usually significantly reduced only if 

thinning is accompanied by reducing and altering the arrangement of surface fuel created by the 

thinning operation (Graham et al. 1999, 2004). 

Prescribed burning is often used to reduce surface 

fuel. The effectiveness of prescribed fire depends 

on weather, initial fuel conditions, and skill of the 

fire manager. It can be safely conducted only if 

the probability of crown-fire initiation is low. This 

means that ladder fuel must be minimal, a condition that may exist only after thinning. Retention 

of larger, more fire-resistant trees in silvicultural 

prescriptions reduces fire damage to the overstory even if damage is high to smaller, residual 

(and less fire-resistant) trees in a subsequent fire 

(Weatherspoon and Skinner 1995). In some cases, 

removal of trees from the canopy and understory 

could conceivably increase surface wind movement (Albini and Baughman 1979) and facilitate 

drying of live and dead fuel (Pollet and Omi 2002), 

although effective removal of ladder and surface 

fuel should mitigate these factors by reducing the 

fuel load and potential for fire spread.

Thinning and prescribed fire target different 

components of the fuelbed of a given forest stand 

or landscape (Peterson et al. 2003). Thinning is 

potentially effective at reducing the probability of 

crown-fire spread, and is precise in that specific 

trees are targeted and removed from the fuelbed. 

Thinning is expensive and poses a challenge 

for handling large amounts of woody material, 

much of which may be unmerchantable. Prescribed burning is a less precise management 

tool, although it can be highly effective at reducing surface fuel, creating gaps, and in some cases 

reducing ground fuel.

Prescribed burning affects potential fire behavior 

by reducing fuel continuity on the forest floor, 

thereby slowing fire spread rate, reducing fire intensity, and reducing the likelihood of fire spreading into ladder fuel and the crown. Prescribed fire 

is typically cheaper per unit area than thinning 

and in some cases can be used to reduce stem 

density and ladder fuel by killing (mostly) smaller 

trees. This has proven to be effective as the sole 

means of fuel treatment in the mixed-conifer forest 

of the southern Sierra Nevada, California (Kilgore 

and Sando 1975, McCandliss 2002, Stephenson et 

al. 1991), and may be effective in other Western 

forests if carefully applied, particularly in stands 

with large, fire-resistant trees. However, potential 

secondary effects pose management challenges. 

Prescribed fires may kill individual trees and 

clumps of trees that are not targeted for removal. 

Fallen dead branches increase fine fuel that helps 

propagate surface fire, and fallen boles add to the 

potential for energy release and smoke production. 

Prescribed fires create smoke that decreases air 

quality in local communities and cause charring 

that affects esthetic qualities of the residual stand 

and landscape. 

The type and sequence of fuel treatments depend 

on the amount of surface fuel present; the density 

of understory and midcanopy trees (Fitzgerald 

2002); long-term potential effects of fuel treatments on vegetation, soil, and wildlife; and shortterm potential effects on smoke production (Huff 

et al. 1995). In forests that have not experienced 

fire for many decades, multiple fuel treatments are 

often required. Thinning followed by prescribed 

burning reduces canopy, ladder, and surface fuel, 

thereby providing maximum protection from 

severe fires in the future. Given current accumulations of fuel in some stands, multiple prescribed 
11

fires—as the sole treatment or in combination 

with thinning—may be needed initially, followed 

by long-term maintenance burning or other fuel 

reduction (e.g., mastication), to reduce crown-fire 

hazard.

Evidence for Fuel Treatment 

Effectiveness

The majority of the scientific literature supports 

the effectiveness of fuel treatments in reducing 

the probability of crown fire (e.g., Agee 1996; 

Edminster and Olsen 1996; Helms 1979; Kilgore 

and Sando 1975; Martinson and Omi 2002; Omi 

and Martinson 2002; Pollet and Omi 2002; Scott 

1998a, 1998b; van Wagtendonk 1996; Wagle and 

Eakle 1979; Weatherspoon and Skinner 1995). 

Fuel loading may determine fire severity in historically low and moderate fire-severity regimes, but 

because the relative influence of fuel and weather 

differs between forest ecosystems (Agee 1997), it is 

difficult to develop precise quantitative guidelines 

for fuel treatments. A majority of the evidence 

supporting the effectiveness of fuel treatments for 

reducing crown-fire hazard is based on informal 

observations (Brown 2002, Carey and Schumann 

2003), postfire inference (Omi and Kalabokidis 

1991, Pollet and Omi 2002), and simulation modeling (Finney 2001, Stephens 1998).

Observations from the Hayman Fire in Colorado 

(Graham 2002) suggest that prescribed fire treatments effectively reduced fire behavior on relatively gentle slopes, with crown fires diminishing 

to surface fires in stands with lower stem densities 

and surface fuel on days when weather conditions 

were less extreme. The results of fuel treatments 

were less clear at locations that burned when fire 

weather was extreme. Observations from the Cone 

Fire in California in 2002 (Agee and Skinner, in 

press) also suggest that past thinning treatments 

can reduce crown fires to surface fires.

Empirical studies comparing on-the-ground effects 

of fire in treated versus untreated stands under 

extreme fire weather conditions and on steep 

slopes are rare (Pollet and Omi 2002). In response 

to the lack of knowledge in this area, the Fire and 

Fire Surrogates study (http://www.fs.fed.us/ffs) 

was developed through the Joint Fire Science 

Program to quantify the consequences and tradeoffs of alternative fire and thinning treatments. 

Although the small number of treatments (control, 

thinning, fire, thinning plus fire) are not comprehensive of the diverse forest landscapes now being 

considered for fuel treatments, the study will provide valuable new empirical data that can inform 

future fuel treatment decisions.

The Role of Fire Weather

Fire weather is often perceived at different scales. 

Weather at small spatial and temporal scales

regulates fuel moisture content, which influences 

diurnal and day-to-day variation in flammability. Temperature, relative humidity, and wind are 

monitored by fire managers throughout the fire 

season to determine fire danger and the potential 

for flammability and fire spread during wildfires 

and prescribed fires. Weather at broad spatial 

and temporal scales, or climatology, often controls extreme fire behavior (e.g., crown-fire spread) 

and the occurrence of large fires (e.g., Flannigan et 

al. 2000), although this generalization varies considerably among biogeographic regions (Gedalof 

et al., in press). The relative influence of fuel and 

climatology has been poorly quantified for most 

forest ecosystems and regions of the United States.
12

Extensive scientific data exist on key aspects of 

fuel, topography, and weather that influence fire 

behavior and severity, especially for surface fire. 

However, owing to the logistical constraints of 

working in wildfires, applicability of empirical 

data and current theory is more limited for ex

-

treme fire weather conditions (Agee 1997). For ex

-

ample, a comparison of BEHAVE (Andrews 1986) 

and the Fire Behavior Prediction (FBP) model 

(Forestry Canada Fire Danger Group 1992) in 

Canadian mixed-wood boreal forest showed that 

FBP was more sensitive to variation in weather, 

and BEHAVE was more sensitive to variation in 

vegetation (Hely et al. 2001). This disparity in how 

fire behavior is modeled in the two common sys

-

tems used in North America indicates a disparity 

in the basis for describing the interaction of fuel 

and weather–empirical for FBP, laboratory based 

for BEHAVE.

Large fires tend to occur most often during and 

following periods of dry weather that lower fuel 

moisture (e.g., Agee 1997, Heyerdahl et al. 2001). 

Dry weather and the potential for ignitions are 

more common during distinct climatic modes, 

such as high-pressure blocking ridges (Gedalof 

et al., in press). In addition, temporal variation in 

large fires in the West is at least partially con

-

trolled by variation in long-term climatic pat

-

terns. The occurrence of large fires in ponderosa 

pine forests of the Southwest (Allen et al. 2002, 

Swetnam and Betancourt 1990) and ponderosa 

pine-Douglas-fir forests of the southern Rocky 

Mountains (Veblen et al. 2000) is related to 3- to 

7-year phases of the El Niño Southern Oscillation 

(ENSO), whereas large fires in ponderosa pineDouglas-fir forests of the inland Pacific Northwest 

are related to 20- to 40-year phases of the Pacific 

Decadal Oscillation (PDO) (Gedalof et al., in 

press; Hessl et al. 2004). Extreme fire weather is 

more common during warm phases of the ENSO 

and PDO, and along with steep slopes, creates the 

conditions that facilitate rapid spread of crown fire 

and long-distance transport of burning embers 

(spotting).

Integrating Tools to Provide Quanti-

tative Fuel-Treatment Guidelines 

Management of fuel across large landscapes is 

required to effectively reduce the area and severity 

of fires, as well as effects on local communities. 

In addition, because a small proportion of fires 

(approximately 1 percent) is responsible for as 

much as 98 percent of the fire area (Strauss et al. 

1989), managers need fuel treatment options that 

are effective under extreme fire weather and in 

steep mountain topography–conditions under 

which crown fire spreads most rapidly and burns 

most severely.

Silvicultural Thinning

Silvicultural options for fuel treatment are sum-

marized in Graham et al. (1999) and Fitzgerald 

(2002), which provide visual displays of thinning 

treatments and explain how treatments address 

fuel loading. Thinning, the removal of specific 

components of the tree stratum, is used to modify 

fire hazard, improve growth and vigor of residual 

trees, and promote certain types of wildlife habi

-

tat. Several thinning methods exist (Graham et al. 

1999): (1) crown thinning, (2) low thinning, (3) 

selection thinning, (4) free thinning, (5) geometric 

thinning, and (6) variable-density thinning. The 

effects of thinning on forest canopy components 

are compared in table 1, and visualizations of an 

unthinned stand (fig. 5) are compared to three 

thinning treatments (figs. 6–8).
13

Figure 5—A mixed-conifer stand from Pack Forest, Eatonville, Washington. Initial stand condition is 278 trees 

per acre, basal area 376 ft2

•ac-1, average diameter 12 in, comprising Douglas-fir, western hemlock (Tsuga 

heterophylla), western redcedar (Thuja plicata), red alder (Alnus rubra), and black cottonwood (Populus 

trichocarpa). Visualizations in figures 5 through 8 are derived with the Forest Vegetation Simulator.

Table 1—Effects of thinning treatments on key components of canopy structure related to crown-fire hazard

Thinning 

treatment

Canopy 

base height Canopy bulk density Canopy continuity Overall effectiveness

Crown Minimal Lower in upper canopy but minimal 

effect in lower canopy

Lower continuity in upper canopy 

but minimal effect in lower canopy

May reduce crown-fire spread 

slightly but torching unaffected

Low Large increase Large effect in lower canopy, some 

effect in upper canopy depending 

on tree sizes removed

Large effect in lower canopy, some 

effect in upper canopy depending 

on tree sizes removed

Will greatly reduce crown-fire 

initiation and torching

Selection None Lower in upper canopy but minimal 

effect in lower canopy

Lower continuity in upper canopy 

but minimal effect in lower canopy

May reduce crown-fire spread 

slightly if many trees removed 

but torching unaffected 

Free Small to moderate 

increase, depending 

on trees removed

Small to moderate decrease 

throughout canopy, depending 

on trees removed

Small to moderate decrease 

throughout canopy, depending 

on trees removed

May reduce crown-fire spread 

slightly if many trees removed; 

torching reduced slightly

Geometric None Small to moderate decrease 

throughout canopy, depending 

on spacing and species composition

Small to moderate decrease 

throughout canopy, depending on 

spacing and species composition

Crown-fire spread and initiation 

reduced if spacing is sufficiently 

wide; torching reduced

Variable 

density

Increase in patches where 

trees are removed

Decrease in patches where trees 

are removed

Moderate to large decrease Crown-fire spread reduced, crownfire initiation reduced somewhat; 

torching reduced somewhat
14

Crown thinning (thinning from above) (fig. 6) 

removes trees with larger diameters but favors the 

development of the most vigorous trees of these 

same size classes. Most of the trees that are cut 

come from the codominant class, but any intermediate or dominant trees interfering with the 

development of residual trees (sometimes termed 

crop trees if timber production is an objective) are 

also removed. Thinning from above focuses on 

removal of competitors rather than eliminating all 

suppressed trees.

Low thinning (thinning from below) (fig. 7) 

primarily removes trees with smaller diameters. 

This method mimics mortality caused by intraspecific and interspecific competition or abiotic 

factors such as wildfires. Thinning from below 

primarily targets intermediate and suppressed 

trees, although codominant and dominant trees 

are not exempt from harvest. If codominant and 

dominant trees are removed, all smaller, intermediate, and overtopped trees are also removed 

(Smith et al. 1997). Often, diameter limits are used 

to establish cutting targets. For example, a thinning prescription may call for all trees of less than 

9 in diameter to be removed.

Selection thinning removes dominant trees with 

the potential objective of stimulating the growth 

of smaller trees. This practice, commonly called 

“high grading,” removes the most economically 

valuable trees. This thinning method has limited 

applicability in forest management programs with 

multiple objectives (e.g., structural diversity, wildlife habitat) because it limits future stand options.

Figure 6—The mixed-conifer stand from figure 5, showing the results of crown thinning.
15

Figure 7—The mixed-conifer stand from figure 5, showing the results of low thinning; all stems of less than 9 in 

diameter were removed.

Free thinning (fig. 8) primarily favors selected 

individual trees in a stand while the rest of the 

stand remains untreated. Cuttings are designed to 

release residual trees from competition regardless 

of their position in the crown canopy. The method 

is commonly used to increase structural diversity 

in forest stands.

Geometric thinning removes trees based on 

predetermined spacing (e.g., 6- by 6-ft spacing) 

or other geometric pattern, with little regard for 

their position in the crown canopy. This type of 

thinning is often applied in young plantations with 

high density, and employed only in the first thinning of a stand. Space thinning and row thinning 

can be used to accomplish geometric thinning. 

Variable-density thinning combines thinning 

from below and one or more of the other silvicultural thinning techniques by removing trees 

from some patches and leaving small stands of 

trees in other patches. This technique reduces fuel 

continuity within the canopy, thereby reducing 

crown-fire hazard. For any target stem density, 

variable-density thinning generally increases 

spatial heterogeneity of trees and canopy structure. 

This technique can promote better habitat characteristics for certain types of plants and animals at 

small and large spatial scales

Graham et al. (1999) provided examples of how 

specific thinning treatments affecting stand 

density, canopy base height, and canopy bulk 

density can be linked with fire behavior fuel 
16

models (Anderson 1982), which are standardized 

representations of surface fuel sizes and mass. This 

approach determines if surface fire will propagate 

to crown fire, thus providing a rough estimate of 

the likelihood of crown fire following fuel 

treatments.

Scott and Reinhardt (2001) provided the conceptual and quantitative framework for a more 

detailed analysis of the potential for transitions 

from surface fire to crown fire. The Fire and Fuels 

Extension to the Forest Vegetation Simulator 

(Reinhardt and Crookston 2003) incorporates 

much of this analytical capability. It allows users 

to enter current stand and surface fuel conditions; 

simulate thinning treatments, surface-fuel treatments, and fire; and examine the effects of these 

treatments on surface fuel, canopy fuel, and 

potential fire behavior over time. Indices of 

crown-fire hazard (“torching index” and “crowning index,” Scott and Reinhardt 2001) are provided 

to help assess the effectiveness of fuel treatments 

on crown-fire potential (e.g., Fiedler and Keegan 

2002).

Quantifying Fire Hazard and Fire Potential

Accurate quantification of fuel in the canopy and 

shrub/small tree strata is necessary to understand 

the combustion environment of crown fire (Cruz 

et al. 2003, Scott and Reinhardt 2001). Potentially 

effective techniques for reducing crown-fire occurrence and severity are to (1) increase canopy 

base height, (2) reduce canopy bulk density (Agee 

1996, Scott and Reinhardt 2001), (3) reduce forest 

canopy continuity (Cruz et al. 2002, Scott and 

Reinhardt 2001, van Wagner 1977), and (4) reduce 

surface fuel (Graham et al. 2004).

Figure 8—The mixed-conifer stand from figure 5, showing the results of free thinning.
17

With the caveat that few empirical data are available that quantify the effects of specific fuel treatments on fire behavior, objective and quantifiable 

fuel-treatment criteria will assist fire managers 

and silviculturists in achieving desired conditions 

for fuel to reduce fire hazard. Desired conditions 

for canopy base height, crown bulk density, and 

continuity depend on management objectives for 

fuelbeds and crown-fire hazard. For example, fire 

managers often make assessments of potential fire 

behavior for specific fire weather, such as the 

50th-, 90th-, and 97th-percentile weather severity, 

or for moistures of 1-, 10-, and 100-hr timelag 

fuels (equivalent to fuel size classes of <0.25 in, 

0.25 to 1.0 in, and 1.0 to 3.0 in, respectively). In 

addition, desired conditions must be adjusted for 

slope, because even greater fuel reductions are 

needed on steep slopes owing to convective winds 

and heating and intensification of fire behavior as 

fire spreads upslope.

Canopy base height should be considerably 

higher than the height of expected flame lengths 

for a specified fuelbed in order to avoid torching 

and potential crown-fire initiation (Scott 2002, 

Scott and Reinhardt 2001) (fig. 9). For many dry 

forests, this value may be 20 feet or more (Jain et 

al. 2001). Using the flame length for the worst case 

fire weather (e.g., 97th-percentile weather severity) 

as a standard would be the least risky option. The 

required reduction in stem density and basal area 

will differ considerably between stands, depending on initial stem density and canopy structure. 

Target values of canopy base height can be inferred 

from canopy fuel descriptions for various forest 

types (Cruz et al. 2003, Reinhardt 2004).

Figure 9—Critical flame length is less than canopy base height when canopy base height is greater 

than about 3 ft. The lines represent foliage moisture content (FMC) of 80 percent, 100 percent, and 120 

percent. (From Scott and Reinhardt 2001)
18

Canopy bulk density should be maintained below 

a critical threshold (a function of fire weather and 

fire rate of spread) such that a crown fire can make 

a transition back to a surface fire. This threshold 

is not well defined, although canopy bulk density 

<0.10 kg·m-3 (= 0.0062 lb·ft-3, canopy bulk density 

by convention is always expressed in metric units) 

seemed to be sufficient in the 1994 Wenatchee 

Fire in <100-year-old ponderosa pine-Douglas-fir 

stands on the east side of the Cascade Range of 

Washington (Agee 1996). The required reduction 

in stem density and basal area will differ considerably between stands, depending on initial 

stem density and canopy structure (fig. 10). For a 

ponderosa pine stand that has a dense understory 

and has not experienced fire for many decades, it 

may be necessary to remove 75 percent or more of 

the stems to achieve the target bulk density. Target 

values of canopy bulk density can be inferred from 

canopy-fuel descriptions for various forest types 

(e.g., Cruz et al. 2003, Reinhardt 2004).

Basic fire behavior principles and forest allometric 

relationships can be used to establish critical levels 

of canopy bulk density below which crown-fire 

initiation and spread are unlikely. These levels, 

in combination with information on fire weather, 

surface fuel data, and stand data, can be used to 

define a stand that is unlikely to generate or allow the spread of crown fire (Agee 1996). Crown 

bulk densities were calculated for ponderosa pine, 

Douglas-fir, and grand fir, associated with various 

combinations of mean stem diameter and stem 

density (Agee 1996) (table 2).

Figure 10—Vertical profile of canopy bulk density in a Sierra Nevada mixed conifer stand. Effective canopy bulk density 

is considered to be the maximum 3-m running mean (0.21 kg • m-3 in this stand). Canopy bulk density varies greatly 

depending on species, stand age, and stem density. The vertical distribution shown in this example is typical of dense, 

multistoried stands. Note that canopy bulk density by convention is always expressed in metric units.
19

Table 2—Canopy bulk density by diameter class and density 

for ponderosa pine (PP), Douglas-fir (DF), and grand fir (GF)

Mean 

diameter

Stem density (trees per acre)

Species 120 250 500 1,000 2,000

Inches Kilograms per cubic meter

0.5 PP 0.001 0.002 0.003 0.009 0.018

DF .002 .003 .005 .011 .022

GF .002 .003 .007 .014 .027

3.0 PP .003 .007 .014 .028 .055

DF .004 .007 .014 .028 .056

GF .006 .012 .024 .047 .094

8.0 PP .005 .010 .021 .041 .083

DF .009 .017 .034 .068 .136

GF .008 .016 .033 .066 .132

12.0 PP .010 .020 .041 .082 .164

DF .012 .025 .049 .099 .198

GF .013 .026 .052 .103 .103

18.0 PP .011 .023 .047 .248 .361

DF .019 .039 .078 .191 .252

GF .023 .047 .095 .247 .247

Source: Agee 1996.

Canopy bulk densities are generally lowest for 

ponderosa pine and highest for grand fir. Differences between species are typically not as great 

as differences between densities and size classes. 

However, fire tolerance is another matter, and 

thin-barked species such as grand fir are sensitive 

to surface fire, whereas thick-barked species such 

as ponderosa pine are not. Therefore, canopy 

bulk density is just one factor to consider in 

thinning prescriptions; the appropriate threshold 

is subjective and depends on fire weather conditions and rate of fire spread. Because table 2 

represents idealized single-species stands with 

uniform diameter, uniform density, and a single 

canopy stratum, caution should be used in 

applying these values; empirical data for actual 

stands should be used if available.

Canopy continuity is more difficult to quantify 

and is a more subjective fuel-treatment target. The 

general objective is to reduce physical contact of 

tree canopies and fire spread through the canopy. 

During extreme fire weather, fire can spread 

through horizontal and vertical heat flux and 

spotting from embers, so relatively wide spacing of 

canopies is necessary to effectively reduce crownfire hazard. An example of a field-based rule is 

that the distance between adjacent tree crowns 

should be the average diameter of the crown of 

codominant trees in the stand.

Crown competition factor (total crown base area 

divided by stand area), which is correlated with 

canopy bulk density, may hold promise as a field 

measurement that represents crown fuel. For 

example, Jain et al. (2001) suggested that stands 

with a crown competition factor <140 have canopy 

densities low enough to greatly reduce probability 

of crown fire. Additional empirical data are needed 

to determine how well this parameter works as a 

guideline for thinning.
20

An alternative approach, the Fuel Characteristic 

Classification System (FCCS) estimates quantita

-

tive fuel characteristics (physical, chemical, and 

structural properties) and probable fire parameters 

from comprehensive or partial stand inventory 

data, and allows users to access existing fuelbed 

descriptions or create custom fuelbeds for any lo

-

cation in the United States (Sandberg et al. 2001). 

A given set of fuel data can be associated with 

approximately 120 combinations of surface-fire 

potential, extreme-fire potential, and fire-effects 

potential. A three-digit code is used to represent 

(1) surface-fire potential (reaction potential, 

spread potential, and flame-length potential), 

(2) extreme-fire potential for canopy and shrub 

fuel (torching potential, dependent crown-fire 

potential, independent crown-fire potential), and 

(3) fire-effects potential (for flame available, 

smoldering available, and residual available fuel). 

The FCCS contains empirical fuelbed data from 

throughout the United States compatible with 

forest stand inventory data used by silviculturists. 

These fuelbeds can be linked with specific fuel 

treatments at any spatial resolution. The FCCS will 

be available online in 2005.

Assessing Large-Scale Fuel 

Conditions

Effective fuel treatment programs must consider 

the spatial pattern of fuel across large landscapes 

(e.g., Hessburg et al. 2000) because multiple 

stands and fuel conditions are involved in large 

fires (Finney 2001). Fire behavior under extreme 

fire weather may involve large areas of fuel, mul

-

tiple fires, and spotting, so a “firesafe” landscape 

needs to encompass hundreds to thousands of 

acres with desired fuel conditions strategically 

located in any particular management unit (Finney 

2003). Treating small or isolated stands without 

assessing the broader landscape may be ineffective 

in reducing large-scale crown fire.

The efficacy of fuel treatments across large land

-

scapes can be visualized by using spatially explicit 

data on fuel and fire hazard generated by manage

-

ment tools such as the Landscape Management 

System (LMS), which automates stand projections 

and manipulations, summarizes stand-level at

-

tributes, and displays associated graphs and tables 

(McCarter et al. 1998). The LMS uses stand inven

-

tory data (species, height, diameter, stem density), 

geospatial data, and forest growth models to 

project forest vegetation succession and changes in 

landscape pattern. All variants of the Forest Veg

-

etation Simulator (FVS) (Crookston 1990, USDA 

FS 2004) and ORGANON (Hann et al. 1997, OSU 

2004) are embedded within the system.

Silvicultural treatments can be implemented in 

LMS at designated times during a planning cycle 

(e.g., 50-year projection). Stand treatments include 

thinning to target basal area (BA), stand density 

index (SDI), or trees per acre (TPA). Thinning can 

be executed from above, below, proportionally, or 

within specific diameter limits. The system also 

has the ability to add new records (regeneration 

or ingrowth files). The effects of treatments can 

be readily analyzed with graphs, tables, and stand 

and landscape visualizations for any period during 

the planning cycle.

The LMS or another analysis tool can be used to 

display spatial patterns of forest structures and 

fuel across a landscape for existing conditions 

compared to patterns produced by various fueltreatment scenarios (fig. 11). Fuel conditions can 

be quantified with the FCCS, fuel models, or other 
21

Figure 11—Example of how a landscape analysis system can be combined with fuel classification to assess spatial fuel patterns.
22

fire-hazard parameters. By scanning across spatial 

patterns, fire managers can determine priority 

areas for fuel treatments and identify blocks of 

stands that need treatment to achieve desired fuel 

conditions. Integrating basic landscape analysis 

with fuel-treatment prescriptions for specific 

stands may be the most effective approach for 

managing fuel and reducing crown-fire hazard at 

large spatial scales.

Simulation modeling can also be used to predict 

propagation of fire at broad spatial scales. The 

primary tool used to model fire spread, includ

-

ing crown fire, for forest landscapes is FARSITE 

(Finney 1998). This program integrates geospatial 

fuel data, climatic data, and fire behavior mod

-

eling (BEHAVE, Andrews 1986) to predict fire 

spread. Although FARSITE requires large databas

-

es, simulation modeling skill, and good computer 

resources, it is a powerful tool for simulating the 

spread of fire across large landscapes (e.g., Finney 

2003), assuming that spatially explicit fuel data 

and good weather data are available.

The use of a landscape analysis tool can also be effective in scheduling fuel treatments over time. For 

example, the FVS and the Fire and Fuels extension 

of FVS (Reinhardt and Crookston 2003) can be 

used to quantify vegetation and fuel succession 

following fire or fuel treatments. By choosing a 

target for crown-fire hazard (e.g., a specific FCCS 

code or fuel model) above which hazard is deemed 

unacceptable, fuel treatments can be scheduled to 

always remain below the management threshold. 

Following initial thinning and prescribed burning 

to reduce high fuel accumulations, frequent pre

-

scribed burning (say, every 5 to 20 years) may be 

sufficient to control tree regeneration and surface 

fuels. If this is not desirable or practical, thinning 

can be scheduled at desired intervals, perhaps ac

-

companied by prescribed fire, to reduce ingrowth 

of ladder fuels. Scheduling of fuel treatments will 

differ by species, elevation, aspect, climatic zone, 

and soil fertility. Broad spatial perspectives and 

tools are the key to planning and implementing 

management for a fire-resilient landscape.

Using Scientific Principles for 

Adaptive Fuel Management

Forest ecosystems are inherently complex, and the 

effectiveness of site-specific modification of fire 

hazard is directly proportional to the quantity and 

quality of local data on forest structure and fuel. 

Fire behavior modeling is reasonable for surface 

fires and small spatial scales, but is in its infancy 

for crown fires and large spatial scales, and our 

understanding of the interaction of fuel, topog

-

raphy, and weather is better for small scales and 

moderate fire weather than for large scales and 

severe fire weather. In the face of this complexity, 

it is important to focus on basic scientific princi

-

ples that will aid decisionmaking and guide future 

data collection (table 3).

These basic principles of fire resilience can be 

applied quantitatively as well as qualitatively if 

adequate data are available. The relative impor

-

tance of each principle may vary depending on 

management objectives and the specific location of 

fuel treatments (e.g., forests adjacent to structures 

and local communities versus forests in a wilder

-

ness area). One approach is to target desired fuel 

conditions that will achieve a specific fire hazard 

or predicted fire behavior outcome for specific fire 

weather severity.

The relationship between fuel treatments and 

wildfires is based on documented scientific 
23

principles but a limited empirical database. Appropriate types of thinning and subsequent residue 

treatment are clearly useful in reducing surfaceand crown-fire hazards under a wide range of 

structural, fuel, and topographic conditions. Steep 

slopes and extreme fire weather will always be a 

challenge for fire management and require higher 

relative removal of fuel to reduce fire hazard. 

Resource managers need to use the best information available and expert opinion on local conditions, as well as clearly state the level of acceptable 

risk relative to treatments for any particular forest 

stand or landscape. The growing empirical database on how forest structure affects large wildfires 

will inform adaptive fuel management and provide 

new quantitative insights in the years ahead.

Adherence to four basic but challenging points will 

increase the effectiveness of adaptive fuel management. First, we need high-quality empirical data 

on fuel and geographic information system tracking of changes in fuel over time. Second, fire managers, silviculturists, and other resource specialists 

need to work together to develop prescriptions that 

effectively reduce fire hazard and achieve other 

resource objectives. Third, local management units 

need to monitor posttreatment fuel conditions and 

the effectiveness of fuel treatments when wildfire 

occurs. Finally, a rigorous schedule of periodic fuel 

treatments should be implemented to maintain the 

desired level of fire hazard and other conditions.

Acknowledgments

This report was subjected to peer review by 

three anonymous reviewers. We thank those 

reviewers, R. Gerald Wright for oversight of the 

review process, members of the national Fuels 

Planning project, and members of the University 

of Washington Fire and Mountain Ecology Laboratory for helpful comments on previous drafts. 

Kelly O’Brian and Lynn Starr provided valuable 

assistance with editing. Funding was provided by 

the U.S. Department of Agriculture, Forest Service, 

Fire and Aviation Management staff, through the 

national Fuels Planning: Science Synthesis and 

Integration Project.

Metric Equivalents

When you know: Multiply by: To find:

Feet (ft) 0.3048 Meters (m)

Acres (ac) .405 Hectares (ha)

Pounds per acre 1.12 Kilograms 

 (lb•ac-1) per hectare 

 (kg•ha-1)

Pounds per cubic 16.2 Kilograms

 foot (lb•ft-3) per cubic 

 meter (kg•m-3)

Table 3—Principles for fire-resilient forests

Principle Effect Advantage Concerns

Reduce surface fuel Reduces potential flame length Control easier, less torching Surface disturbance less with 

fire than other techniques

Increase canopy base 

height

Requires longer flame length 

to begin torching

Less torching Opens understory, may allow 

surface wind to increase

Decrease crown density Makes tree-to-tree crown fire 

less probable

Reduces crown-fire potential Surface wind may increase, 

surface fuel may be drier

Retain larger trees Thicker bark and taller crowns Increases survivability of 

trees

Removing smaller trees is 

economically less profitable

Source: Agee 2002b.
24

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