United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

United States

Department of the

Interior

Bureau of Land

Management

General Technical

Report

PNW-GTR-463

September 1999

The Effects of Thinning and

Similar Stand Treatments on

Fire Behavior in Western Forests

Russell T. Graham, Alan E. Harvey, Threasa B. Jain,

and Jonalea R. Tonn
Preface

The Interior Columbia Basin Ecosystem Management Project was initiated by the Forest Service

and the Bureau of Land Management to respond to several critical issues including, but not limited

to, forest and rangeland health, anadromous fish concerns, terrestrial species viability concerns, and

the recent decline in traditional commodity flows. The charter given to the project was to develop a

scientifically sound, ecosystem-based strategy for managing the lands of the interior Columbia River

basin administered by the Forest Service and the Bureau of Land Management. The Science Integration Team was organized to develop a framework for ecosystem management, an assessment of the

socioeconomic and biophysical systems in the basin, and an evaluation of alternative management

strategies. This paper is one in a series of papers developed as background material for the framework,

assessment, or evaluation of alternatives. It provides more detail than was possible to disclose directly

in the primary documents.

The Science Integration Team, although organized functionally, worked hard at integrating the approaches, analyses, and conclusions. It is the collective effort of team members that provides depth

and understanding to the work of the project. The Science Integration Team leadership included deputy

team leaders Russel Graham and Sylvia Arbelbide; landscape ecology—Wendel Hann, Paul Hessburg,

and Mark Jensen; aquatic—Jim Sedell, Kris Lee, Danny Lee, Jack Williams, Lynn Decker; economic—

Richard Haynes, Amy Horne, and Nick Reyna; social science—Jim Burchfield, Steve McCool, Jon

Bumstead, and Stewart Allen; terrestrial—Bruce Marcot, Kurt Nelson, John Lehmkuhl, Richard

Holthausen, and Randy Hickenbottom; spatial analysis—Becky Gravenmier, John Steffenson, and

Andy Wilson.

Thomas M. Quigley

Editor

United States

Department of

Agriculture

Forest Service

United States

Department of

the Interior

Bureau of Land

Management

Authors

RUSSELL T. GRAHAM is a research forester, ALAN E. HARVEY is a research plant pathologist,

THERESA B. JAIN is a forester, and JONALEA R. TONN is a forester, Rocky Mountain Research

Station, 1221 South Main, Moscow, ID 83843. This paper was prepared in response to issue raised as

part of the Interior Columbia Basin Ecosystem Management Project.
Abstract Graham, Russell T.; Harvey, Alan E.; Jain, Theresa B.; Tonn, Jonalea R. 1999.

The effects of thinning and similar stand treatments on fire behavior in Western

forests. Gen. Tech. Rep. PNW-GTR-463. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station. 27 p.

In the West, thinning and partial cuttings are being considered for treating millions 

of forested acres that are overstocked and prone to wildfire. The objectives of these

treatments include tree growth redistribution, tree species regulation, timber harvest,

wildlife habitat improvement, and wildfire-hazard reduction. Depending on the forest

type and its structure, thinning has both positive and negative impacts on crown fire

potential. Crown bulk density, surface fuel, and crown base height are primary stand

characteristics that determine crown fire potential. Thinning from below, free thinning,

and reserve tree shelterwoods have the greatest opportunity for reducing the risk of

crown fire behavior. Selection thinning and crown thinning that maintain multiple

crown layers, along with individual tree selection systems, will not reduce the risk of

crown fires except in the driest ponderosa pine (Pinus ponderosa Dougl. ex Laws.)

forests. Moreover, unless the surface fuels created by using these treatments are

themselves treated, intense surface wildfire may result, likely negating positive

effects of reducing crown fire potential. No single thinning approach can be applied to

reduce the risk of wildfires in the multiple forest types of the West. The best general

approach for managing wildfire damage seems to be managing tree density and

species composition with well-designed silvicultural systems at a landscape scale

that includes a mix of thinning, surface fuel treatments, and prescribed fire with

proactive treatment in areas with high risk to wildfire.

Keywords: Silviculture, forest management, prescribed fire, selection, forest fuels,

crown fire.
Contents 1 Introduction

1 Thinning Methods

3 Thinning

9 Regeneration Methods

12 Resulting Fire Behavior

15 Thinning and Fire Behavior

21 Thinning and Nutrition

22 Conclusion

23 Literature Cited
Thinning Methods

Introduction Catastrophic wildfire, fire hazard, fire risk, resource damage, and loss of human lives

and property are only some of the issues that address the use and occurrence of

fires in Western wildlands. Wildfires are common in both forests and rangelands of

the West. Over 95 percent of these fires are extinguished when they are small (less

than 2 acres). The 2 to 5 percent that are not suppressed burn 95 percent of the

area (Dodge 1972). Because of these issues, there is strong sentiment for treating

fuel through thinning and prescribed burning to restore wildlands to their former 

character (Babbitt 1997, Mutch 1994).

Successful fire exclusion over the past 60 to 70 years has contributed to greater

stand densities and an increase in crown fire potential in many forests of the West

(Mutch 1994). In addition, forests have changed from fire-adapted species to species

more susceptible to fire that tend to form unhealthy stands prone to large-scale wildfires, as well as increased outbreaks of insects and diseases (McCool and others

1997). Salvage logging and thinning have been suggested as appropriate preburn

treatments before prescribed fire can be safely reintroduced into these dense forests

(Mutch and others 1993). Private timber companies demonstrated that thinning and

removing diseased and dying trees can lower fire losses to a point where they can

reasonably self-insure their tree farms (Schott 1994). In contrast, DellaSala and 

others (1995) argue that intensive salvage, thinning, and many other logging activities do not reduce the risk of catastrophic fires. Bessie and Johnson (1995) indicate

that regional droughts and high winds play a greater role in fire behavior than forest

age and fuel loads in high-elevation subalpine fir (Abies lasiocarpa (Hook.) Nutt.)

forests. Turner and others (1994) raise doubts about the effectiveness of intensive

fuel reductions as “fire-proofing” measures. During the extreme fire season of 1967,

however, intensity of fires burning on the Flathead National Forest in western

Montana decreased from crown to surface fires when they encountered thinned

areas (Cron 1969). In addition to these well-documented and contrasting views on

the effect of thinnings on fire behavior, there are many other descriptions, interpretations, and controversies regarding how “thinnings” affect subsequent wildfire or prescribed fire behavior in the “soft” literature. To provide more precise predictive power,

the approach we use to address the thinning-fire issue is first to describe forest

treatments defined as thinnings, and those that could be interpreted as thinnings,

and then show how fires would behave in resulting stand structures, compositions,

and fuels created by well-defined treatments. Predictions are based on a variety of

literature available for western conifer forests. 

Depending on the forest type and biophysical setting, hundreds to tens of thousands

of seedlings per acre can naturally regenerate after a disturbance in the inland West

(Haig and others 1941, Pearson 1950). Even with such high stand densities, at 100

to 150 years old, only 100 to 200 stems per acre remain (Haig 1932, Meyer 1938).

This reduction is caused by intertree competition, wind, snow, ice, diseases, insects,

fire, or a combination of these important mortality factors (Haig and others 1941,

Oliver and Larson 1990). These stocking reductions allowed the site’s growth potential to be concentrated on fewer stems producing fewer but larger trees. The efficiency

at which mortality factors reduce the number of stems on a site depends on the 

disturbance, forest type, and biophysical setting. Individual lodgepole pine (Pinus

contorta Dougl. ex Loud.), ponderosa pine (Pinus ponderosa Dougl. ex Laws.), and

interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), trees in many areas do

1
not readily succumb to intertree competition, often causing stagnated stands with

thousands of stems per acre. Likewise, in the mixed-conifer forests of the Cascade

Range and northern Rocky Mountains, dense stands of shade-tolerant western 

hemlock (Tsuga heterophylla (Raf.) Sarg.), grand fir (Abies grandis (Dougl. ex D.

Don) Lindl.) and western redcedar (Thuja plicata Donn ex D. Don) are common.

Throughout much of the intermountain West, fire was a major mortality factor that

thinned stands and selected for fire-resistant species, but fire suppression has aided

in the development of large expanses of such dense stands (Hann and others 1997).

Most of the forests dominated by ponderosa pine historically had a large component

of large ponderosa pine (Covington and Moore 1994, Hann and others 1997). Because of fire-suppression efforts, the once frequent (20 years or less) low-intensity

surface fires no longer clean the forest floor of fine fuels (3 inches in diameter or

less) and kill patches or individual seedlings and saplings. Resulting forest structures

and compositions are now often dominated by many suppressed and intermediate

grand firs, white firs (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.) and Douglasfirs (Arno 1980, McCool and others 1997). In addition to fire suppression, many of

these forests were subjected to the removal of the dominant ponderosa pine through

commercial timber harvest (McCool and others 1997).

In addition to natural events that reduce density of forest stands, forest management

through application of thinnings also can alter species composition and stand structure. Depending on the objectives, thinnings can be applied to forest stands for various reasons. Classically, thinning is defined as “cuttings made in immature stands in

order to stimulate the growth of trees that remain and to increase the total yield of

useful material from a stand” (Smith 1962). But, often any kind of partial cutting such

as cleaning, weeding, liberation, preparatory, improvement, sanitation, and selection

cuttings is termed thinning, especially outside the field of silviculture, and all reduce

the number of stems in a forest stand. They could be applied to increase forage for

both wildlife and livestock, change tree species composition to create more diseaseand insect-resistant stands, harvest timber products, or alter wildfire behavior.

Thinning treatments have the potential to alter fire behavior but, depending on how

these intermediate removals are applied, will not necessarily result in compositional

or structural changes similar to those produced by nonlethal and mixed-fire disturbances of the native system (Hann and others 1997).

Ground, surface, and crown are the three types of fires most often recognized

(Brown and Davis 1973). Surface and crown fires both historically and currently

occur in the intermountain West. The intensity (the rate at which fuel is consumed

and heat generated) and severity (the damage to both abiotic and biotic forest components) of surface and crown fires depends on species composition, available fuel,

fuel arrangement, fuel moisture content, weather, and the physical setting. Depending

on how these variables are combined, fires can range from the low-intensity and lowseverity fires that historically occurred in ponderosa pine forests to intense, severe,

stand-replacing fires more typical of lodgepole pine or moist, long fire cycle forests.

Although stand treatments cannot alter all variables that influence fire behavior, they

2
Thinning

can, directly or indirectly influence species composition, available fuel, fuel arrangement, fuel moisture, and surface winds. Thus, depending on the nature of the thinning, all these factors can be used to change posttreatment wildfire or prescribed fire

behavior. To change landscape-scale wildfire behavior and effects, treatments must

alter the typically large connected matrix of susceptible patches (stands) that occur in

high-risk watersheds (Hann and others 1997, Hessburg and others 1994, Huff and

others 1995).

The classic objective of thinning is to redistribute growth potential to fewer trees past

the sapling stage, leaving a stand with a desired structure and composition. In general, five methods of thinning are recognized:

1. Low, or thinning from below

2. Crown, or thinning from above

3. Selection, or diameter-limit thinning

4. Free thinning

5. Mechanical thinning (Nyland 1996, Smith and others 1997).

Most often, forest stands do not develop with one canopy. Because of individual tree

species, microsite differences, and local disturbances, multiple crown classes usually

develop. Four are specifically recognized and used to describe different stand structures (Smith 1962).

Dominant: Trees with crowns extending above the general crown layers receiving full

light from above and partly from the sides.

Codominant: Trees with crowns forming the general level of cover and receiving full

light from above but comparatively little from the sides.

Intermediate: Trees shorter than the preceding with crowns extending into the crowns

formed by dominant and codominants, receiving little direct light from above and

none from the sides.

Suppressed: Trees with crowns entirely below the general level of cover, receiving no

direct light from above or the sides—overtopped.

These crown classes are used to describe the trees removed in different types of

thinnings.

Low thinning (thinning from below) is when trees are removed from the lower canopy,

leaving large trees to occupy the site (table 1). This method mimics mortality caused

by intertree competition or surface fires and concentrates site growth potential on

dominant trees. Low thinnings primarily remove intermediate and suppressed trees,

but heavy thinnings also can remove many in the codominant crown class. (fig. 1).

Low thinnings not only remove understory canopies but also can alter species compositions. Usually, different tree species have characteristic development rates that

result in individual species dominating specific canopy layers. For example, in many

3
Table 1—Trees removed during different intensities of low thinning

Intensity Trees removed

Very light Poorest overtopped

Light Overtopped and poorest intermediate

Moderate Overtopped and intermediate

Heavy Overtopped, intermediate, and many codominant

Source: Smith 1962.

4

Figure 1—A 120-year-old conifer stand containing a mixture of dominant (D), codominant (C), intermediate

(I), and suppressed (S) trees thinned from below (low thinning) to three different intensities.
areas of the West, ponderosa pine primarily occupies the dominant canopy layers,

whereas shade-tolerant grand fir, white fir, or Douglas-fir occupy the intermediate and

suppressed layers. A low thinning in these stands therefore favors the development

of the dominant and codominant ponderosa pine (fig. 1). Depending on the desired

stand structure, low thinnings can remove few to many trees. Also, thinnings need

not create regular spacings but rather can vary both the number and clumping of

residual trees. Low thinnings (thinning from below), therefore, create various stand

structures and compositions, depending on the forest type and biophysical setting.

Crown thinning, or thinning from above, reduces crowding within the main canopy.

Dominant and codominant trees are removed to favor residual trees in these same

classes. This method is often used to remove selected species in the dominant and

codominant crown classes that are competing with more desirable species (Nyland

1996). This method keeps vertical structure in place, which is often desirable for

wildlife species. Also, intermediate and suppressed shade-tolerant species, such

as western redcedar and grand fir, often respond to release if they have adequate

crowns (Ferguson and Adams 1980, Graham 1982). As with low thinning, crown thinning can create various stand structures and compositions while retaining vertical

structure (fig. 2).

Selection thinning removes dominant trees to favor smaller trees. This method is

often applied by removing trees over a certain diameter. Diameter-limit cuts that continually remove the largest trees may well be dysgenetic and can be a disguise for

high grading (removing trees of high economic value). By removing the current value

from a stand, future options often can be limited, and the only recourse for the future

may be to regenerate. Stand structures and species compositions created by using

selection thinning are limited and, in general, favor shade-tolerant species or trees

occupying the intermediate and suppressed crown classes. Often the stands created

by selection thinnings are prone to epidemics of insects and diseases. Compared to

the other thinning methods, selection thinning is less useful because of the limited

stand structures and compositions it can create (fig. 2).

Free thinning, sometimes called crop-tree thinning, primarily releases selected trees.

This method favors specific trees, whereas the remainder of the stand goes untreated. Depending on what is presented in various portions of a stand (tree spacing,

species, vertical structure, etc.), the thinning criteria can be highly flexible, producing

stands with large amounts of diversity. It can be used in any of the crown classes for

releasing specific trees. This method has the most flexibility for creating various stand

structures and compositions (fig. 2).

Mechanical thinning removes trees based on specified spatial arrangements (Nyland

1996). This method is often applied in plantations where every other row or every

other tree in a row is removed. Such rigid thinning is easy to apply, but the stands

created often lack diversity in either structure or composition. This method also

resembles strip thinning, where a strip of trees is removed. Mechanical thinning is

well suited for timber production on uniform sites but has limited value for producing

conditions that meet other resource values.

5
6

Figure 2—A 120-year-old conifer stand containing a mixture of dominant (D), codominant (C), intermediate

(I), and suppressed (S) trees receiving a crown, selection, and free thinning.

Other intermediate treatments often termed “thinning” are types of release cuttings

usually applied to sapling-sized trees (fig. 3). These precommercial thinnings usually

produce no products with the exception of fencing material or other specialty products. Cleaning usually refers to the removal of one species to favor another. This is

often the case where a hardwood (such as quaking aspen, Populus tremuloides

Michx. or alder, Alnus spp.) is removed to release a conifer, like western white pine

(Pinus monticola Dougl. ex D. Don) or western larch (Larix occidentalis Nutt.).

Weeding can mean releasing conifer seedlings from competing vegetation, or it might
also denote the removal of vegetation competing with favored trees. Weedings and

cleanings mold future stand structure, determining future species composition and

individual tree growth.

Liberation cuttings release sapling-sized trees from older, overstory trees (fig. 4). 

This might occur when planted regeneration or advanced regeneration developing

after a wind or ice storm requires protection. The large overstory trees can protect

young seedlings from damaging agents early, and then be removed, when saplings

no longer need protection. Liberation cuts have limited use for molding different stand

structures and compositions. Such cuttings might become more common if reserve

seed-tree and shelterwood systems are used to maintain cover while regenerating

new stands.

Improvement and salvage cuttings are designed to remove specific, undesirable

trees from a stand. Such “sanitation” might remove damaged trees, snags, or trees

susceptible to a certain disease or insect. Often this method is used to remove trees

damaged by wind or snow, especially if they might encourage the buildup of pests,

like Ips spp. Similar to sanitation cuttings, salvage cuttings remove dead or dying

7

Figure 3—A sapling sized stand of conifers and hardwoods cleaned to favor the conifers.
8

Figure 4—A young stand of conifers overtopped by large undesirable trees released by a liberation cut.

Figure 5—A mature, mixed-species stand containing various crown classes. In addition, it has several

dead and damaged trees that were removed by a sanitation-salvage cutting.
Regeneration

Methods

trees killed by fire, insects, or disease (fig. 5). Salvage cuttings usually address financial rather than ecological needs (Nyland 1996) even though they are often promoted

for restoring drought- and disease-prone forests to more typical mixes of fire-tolerant

species (McCool and others 1997). In general, these methods have little impact on

overall structure and composition in the short term, but if repeated, they tend to

remove value from the stand.

Depending on growth, thinnings to control density can occur several times during the

life of a stand. The timing and intensity of each can provide for many different stand

structures depending on the management objectives. For example, stocking charts

(charts defining tree sizes in a stand at various ages and densities) can be used to

determine timing and intensity of intermediate treatments for producing timber (Smith

and others 1997). Also, thinning regimes can be designed for producing forest structures desired for wildlife (Reynolds and others 1992). Depending on the forest type,

its growth rate and desired stand structure, six or more thinnings might be applied

within a 100-year period. 

Thinning or other intermediate cuttings are fundamentally methods for controlling

stand composition and structure to produce desired forest conditions. Intermediate

treatments include all of the above “thinnings.” They are intermediate because they

occur between the time a stand is regenerated and “final” harvest. Now, more than

ever, there is often little distinction between the effects of intermediate treatments

and regeneration methods. Under the classic definition of seed-tree and shelterwood

regeneration methods, overstory trees are removed once regeneration is secured.

But now, because of watershed, wildlife, or scenic values, reserve tree shelterwood

and reserve tree seed-tree methods often are used (Reynolds and others 1992).

With reserve tree systems, an overstory component is maintained throughout the life

of the regenerated stand to provide high forest structure, future snags, and future

coarse woody debris. Such reserve systems are often termed irregular shelterwoods,

delayed shelterwoods, or extended shelterwoods. Depending on the size, number,

spacing, and species of reserve trees, the stands created can easily resemble those

maintained by thinnings. This is especially true if preparatory cuts are used in a shelterwood system or the cutting units are small. A preparatory cut removes part of the

stand to increase tree vigor, wind firmness, and seed potential. Depending on the

intensity of these cuts, they also can resemble thinnings. Also, the reserve trees left

in shelterwoods can be grouped and selected by species or size, creating various

stand structures (fig. 6).

Stands managed with an individual tree selection system also can resemble thinned

stands. The selection system creates and maintains stands with three or more age

classes that require multiple entries ranging from 10 to 40 years (Graham 1989,

Nyland 1996). In addition to removing trees in the dominant and codominant crown

classes, selection systems remove trees in the suppressed and intermediate classes

also, so that an uneven-aged, multilayered stand is maintained. Stands managed by

using the individual tree system could easily resemble stands thinned by using

crown, free, or selection thinnings (fig. 7). As with thinnings, a wide variety of stand

9
10

Figure 6—A mature, mixed-species stand regenerated with a shelterwood. A preparatory cut to improve

seed production and wind firmness was used before the seed cutting. Also, a group shelterwood is 

demonstrated.
structures and compositions can be created. In most cases, the selection system

favors development of stands containing shade-tolerant species with high vertical

structures. Selective cutting, creaming, culling, high grading, diameter cutting, and

maturity selection removals often are termed selection but are actually economic harvests with little or no silvicultural or biological basis (Nyland 1996).

There are many different kinds of thinnings, thinning regimes, reserve tree regeneration methods, and combinations that create a plethora of stand structures and compositions to meet various objectives. Because there is no single method or type of

thinning, there is no single structure or composition created by thinnings. Thinning

defines a set of intermediate treatments applied to forest stands to create varying

11

Figure 7—A mixed-species stand managed by using the individual tree selection system on a cutting cycle 

of 30 years.
12

Resulting Fire

Behavior

compositions and structures, but there are other types of partial cuttings that remove

trees. Depending on how a regeneration method (shelterwood, etc.) is applied, it

too can create stands with various compositions and structures. If nontraditional

regeneration methods, typified by reserve tree shelterwoods, are combined with free

thinning or even thinning from below, stands can be created and maintained that

meet various management objectives from wildlife habitat improvement to watershed

maintenance. Because there are many different stand compositions and structures

possible from thinnings and regeneration methods, there are at least as many ways

these stands will respond to wildfire or prescribed fire. As mentioned earlier, thinnings

can directly or indirectly alter the amount, kind, and moisture of fuel, all key ingredients of future fire behavior.

Fuel models—Fire behavior depends on forest density, composition, amount of 

surface fuel, its arrangement, moisture content, prevailing weather, and physical setting. To characterize surface fire behavior, 13 fire behavior fuel models are available

that describe the fuel complex, fuel loading, fuel bed depth, and moisture of extinction (upper limits of fuel moisture beyond which a fire will no longer spread with a 

uniform front) in dead and live fuels for grass, shrub, timber, and logging slash groups

(Albini 1976) (table 2). These models in combination with dead and live fuel moisture

content, slope angle, and wind speed provide a basis for predicting both fire spread

rate (chains per hour) and intensity (flame length) (Anderson 1982, Rothermel 1983). 

Wind—The standard height for wind measurements used by land management

agencies in the United States is 20 feet above the vegetation. All fires in surface

fuels burn below the 20-foot height, and because wind is slowed by friction near the

surface and overstory vegetation, the 20-foot wind speed must be adjusted to correctly predict fire behavior near the surface (Rothermel 1983). Depending on the 

vegetation cover and exposure, 20-foot wind speed reduction factors range from 0.1

to 0.6 to arrive at midflame wind speeds (horizontal wind speed at midflame height)

(Albini 1976, Rothermel 1983). For example, the 20-foot wind speed must exceed 50

miles per hour for midflame wind speeds to reach 5 miles per hour within a dense

stand (0.1 adjustment factor). In contrast, in an open stand (0.3 adjustment factor),

the same midflame wind speeds would occur at only a 16-mile-per-hour wind at 

20 feet.

Crown fire—Surface fire intensity (flame length), crown base height, and moisture

content of the live foliage determines crown ignition (Van Wagner 1977). For example, crowns with 75 percent moisture (which might occur in the late fall) and a base

height of 10 feet would ignite if flames from a surface fire exceeded 5 feet (Alexander

1988) (fig. 8). Fires this intense (5 feet flame length) would be possible in stands represented by fuel model 10, 12, or 13 when driven by 5 mile per hour midflame winds

(table 2). Even though a surface fire might ignite tree crowns, however, the resulting

crown fire is not necessarily sustained.

Crown fire spread—Whether crown ignition is sustained or not is determined by rate

of spread and crown bulk density (foliage weight in pounds per square foot divided

by the average live crown length) (Alexander 1988, Van Wagner 1977). Wind and

slope determine potential crown fire spread rate (Rothermel 1991), and species composition and structure control crown bulk density. In general, as crown bulk density
13

Table 2—Description of the timber and slash fuel models used in predicting fire behavior

Dead fuel loading

Fuel Live Fuel bed Rate of Flame Moisture ofmodel Typical fuel complex <0.25 in 0.25-1 in 1-3 in fuel depth spreada length extinctionb

Tons/acre Feet Chains/hr Feet Percent

Timber grass:

2 Timber (grass and understory) 2.00 1.00 0.50 0.50 2.5 35.0 6.0 15

Timber litter:

8 Closed timber litter 1.50 1.00 5.51 .2 1.6 1.0 309 Hardwood litter 2.92 .41 .15 .2 7.5 2.6 25

10 Timber (litter and understory) 3.01 2.00 5.01 2.00 1.0 7.9 4.8 25

Slash:

11 Light logging slash 1.50 4.51 5.51 1.0 6.0 3.5 1512 Medium logging slash 4.01 14.03 16.53 2.3 13.0 8.0 2013 Heavy logging slash 7.01 23.04 28.05 3.0 13.5 10.5 25

a Rate of spread for 5-mile-per-hour midflame wind speeds and fine fuel moisture content of 8 percent. b Moisture of extinction in dead fuels: upper limits of fuel moisture beyond which a fire will no longer spread with a uniform front.

Source: Albini 1976 and Anderson 1982.
14

Figure 8—The flame lengths (fire intensities) required to ignite conifer crowns having different base

heights for live foliage moisture contents of 75, 100, and 125 percent (Alexander 1988, Van Wagner 1977).

Figure 9—The rate of spread required for crown fires to be sustained as determined by stand crown bulk

density (Alexander 1988, Van Wagner 1977).
Thinning and 

Fire Behavior

increases, lower rates of spread are required to sustain the fire (fig. 9). For example,

during a normal summer, a crown fire burning on a 50-percent slope driven by a 20-

mile-per-hour wind would spread at about 62 feet per minute (fig. 10). A crown fire

would be sustained at those spread rates, except in stands with crown bulk densities

less than 0.01 pound per cubic foot (fig. 9). Thus, the primary stand attributes that

control a fire’s behavior are surface fuel condition, crown bulk density, and crown

base height. All three attributes can be directly managed by thinning or other similar

forest treatments.

As noted, there are many stand treatments similar to thinnings that may or may not

be thinnings. All alter the stand characteristics that directly influence fire behavior.

The crowns of trees removed during treatments may significantly contribute to surface fuels (slash). These fuels have a major impact on expected fire intensities

depending on whether and how they are treated. The moisture content of surface

fuels differs as a function of high forest cover similar to midflame wind speeds.

Crown bulk density is the primary controlling factor of crown fire behavior, and it

depends on both species composition and stand density. Crown base height also

depends on species and growth history (including density and many other stand

characteristics that affect crown dimension). Depending on the type, intensity, and

extent of thinning, or other treatment applied, fire behavior can be improved (less

severe and intense) or exacerbated.

15

Figure 10—Crown fire spread rates for different slopes and wind speeds. These spread rates are for fires

burning in fuel model 10 during a normal summer (Rothermel 1991).
16

To show how different stand characteristics affect fire behavior, we assigned specific

characteristics to the different stand structures displayed in figures 1-7. In much of

the interior Northwest, forests contain mixes of ponderosa pine, Douglas-fir, and

grand fir. Often these stands contain dominant and codominant ponderosa pine,

grand fir, and Douglas-fir that were historically cleaned by fire. Such stands now, in

addition to the dominants and codominants, contain large amounts of ladder fuel

(fuel that can carry surface fires into overstory crowns) in the form of intermediate

and suppressed fir. These types of stands are displayed in figure 1 and described in

table 3. One such hypothetical stand contains 158 trees per acre, has a crown base

height of 4 feet, crown bulk density of 0.013 pound per cubic foot, and 12 tons of

fuels per acre, 1 foot in depth (fuel model 10, table 2). Crown bulk densities were

determined from foliage weights estimated by using Brown’s (1978) equations. 

In such a stand, if a fire in the surface fuels is driven by 5-mile-per-hour midflame

winds, it will produce 5-foot flames (Anderson 1982) (fuel model 10, table 2).

Because the crown base height of the stand is only 4 feet, crown ignition will occur

even with crown moisture contents as high as 125 percent (fig. 8). To produce 5-mileper-hour midflame wind speeds, the 20-foot wind speeds in a stand with these densities must exceed 50 miles per hour (Rothermel 1983). Winds of that speed would

produce crown fire spread rates over 185 feet per minute, even on flat ground (fig.

10) (Rothermel 1991). A crown fire spread rate of this magnitude would ensure a

crown fire is sustained. A crown fire would burn this stand (crown bulk density of

0.013 pound per cubic foot) if spread rates exceeded 46 feet per minute (fig. 9). 

Thinning from below—If that stand (fig. 1 and table 3) was moderately thinned 

from below and all intermediate and suppressed grand fir ladder fuels (small trees in

the understory) removed, the crown base height would increase to 40 feet, and the

crown bulk density would decrease to 0.006 pound per cubic foot. Further, if the 

tops and limbs of the cut grand fir were removed and the surface fuel loadings not

increased, surface fires would produce 10-foot flames and not ignite the crowns (fuel

model 10, table 2). The increased flame lengths in the thinned stand, compared to

the unthinned stand, are the result of higher wind speeds that occur in thinned

stands. For example, in a closed stand, a 50-mile-per-hour 20-foot wind would create

5-mile-per-hour midflame wind speeds; whereas in a thinned stand, the same midflame wind speeds would be created by just 16- to 25-mile-per-hour 20-foot winds

(Rothermel 1983). In contrast, if the cut grand fir were not removed or treated, subsequent fire intensities could ignite the crowns (fig. 8). The increased fire intensities

(14-foot flames) would be facilitated by the increased fuel loads (fuel model 12 compared to fuel model 10) (table 2). Also, dead fuel in the thinned stand would be drier

than similar fuels in the unthinned stand (Rothermel 1983). 

In the moderately thinned stand (fig. 1, table 3), a crown fire ignited from outside

sources (such as a crown fire in an adjoining stand), would cause a sustained crown

fire only if spread rates exceeded 100 feet per minute (fig. 9). Stands exhibiting these

crown bulk densities (0.006 pound per cubic foot) would be relatively protected from

crown fires. In our example, the moderate and free thinning would produce these

conditions as would the shelterwood cutting (table 3). Such densities would be representative of some common, historical stand structures (fig. 6 and table 3). 
17

Table 3—Characteristics of a hypothetical mixed ponderosa pine stand about

120 years old growing on a mixed-conifer site with different intermediate treatments and a reserve shelterwooda

Uncut stand Light thinning

Attribute D.-fir G.fir P.pine D.-fir G.fir P.pine

Trees per acre 8 112 38 8 20 38

Crown length (ft) 32 10 30 32 18 30

Mean height (ft) 108 23 81 108 43 81

Crown base height (ft)b 77 4 40 77 25 40

Total trees (per acre)c 158 66

Crown bulk density (lb/ft)c 0.013 0.007

Moderate thinning Heavy thinning

Trees per acre 8 0 38 6 0 30

Crown length (ft) 32 -- 30 33 -- 32

Mean height (ft) 108 -- 81 103 -- 82

Crown base height (ft)b 77 -- 40 71 -- 50

Total trees (per acre)c 46 36

Crown bulk density (lb/ft)c 0.006 0.005

Crown thinning Free thinning

Trees per acre 4 108 30 6 23 30

Crown length (ft) 29 9 32 31 19 32

Mean height (ft) 104 23 82 111 52 83

Crown base height (ft)b 75 4 51 80 33 51

Total trees (per acre)c 142 59

Crown bulk density (lb/ft)c 0.010 0.006

Selection thinning Shelterwood

Trees per acre 3 112 19 0 0 15

Crown length (ft) 29 12 28 -- -- 35

Mean height (ft) 102 26 84 -- -- 84

Crown base height (ft)b 73 4 49 -- -- 49

Total trees (per acre)c 134 15

Crown bulk density (lb/ft)c 0.007 0.002

a These characteristics describe the stands displayed in figures 1, 2, and 6. b The lowest crown base height that occurs in the stand is the value assigned to the entire stand. c Total trees per acre and crown bulk density are values for the entire stand.
18

Thinnings in general will lower crown bulk densities and redistribute fuel loads significantly, thus decreasing fire intensities if the surface fuels are treated (Agee 1993,

Alexander 1988, Alexander and Yancik 1977). These removals have been shown to

be effective in reducing crown fire potential, especially around homes (Coulter 1980,

Dennis 1983, Rothermel 1991, Schmidt and Wakimoto 1988). Because of drier fuels

(fuels are more exposed to wind and heat) and increased wind speeds that occur in

thinned stands, it is critical that they be treated to minimize fire intensity. In California,

plantations where surface fuels were treated had substantially less damage from

wildfires compared to untreated plantations that burned completely and severely

(Weatherspoon and Skinner 1995).

Species composition—By using the same characteristics to describe a mixed 

stand of grand fir, Douglas-fir, and western white pine, the crown bulk density is

0.006 pound per cubic foot compared to 0.013 pound per cubic foot for the stand

dominated by ponderosa pine (table 4). To sustain a crown fire in this uncut western

white pine stand, a spread rate of 100 feet per minute would be required to sustain a

crown fire compared to only a 46-feet-per-minute rate of spread required for sustaining a crown fire in the ponderosa pine stand (fig. 9). If the stand was dominated by

western larch instead of western white pine, however, it would have even lower

crown bulk densities (Brown 1978, Rothermel 1983). Thus, crown fires would be

more difficult to sustain in western white pine- and western larch-dominated stands

than in stands dominated by other species. 

A western white pine stand heavily thinned from below by removing the grand fir, 

and some codominant trees to a density of 36 trees per acre would result in a stand

crown bulk density of 0.002 pound per cubic foot (table 4). A crown fire spread rate 

of 300 feet per minute would be required for a fire to be sustained, effectively removing the crown fire threat from these stands (fig. 9). These low crown bulk densities

are the result of western white pine crowns that tend to be narrower and shorter than

crowns of most of its associates, with the exception of western larch. 

All methods of thinning could reduce the number of trees to a point where crown fires

would be difficult to initiate or sustain, but these conditions may not meet many present multiresource values. Because crown thinning and selection thinnings leave

suppressed and intermediate trees, crown base heights remain low and crown bulk

densities could remain high thus not decreasing the potential for crown fire (tables 3

and 4, fig. 2). In contrast, free thinning, could be effective at decreasing crown fire

risk depending on thinning intensity. But the stand treated by using free thinning in

our example would still be prone to crown fire initiation and spread (tables 3 and 4,

fig. 2).

Other immediate treatments—Sanitation and salvage harvests would do little to

minimize crown fire initiation or spread because crown bulk densities would likely

always exceed 0.006 pound per cubic foot (fig. 5). Likewise, ladder fuels and low

crown base heights would exist in most stands receiving salvage cuttings. Thus salvage and sanitation harvests would probably not significantly change the potential

fire characteristics even if the surface fuels were treated. 
19

Table 4—The characteristics of a hypothetical mixed western white pine stand

about 120 years old growing on a mixed-conifer site with different intermediate

treatments and a reserve shelterwooda

Uncut stand Light thinning

Attribute D.-fir G.fir P.pine D.-fir G.fir P.pine

Trees per acre 8 112 38 8 20 38

Crown length (ft) 32 10 33 32 18 33

Mean height (ft) 108 23 130 108 43 130

Crown base height (ft)b 77 4 97 77 25 97

Total trees (per acre)c 158 66

Crown bulk density (lb/ft)c 0.006 0.003

Moderate thinning Heavy thinning

Trees per acre 8 0 38 6 0 30

Crown length (ft) 32 -- 33 32 -- 36

Mean height (ft) 108 -- 130 103 -- 136

Crown base height (ft)b 77 -- 97 71 -- 100

Total trees (per acre)c 46 36

Crown bulk density (lb/ft)c 0.003 0.002

Crown thinning Free thinning

Trees per acre 4 108 30 6 23 30

Crown length (ft) 29 9 31 31 19 36

Mean height (ft) 108 23 125 111 52 136

Crown base height (ft)b 75 4 94 80 33 100

Total trees (per acre)c 142 59

Crown bulk density (lb/ft)c 0.004 0.002

Selection thinning Shelterwood

Trees per acre 3 112 19 0 0 15

Crown length (ft) 29 12 27 -- -- 39

Mean height (ft) 102 26 115 -- -- 147

Total trees (per acre)c 134 15

Crown bulk density (lb/ft)c 0.003 0.001

a These characteristics describe the stands displayed in figures 1, 2, and 6. b The lowest crown base height that occurs in the stand is the value assigned to the entire stand. c Total trees per acre and crown bulk density are values for the entire stand.
Cleanings and weedings (precommercial thinnings) in sapling-sized stands can influence fire behavior by favoring species with light crowns (western larch and western

white pine). In addition, cleaning plantations by removing brush has successfully

reduced damage from wildfires in California (Van Wagner 1968). These treatments

can space trees, allowing stands with low crown bulk densities to develop. 

Precommercial thinnings to reduce either competition or favor ponderosa pine over

Douglas-fir or grand fir in dry ecosystems generally seems likely to improve the

health of these forests; the situation in moist forests may be more demanding. Moist

forests evolved as dense stands largely dominated by seral species, especially western white pine and western larch. Native pest actions and periodic fire “thinned” late

seral species (grand fir, white fir, western hemlock, and western redcedar) continuously over time. In the absence of a significant component of western white pine and

western larch, simple thinning of a late seral stand to reduce competition or crown

bulk densities may not decrease activities of insects and pathogens or select appropriate genotypes, either of which could offset any reduction in crown fire potential

within a relatively short time. Thus, thinning in moist forests should be approached

carefully. Any approach to reduce crown fire potential and improve health should be

tied to the active restoration of early seral species, especially the western white pine

and western larch. Precommercial thinnings not only mold a stand’s future composition and structure but usually produce large quantities of fine fuels. Fuel models 12

and 13, with over 30 tons per acre of fuel, often are used to describe the slash created by precommercial thinnings (table 2) (Anderson 1982).

Fire intensity in thinned stands is greatly reduced if thinning is accompanied by reducing the surface fuels created by the cuttings. Fire has been successfully used to

treat fuels and decrease the effects of wildfires especially in climax ponderosa pine

forests (Deeming 1990; Wagel and Eakle 1979; Weaver 1955, 1957). In contrast,

extensive amounts of untreated logging slash contributed to the devastating fires 

during the late 1800s and early 1900s in the inland and Pacific Northwest forests.

These catastrophic fires led to both laws and policies governing the treating of slash

after timber harvesting (Brown and Davis 1973, Deeming 1990). These initiatives led

to several methods, in addition to fire, for treating fuels including cutting, scattering,

piling, clearing, crushing, and disking (Brown and Davis 1973).

Silvicultural systems—A series of forest treatments or a silvicultural system that

maintains multiple forest canopies and high crown bulk densities is unlikely to

decrease the potential for crown fire behavior. Individual tree selection systems that

remove and tend trees on cutting cycles of 10 to 40 years will likely maintain stands

prone to crown fire behavior (fig. 7). (Pure, climax stands of ponderosa pine would

be the exception.) In mixed-conifer stands, crown bulk densities would remain high,

crown base heights would be low, and fine fuels would be continually generated. In

addition, these silvicultural systems favor the development of stands dominated by

grand fir, western hemlock, or other shade-tolerant species (Graham 1989, Nyland

1996). These species all tend to have long and heavy crowns creating stands with

high bulk densities (Brown 1978, Rothermel 1983).

Seed-tree and shelterwood regeneration methods and all of their variations have the

potential to reduce the severity and intensity of wildfires. Open stands with low crown

bulk densities would not likely support a crown fire when the regeneration was short

20
Thinning and

Nutrition

(fig. 6, tables 3 and 4). But if the regeneration was not precommercially thinned, the

subsequent development of ladder fuels would increase potential for crown fire

behavior. The stand structures created by group shelterwoods would likely be prone

to crown fire behavior depending on the density and spacing of the groups of leave

trees (fig. 6).

Opinions differ as to the relative threat of changes in inland forests this century, but

there is general agreement that changes have occurred (Baker 1988, Brown 1983).

In general, there has been a shift from open ponderosa pine to closed pine and

Douglas-fir stands in dry ecosystems, and in moist forests, the change has been

from tall western white pine-western larch stands to relatively short, closed grand firwestern hemlock-western redcedar stands (Harvey and others 1994, Mutch and others 1993). These shifts toward late-successional species occurred relatively rapidly,

compressing successional processes. In dry ecosystems with historically frequent fire

regimes, ponderosa pine may not be succeeded by Douglas-fir in 300 to 400 years,

but it has occurred in only 40 years in some locations in southern Idaho (Harvey and

others 1994). Similarly, in western white pine-dominated ecosystems with mixed and

variable fire regimes, early seral species may (or may not, depending on fire frequency) be succeeded by late seral species in 200 to 300 years. In the Coeur d’Alene

Mountains in northern Idaho, however this conversion took less than 50 years

(Moeur 1992).

These changes have altered the nutrient dynamics that historically occurred in the

forests of the inland West. Ponderosa pine, western white pine, and western larch 

all tend to be tall and self-prune, even in moderately dense stands. In addition, the

crowns of western white pine and western larch have low bulk densities and nutrient

content compared to most late-successional species (Minore 1979). These species in

general carry their canopies well above the surface fuels, do not carry crown fires

well, and tend not to have ladder fuels. Thus canopy nutrient stores are relatively well

protected from fire. In contrast, stands dominated by the true firs, Douglas-fir, western hemlock, and western redcedar do not self-prune well, carry large branches low

in the canopy, and have relatively high crown bulk densities with resulting high nutrient concentrations (Brown 1978, Minore 1979). Stands dominated by these species

usually support crown fires well and have generally high nutrient content in tissues

susceptible to loss from fire.

Pines and western larch tend to be deep rooted, whereas the true firs, western 

hemlock, and western redcedar all tend to be relatively shallow rooted with large

numbers of fine roots in the high nutrient content, shallow soil organic layers (Harvey

and others 1986, Minore 1979). Douglas-fir also shows this trend when compared to

ponderosa pine. Thus the nutrients and nutrient turnover tend to be dispersed vertically in soils dominated by early seral species and concentrated near the surface

when the stand is dominated by late seral species. Soil surface nutrient stores and

feeder roots in late seral species-dominated stands (or early seral stands if surface

organics accumulate from fire exclusion) can be at more risk to wildfire, or other disturbance, than the historic pine and western larch-dominated stands. Therefore, the

combination of dense, heavy, nutrient-rich crowns and roots and nutrients concentrated in the surface layers make stands dominated by late seral species at much higher

21
Conclusion

risk to long-term nutrient losses from wildfire and other disturbances than stands

dominated by early seral species. Tree removal from thinnings can also significantly

alter nutrient storage and turnover in the modified stands. For example, any treatment that removes large quantities of branches and needles has the potential to

reduce onsite nutrient reserves. As much as 45 percent of site potassium and 25 

percent of site nitrogen stores can be contained in branchwood, needles, and other

similar materials (Jurgensen and others 1997, Moore 1994). If the site is chronically

short on these nutrients, as many are, intensive removal of these materials is likely to

cause subsequent nutrient shortage. On the other hand, with potentially volatile nutrients like nitrogen and sulfur, high-intensity fire can bring about even greater losses,

especially with nitrogen (Jurgensen and others 1997). Similarly, on low-fertility situations (especially dry sites) where soil organic horizons and coarse woody debris supplies may be low, both intense fire or overzealous removal of fuel to prevent fire can

imperil future site productivity. In most cases where sites are fragile from a nutrient

standpoint, leaving “fines” on the site for even one winter can redistribute nutrients,

including nitrogen, potassium and sulfur, downward into the soil (Harvey and others

1987, Larsen and others 1981, Moore 1994). This downward movement can provide

significant protection from volatilization by fire or physical removal as a fire-modification treatment (Mroz and others 1978). Thus, balancing relative risks with nutrient and

organic matter site legacies is an important aspect of any fuel-management operation.

Thinning and other thinning-like stand treatments can substantially influence subsequent fire behavior at the stand level by either increasing or decreasing fire intensity

and associated severity of effects. Depending on intensity, thinning from below and

possibly free thinning can most effectively alter fire behavior by reducing crown bulk

density, increasing crown base height, and changing species composition to lighter

crowned and fire-adapted species. Such intermediate treatments can reduce the

severity and intensity of wildfires for a given set of physical and weather variables.

But crown and selection thinnings would not reduce crown fire potential. Similarly,

because multiple canopy layers, and shade-tolerant species are favored in stands

where an individual tree selection system is used, crown fire potential would be high.

A possible exception is in the driest of the ponderosa pine forests where low crown

densities can be achieved by using the selection system.

Salvage and sanitation cuttings would not decrease potential for crown fire behavior

unless tree densities were substantially reduced. Stand densities would need to be

similar to the moderately and heavily thinned stands displayed in our examples to

“fire-proof” a stand. In all cases, any intermediate treatment needs to be accompanied by surface fuel modification. If the fine fuels are removed after a winter or

burned with a low-intensity prescribed fire, there is minimal loss of nutrients. The best

success in modifying fire behavior through the use of thinnings throughout the West

is when applied in conjunction with prescribed fire. In climax ponderosa pine forests,

cleaning stands with prescribed fire has been effective in modifying wildfire behavior. 

22
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wildfire in a ponderosa pine vegetation type. Forest Science. 25(1): 123-129.

Weatherspoon, C. Phillip; Skinner, Carl N. 1995. An assessment of factors associated with damage to tree crowns from the 1987 wildfires in northern California. 

Forest Science. 41(3): 123-129.

Weaver, H. 1955. Fire as an enemy, friend, and tool in forest management. Journal 

of Forestry. 53: 499-504.

Weaver, H. 1957. Effects of burning in ponderosa pine. Journal of Forestry. 55: 

133-137.
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Graham, Russell T.; Harvey, Alan E.; Jain, Theresa B.; Tonn, Jonalea R. 1999. The 

effects of thinning and similar stand treatments on fire behavior in Western forests. Gen.

Tech. Rep. PNW-GTR-463. Portland, OR: U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station. 27 p.

In the West, thinning and partial cuttings are being considered for treating millions of forested

acres that are overstocked and prone to wildfire. The objectives of these treatments include

tree growth redistribution, tree species regulation, timber harvest, wildlife habitat improvement,

and wildfire-hazard reduction. Depending on the forest type and its structure, thinning has

both positive and negative impacts on crown fire potential. Crown bulk density, surface fuel,

and crown base height are primary stand characteristics that determine crown fire potential.

Thinning from below, free thinning, and reserve tree shelterwoods have the greatest opportunity for reducing the risk of crown fire behavior. Selection thinning and crown thinning that

maintain multiple crown layers, along with individual tree selection systems, will not reduce

the risk of crown fires except in the driest ponderosa pine (Pinus ponderosa Dougl. ex Laws.)

forests. Moreover, unless the surface fuels created by using these treatments are themselves

treated, intense surface wildfire may result, likely negating positive effects of reducing crown

fire potential. No single thinning approach can be applied to reduce the risk of wildfires in the

multiple forest types of the West. The best general approach for managing wildfire damage

seems to be managing tree density and species composition with well-designed silvicultural

systems at a landscape scale that includes a mix of thinning, surface fuel treatments, and

prescribed fire with proactive treatment in areas with high risk to wildfire.

Keywords: Silviculture, forest management, prescribed fire, selection, forest fuels, crown fire.

The Forest Service of the U.S. Department of Agriculture is

dedicated to the principle of multiple use management of the

Nation’s forest resources for sustained yields of wood, water,

forage, wildlife, and recreation. Through forestry research,

cooperation with the States and private forest owners, and

management of the National Forests and National Grasslands,

it strives—as directed by Congress—to provide increasingly

greater service to a growing Nation.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race,

color, national origin, gender, religion, age, disability, political

beliefs, sexual orientation, or marital or family status. (Not all

prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should

contact USDA's TARGET Center at (202) 720-2600 (voice 

and TDD).

To file a complaint of discrimination, write USDA, Director,

Office of Civil Rights, Room 326-W, Whitten Building, 14th 

and Independence Avenue, SW, Washington, DC 20250-9410

or call (202) 720-5964 (voice and TDD). USDA is an equal

opportunity provider and employer.

Pacific Northwest Research Station

333 S.W. First Avenue

P.O. Box 3890

Portland, Oregon 97208-3890
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