United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

May 2008

Forest Resources 

of the Bighorn 

National Forest

Chris Witt
About the author __________________________

Chris Witt is an ecologist with the Interior West Forest Inventory and Analysis 

Program, Rocky Mountain Research Station in Ogden, Utah.

Contents___________________________________

Page

Description of the Bighorn National Forest....................................................1

Forest land highlights of the Bighorn National Forest..................................2

Nonreserved timberland highlights of the Bighorn National Forest.........8

Inventory methods.............................................................................................11

Documentation...................................................................................................13

For further information.....................................................................................13

Rocky Mountain Research Station

Natural Resources Research Center

2150 Centre Avenue, Building A

Fort Collins, CO 80526-2098
1

Forest Resources of the Bighorn National Forest

Chris Witt

Figure 1—Percent area of accessible forest and nonforest land, Bighorn National Forest, 2000.

The Interior West Forest Inventory and Analysis (IWFIA) 

Program of the USDA Forest Service, Rocky Mountain 

Research Station, as part of our National Forest System 

cooperative inventories, conducted a forest resource inventory on the Bighorn National Forest (Bighorn) using 

a nationally standardized mapped-plot design (for more 

details see section “Inventory methods” page 11). This 

report presents the highlights of this 2000 inventory using 

commonly requested variables and summaries. The data 

could be summarized in other ways for different purposes 

(see “For further information” on the inside back cover). 

The information presented in this report is based solely on 

the IWFIA inventory sample (USDA 1999). Supplementary 

documentation and inventory terminology can be located 

in USDA (2002). Additional data collected by the Bighorn 

National Forest and used separately or in combination with 

IWFIA data may produce varying results. Changes since 

the inventory, such as the impact of recent disturbances 

on the Forest have not been incorporated into this report. 

Annual inventories will soon replace periodic inventories 

to help monitor these changes at shorter intervals.

Description of the Bighorn 

National Forest

The Bighorn National Forest covers 1,107,671 acres 

(USDA 2000), of which 66 percent is forest land, 33 percent 

is non-forest, and less than 1 percent is water (figure 1). 

Seventeen percent of the total area on the Bighorn National Forest is in reserved designation. This report describes the characteristics of the forest land sampled on 

the Bighorn. “Forest land” is defined as land that is at least 

10 percent stocked (or formerly stocked) with live tally 

tree species. Furthermore, forest land must be greater 

than 1 acre in area and at least 120 feet wide. Forest land 

can be divided into “timberland” and “woodland”, based 

on the tree species present. Timberland is forest land 

that consists of tree species used primarily in the wood 

products industry, such as lodgepole pine and Engelmann 

spruce. Woodland is made up of tree species that often 

exhibit multi-stemmed growth forms and as such are not 

typically used in the wood products industry. Examples of 

woodland species are Rocky Mountain juniper, and certain 

species of oak. Only 2 percent of the forest land on the 

Bighorn is classified as woodland, and these woodlands 

are made up entirely of juniper forest type. The first part 

of this report focuses on forest resources of all forest land 

administered by the Bighorn National Forest, including 

reserved lands. The subsequent section will address nonreserved timberland.

66%

34%

Accessible forest

Nonforest and water
2

Forest land highlights of the 

Bighorn National Forest

Forest type—Forest resources are often described using 

a forest type classification. Forest type refers to the predominant tree species in a stand, based on plurality of tree 

stocking. Stocking is an expression of the extent to which 

growing space is effectively utilized by live trees.

Forest types are dynamic and can change slowly through 

forest succession, or rapidly due to disturbances such as 

logging, fire, or insect and disease epidemics. Figure 2 

presents the distribution of forest land area on the Bighorn National Forest by forest type. The lodgepole pine 

forest type is the most common at 49 percent, followed 

by Engelmann spruce/subalpine fir mix (spruce-fir) at 

20 percent. The Engelmann spruce forest type comprises 

almost 13 percent of the forest land area; Douglas-fir, 10 

percent; ponderosa pine, 3 percent; juniper, 2 percent; 

limber pine, just under 2 percent; and aspen, 1 percent.

Tree and stand size—The size distribution of trees is an 

indicator of structural diversity. Figure 3 displays the tree 

size distribution by diameter class on the Bighorn National 

Forest. Overall, this shows a typical diameter distribution 

with a higher number of small trees than large trees. Trees 

often reproduce prolifically, but thin out naturally over 

time due to competition for resources.

Stand-size class is a measure of forest land based on 

the dominant diameter-size of live trees that contribute 

Figure 2—Percent of forest land area by forest 

type, Bighorn National Forest, 2000.

Figure 3—Number of live trees on forest land by diameter class, Bighorn National Forest, 2000.

0

20

40

60

80

100

120

140

160

1-2.9 3-4.9 5-6.9 7-8.9 9-10.9 11-12.9 13-14.9 15-16.9 17-18.9 19-20.9 21-22.9 23-24.9 25-26.9 27-28.9 29+

Diameter class (inches)

Million trees

0% 10% 20% 30% 40% 50% 60%

Aspen

Limber pine

Juniper

Ponderosa pine

Douglas-fir

Engelmann spruce

Spruce-fir

Lodgepole pine

Forest type

Percent of forest land area
3

to stand stocking. Large trees are timber softwoods and 

all woodland tree species 9.0 inches diameter and greater, 

and timber hardwoods 11.0 inches diameter and greater; 

medium trees include timber softwoods and all woodland 

tree species 5.0 to 8.9 inches diameter, and timber hardwoods 5.0 to 10.9 inches diameter; and saplings/seedlings 

comprise all trees under 5.0 inches diameter. Most nonstocked stands are those that have been recently disturbed 

by tree cutting, forest fire, or other large-scale change. 

For tree stocking, fewer large-diameter trees compared 

to small-diameter trees are required to fully stock a site. 

Figure 4 shows a breakdown of forest land on the Bighorn 

National Forest by area and stand-size class. Fifty-one 

percent of the stands have a majority of stocking from 

large trees, while 2 percent are nonstocked.

Figure 5 shows the area of forest land by forest type and 

stand-size class on the Bighorn National Forest. The most 

common forest type in the large tree class is lodgepole 

pine, accounting for 41 percent of the large tree stands. 

The spruce-fir type makes up 22 percent of the large tree 

class and is the second most common type in this class. 

Lodgepole pine type dominates the medium tree class as 

well, accounting for 64 percent of this 

size class.

Number of live trees—Another way to 

assess forest diversity is by examining 

the composition of forest land by tree 

diameter and species. Figure 6 shows 

total number of live trees by species in 

three diameter-size classes. Fifty-seven 

percent of all live trees on the Bighorn 

National Forest are from 1.0 to 4.9 inches 

diameter, 29 percent are from 5.0 to 

8.9 inches diameter, and 14 percent are 

9.0 inches diameter and greater. Lodgepole pine makes up 40 percent of the total 

number of trees; subalpine fir, 30 percent; 

Engelmann spruce, 14 percent; Douglasfir, 10 percent; limber pine, 3 percent; 

and Rocky Mountain juniper, 2 percent. 

Ponderosa pine and aspen comprise the 

remaining 1 percent of live trees on the 

forest.

Elevation is closely correlated with 

variations in local climate. Precipitation 

generally increases with rising elevation, 

while temperature decreases. Aspect 

complicates this general rule; allowing 

relatively warmer- and dryer-site species 

to grow at higher elevations on southand west-facing slopes. These factors 

have a profound effect on competition 

between tree species. The Bighorn 

National Forest displays some distinct 

elevation patterns in tree distribution: 

Subalpine fir and Engelmann spruce 

do well at the highest elevations, while 

0 50 100 150 200 250 300 350 400

Stand-size class

Thousand acres

Large (>= 9 or 11")

Medium (>= 5" and 

< 9 or 11")

Small (< 5")

Nonstocked

0

20

40

60

80

100

120

140

160

180

Lodgepole

pine

Spruce-fir Engelmann

spruce

Douglas-fir Ponderosa

pine

Juniper Limber pine Aspen

Forest type

Thousand acres

Large (>= 9 or 11")

Medium (>= 5" and < 9 or 11")

Small (< 5")

Nonstocked

Figure 4—Forest land area by stand-size class, Bighorn National Forest, 2000.

Figure 5—Area of forest land by forest 

type and stand-size class, Bighorn National 

Forest, 2000.
4

lodgepole pine co-dominates the mid to high elevation 

classes with subalpine fir and Engelmann spruce. Douglasfir dominates the lower elevation stands (Figure 7).

Number and weight of dead trees—Standing and down 

dead trees are important for wildlife and in fire ecology, as 

well as acting as nutrient sinks and erosion controls. Approximately 81 million standing dead trees and 61 million 

0

20

40

60

80

100

120

Lodgepole 

pine

Subalpine-fir Englemann

spruce

Douglas-fir Limber pine Rocky Mtn.

juniper

Aspen Ponderosa

pine

Species

Million trees

1.0"-4.9"

5.0"-8.9"

9.0" and greater

Figure 6—Number of live trees 

1.0” diameter and greater on forest 

land by species and diameter-size 

class, Bighorn National Forest, 

2000.

down dead trees 1.0 inch diameter and greater occur on 

forest land on the Bighorn.

Many organisms rely on standing dead trees for some part 

of their life cycle, but the species, size, and densities required 

for quality habitat vary depending on the species utilizing 

them. Large diameter dead trees are generally scarce relative 

to smaller trees. Considering standing dead trees 11.0 inches 

diameter or larger, an estimated 6.0 per acre occur on forest 

Figure 7—Number of live trees 5.0” 

diameter and greater on forest land by 

species and elevation class, Bighorn 

National Forest, 2000.

0

10

20

30

40

50

60

Lodgepole 

pine

Engelmann

spruce

Subalpine 

fir

Douglas-fir Limber 

pine

Ponderosa

pine

Rocky Mtn.

juniper

Aspen

Species

Million trees

5000-5999 feet

6000-6999 feet

7000-7999 feet

8000-8999 feet

9000-9999 feet

10000+ feet
5

land. Of the very large dead trees (19.0 inches diameter or 

larger) there is an estimated 1.2 per acre. Four timber species 

(lodgepole pine, subalpine fir, Douglas-fir, and Engelmann 

spruce) occur in the 19-inch and larger standing dead tree 

group, with Engelmann spruce being the most common.

Dead material is a component of forest fuel loads. On this 

Forest, about 3.1 million tons of standing dead trees and 3.3 

million tons of down dead trees occur on forest land. This 

estimate includes the merchantable bole and bark of trees 

5.0 inches diameter and greater. Figure 8 shows the weight 

per acre of down dead trees by stand-size class for each 

forest type. For all forest types combined, the small tree 

stand-size class contributes the most dead and down material 

at roughly 5.9 tons per acre, with the sawtimber and poletimber classes accounting for 5.6 and 2.2 tons per acre respectively. 

Overall, lodgepole pine and spruce-fir forest type account 

for roughly 78 percent of the dead and down material on the 

Forest. Aspen forest types in the seedling/sapling stand-size 

class have the largest tonnage of down dead trees per acre 

at 11.16 tons/acre, but this forest type/size class is relatively 

scarce on the Bighorn (roughly 5,500 acres).

Stand age—Stand age for this report is estimated from 

core samples of live trees. The estimate is limited to trees 

with diameters that fall in a stand’s designated stand-size 

class. Many other factors affect the number of sample 

trees available for determining stand age. In general, stand 

age for dense stands that contain more core sample trees 

is more representative than stand age for sparse stands 

that contain less.

Figure 9 displays the percent of forest land area by forest 

type and stand-age class on the Bighorn National Forest. 

Stand age can indicate the duration since the last extensive 

disturbance of the forest overstory. This figure shows the 

81- to 100-year class as the most common on the Forest, 

followed by the 141- to 160-year and 101- to 120-year age 

classes respectively.

Wood volume, biomass, and basal area of live trees—

Estimates of cubic-foot volume and basal area include all 

live trees 5.0 inches diameter and greater. Basal area is the 

cross-sectional area of a tree stem/bole (including bark) 

at the point of diameter measurement. Biomass estimates 

include boles, bark, and branches of 

all live trees 1.0 inches diameter and 

greater. The net volume of wood on 

the Bighorn National Forest is estimated to be in excess of 1.6 billion 

cubic feet. Total biomass of wood 

is estimated at just over 30 million 

tons, and the total basal area is 

0

2

4

6

8

10

12

Spruce-fir Engelmann

spruce

Ponderosa

pine

Lodgepole

pine

Douglas-fir Aspen Juniper Limber pine All types

Forest type

Large (>= 9 or 11")

Medium (>= 5" and < 9 or 11")

Small (< 5")

Nonstocked

Tons per acre

Figure 8—Weight of down dead trees 

5.0” diameter and greater on forest 

land by forest type and stand-size 

class, Bighorn National Forest, 1999.
6

81 percent of Rocky Mountain 

juniper, and 86 percent of 

Douglas-fir volume is in trees 

9.0 inches diameter and greater. Conversely, only 62 percent 

of subalpine fir, 57 percent of 

lodgepole pine, and 10 percent 

of aspen volume is in trees 9.0 

inches diameter and larger.

Another way to look at wood 

volume is by forest type, for 

which estimates per acre can 

be computed along with basal 

area (table 2). These numbers 

include the many different 

species that can occur together 

in each forest type. The highest volume and basal area per 

acre on the Bighorn National 

Forest is in the spruce-fir forest 

type. Volume and basal area 

per acre for aspen, juniper, 

limber pine, and ponderosa 

pine may not be representative due to the small sample 

size. One characteristic of the 

mapped-plot design is that a 

plot may sample more than 

one condition as indicated in 

the last two columns of table 2. 

A forest condition is generally defined as an area of relatively homogeneous vegetative cover that meets the criteria for forest land. Forest type is one of several attributes 

that define and separate conditions identified on the plot.

Stand Density Index—Many factors influence the rate at 

which trees grow and thrive, or die. As tree size and density increase, competition for available resources increases. 

Stand density index (SDI), as developed by Reineke (1933), 

is a relative measure quantifying the relationship between 

number of trees per acre and average stand diameter. The 

concept was developed for even-aged stands, but can 

also be applied to uneven-aged stands composed of three 

or more size classes (Long and Daniel 1990, Shaw 2000). 

Stands may be categorized on the basis of tree size, often 

in terms of their predominant diameter or height class. 

Stands where just one or two size classes dominate are 

called single-storied, or even-aged, because they have a 

structure characterized by a single canopy layer or two 

closely related layers. Stands having a structure composed 

of three or more size classes are called multistoried or 

uneven-aged stands. Both types of structure are important 

in forest diversity.

SDI is usually presented as a percentage of the maximum 

SDI for each forest type (Van Dyck 2002). SDI was computed 

0%

5%

10%

15%

20%

25%

1-20 21-40 41-60 61-80 81-100 101-120 121-140 141-160 161-180 181-200 201-220 221-240 241-260 261+

Stand-age class (years)

Percent of forest land area

Spruce-fir

Ponderosa pine

Lodgepole pine

Limber pine

Juniper

Engelmann spruce

Douglas-fir

Aspen

Figure 9—Percent of forest land area by forest type and stand-age class, Bighorn National 

Forest, 2000.

estimated to be about 77.7 million square feet. Table 1 is a 

breakdown of volume, biomass, and basal area by species.

Figure 10 displays the percent net cubic-foot volume of 

live trees by diameter class. Seventy-one percent of this 

volume is in the 9.0 inches and greater diameter. By species, 

91 percent of Engelmann spruce, 89 percent of ponderosa pine, 

Table 1—Net volume, biomass, and basal area on forest land 

by species, Bighorn National Forest, 2000.

 Volume Biomass Basal area

 (>5.0" dia.) (>1.0" dia.) (>5.0" dia.)

 (Million (Million (Million

Species cubic-feet) tons) square-feet)

Lodgepole pine 764.0 13.7 35.4

Engelmann spruce 445.1 7.0 18.9

Douglas–fir 232.9 4.9 10.0

Subalpine fir 163.6 3.6 9.5

Ponderosa pine 18.9 0.4 1.2

Limber pine 17.5 0.4 1.6

Rocky Mountain juniper 4.3 † 0.9

Aspen 3.2 † 0.3

Total (not exact due 1,649.5 30.1 77.7

 to rounding)

† – Less than 100,000
7

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

5-6.9 7-8.9 9-10.9 11-12.9 13-14.9 15-16.9 17-18.9 19-20.9 21-22.9 23-24.9 25-26.9 27-28.9 29+

Diameter class (inches)

Percent of total volume 

Figure 10—Percent of total net cubic-foot volume of live trees on forest land by diameter class, Bighorn National 

Forest, 1999.

for each location using those maximums, and the results 

were grouped into four classes (Figure 11). A site is considered to be fully occupied at or above 35 percent of SDI 

maximum, which marks the onset of competition-related 

stresses and slowed growth rates (Long 1985). Based on FIA 

sample data, 68.4 percent of all forest stands in the Bighorn 

National Forest are considered to be fully occupied.

Table 2—Net volume per acre, basal area per acre, number of conditions, and condition 

proportions on forest land by forest type, Bighorn National Forest, 2000.

 Net cubic-foot Basal area Number of Condition

Forest type volume per acre sq. ft. per acre conditionsa proportionsb

Spruce–fir 2,679 123 29 25.0

Douglas–fir 2,610 119 16 12.7

Engelmann spruce 2,463 115 19 15.8

Lodgepole pine 2,201 103 68 60.6

Ponderosa pine 959 65 3 3.0

Limber pine 450 59 2 2.0

Juniper 160 26 3 3.0

Aspen 127 12 2 1.5

Total 142 123.6

aNumber of conditions by forest type that were sampled. These numbers are often greater than 

the total number of forested plots by forest type because a plot may sample more than one forest 

condition. bSum of the condition proportions of plots by forest type that were sampled. These numbers are 

often less than the total number of plots by forest type because of nonforest condition proportions 

(from plots containing both forest and nonforest conditions) that are not included here.

Components of change: growth—Another measure of 

forest vigor is net annual growth. Net annual growth is 

the difference between gross annual growth and losses 

due to mortality. Gross annual growth of live trees 

(5.0 inches diameter and greater) of all forest land on 

the Bighorn National Forest is estimated to be just over 

27 million cubic feet, and net annual growth is just over 
8

22 million cubic feet. Gross annual growth is compared 

to mortality for four high volume species in Figure 12.

Components of change: mortality—Field crews assess 

which trees have died in the past 5 years; these trees are 

used to estimate an average annual mortality. Based on 

this estimate, in 1999, 4.8 million cubic feet of wood from 

live trees (5.0 inches diameter and greater) died on the 

Bighorn National Forest, roughly 50 percent of this death is 

attributed to disease. Twenty percent of tree mortality was 

caused by unknown agents, 17 percent by insects, and 13

percent from weather related events. Mortality on all forest land on the Bighorn National Forest is about 18 percent 

of gross annual growth. No species has a larger mortality 

rate than growth rate on the Bighorn National Forest. Aspen has the highest mortality to growth ratio at 73 percent, 

followed by Douglas-fir at 34 percent. The leading specific 

causes of mortality in these two species were disease for 

aspen (58 percent conks, 42 percent cankers) and 

insect damage from bark beetles in Douglas-fir (63 

percent). No mortality was fire related. Lodgepole 

pine (47 percent), subalpine fir (24 percent), and 

Douglas-fir (20 percent) are the top three mortality species, accounting for 91 percent of overall 

mortality.

Understory vegetation—Understory vegetation 

provides forage and cover for wildlife, contributes 

to forest fuel load, and can be an indication of 

the successional stage of the forest community. 

Understory vegetation works in concert with the 

dominant vegetation to provide complexity and 

diversity to forest structure and composition. 

Generally, the more complex and diverse the vegetation component of a forest is, the more diverse 

the wildlife community using it will be. On each 

plot, field crews visually estimated crown canopy 

coverage for four plant groups – tree seedlings/

saplings, shrubs, forbs, and graminoids (See USDA 

1999 for details). Figure 13 shows the average cover 

(as a percentage of total ground cover) of plant 

groups on forest land by forest type. Ponderosa and limber 

pine forest types contain the lowest cover of grasses and 

forbs while Engelmann spruce has the lowest average of 

understory seedling/sapling cover and the spruce-fir forest 

type has the lowest average shrub cover. Conversely, the 

spruce-fir type had the highest average seedling/sapling 

and forb cover at roughly 15 percent and 9 percent, respectively. The juniper forest type has the highest average 

grass cover and the second highest average forb cover, 

despite the well documented allelopathic effects junipers 

have on associated plant communities. The juniper forest 

type also has the highest average combined understory 

cover at 51 percent, followed by Douglas-fir and lodgepole 

pine at 43 percent and 42 percent, respectively. Limber 

pine type has the lowest overall average understory cover 

at 23 percent. Interpretation of data associated with the 

limber pine and juniper forest types should be done with 

caution due to low sample size.

Nonreserved timberland 

highlights of the Bighorn 

National Forest

Reserved lands are those that have been withdrawn 

from management for production of wood products 

through federal statute or administrative designation, such 

as wilderness areas and National Parks. Roughly eleven 

percent (81,361 acres) of the timberland on the Bighorn 

National Forest is reserved, while the remaining 89 percent (636,945 acres) is nonreserved and available for the 

management of timber and wood products. Lodgepole 

pine and s pruce-fir are the only two forest types found on 

reserved lands on the Bighorn National Forest.

0 2 4 6 8 10 12 14

Douglas-fir

Lodgepole pine

Engelmann spruce

Subalpine fir

High volume species

Million cubic feet

Gross growth

Mortality

Figure 12—Gross annual growth of all live trees 5.0”diameter and 

greater compared to mortality for four high-volume species on 

forest land, Bighorn National Forest, 1999.

< 25 25.0-34.9 35.0-59.9 60+

Thousands of acres

180

160

140

120

100

80

60

40

20

0

Percent maximum SDI

Aspen

Douglas-fir

Engelmann spruce

Juniper

Limber pine

Lodgepole pine

Ponderosa pine

Spruce-fir

Figure 11—Area of forest land by Stand Density Index and forest type, 

Bighorn National Forest, 2000. SDI is presented as a percentage of the 

theoretical maximum for each forest type.
9

Forest type—Lodgepole pine accounts for nearly 48 percent of all nonreserved timberland on the Bighorn, making 

it the most common. Spruce-fir (20 percent), Engelmann 

spruce (14 percent), and Douglas-fir (12 percent) make up 

the majority of what remains, while ponderosa pine, limber pine and aspen each account for less than 3 percent 

of the nonreserved timberland found on the Bighorn National Forest.

Dead trees and fuels—The Bighorn has approximately 

23 million standing dead trees and 24 million down 

dead trees 5 inches diameter or greater on nonreserved 

timberland. The merchantable bole and bark of these 

trees is approximately 2.9 tons for the 

standing dead trees and 2.9 tons for 

the down dead trees. Lodgepole pine 

accounts for the largest portion of total 

weight of dead standing (33 percent) 

and dead down (47 percent) trees in 

nonreserved timberland. This is similar 

to the forest as a whole, where Lodgepole pine makes up 37 percent of the 

dead standing trees and 49 percent of 

the down dead trees on Bighorn timberland.

Stand age—In general, stand age on 

nonreserved timberland is similar to 

stand age on all forest land. The 81-100 

year age-class is the most common, followed by 141-160 and 101-120 year ageclasses respectively. This is similar to 

forest land on the Bighorn as a whole.

Wood volume, biomass, and basal 

area of growing-stock trees—Growingstock trees include live timber species 5 inches and greater diameter at 

breast height and meeting specific 

standards of quality and vigor. Of all 

growing-stock trees on nonreserved 

timberland on the Bighorn, 14 percent are 9 inches diameter or greater. Table 3 displays a breakdown of net 

cubic-foot volume, tons of wood biomass, and square 

foot basal area for growing-stock trees 5 inches diameter 

and greater by species on nonreserved timberland for the 

Bighorn. The total net cubic-foot volume of growing stock 

on nonreserved timberland is about 1.5 billion cubic feet. 

Lodgepole pine and Engelmann spruce together account 

for roughly 73 percent of this volume. The total wood 

biomass is estimated at 25.0 million tons, with lodgepole 

pine making up over 45 percent of this amount. Total basal 

area for growing-stock trees on nonreserved timberland is 

0

5

10

15

20

25

Aspen Douglas-fir Engelmann

spruce

Juniper Limber 

pine

Lodgepole

pine

Ponderosa

pine

Spruce-fir

Forest type

Percent cover

Saplngs/seedlings

Shrubs

Grasses

Forbs

Figure 13—Percent cover of four major understory plant groups on forest land by forest 

type, Bighorn National Forest, 1999.

Table 3—Net volume, biomass, and basal area of growing-stock trees 

greater than 5 inches diameter on nonreserved timberland, 

Bighorn National Forest, 2000.

 Volume Biomass Basal area

 (Million (Million (Million

Species cubic-feet) tons) square-feet)

Lodgepole pine 672.8 11.3 31.2

Engelmann spruce 412.0 6.2 17.4

Douglas-fir 229.8 4.5 9.7

Subalpine fir 141.1 2.3 7.9

Ponderosa pine 18.6 0.4 1.2

Limber pine 14.5 0.3 1.2

Aspen 2.9 † 0.2

Total (not exact due to rounding) 1,491.8 25.0 68.7

† less than 100,000 tons

* numbers may not add to total due to rounding
10

0

100

200

300

400

500

600

9-10.9 11-12.9 13-14.9 15-16.9 17-18.9 19-20.9 21-22.9 23-24.9 25-26.9 27-28.9 29+

Diameter class (inches)

Million board feet

Lodgepole pine

Spruce-fir

Engelmann spruce

Douglas-fir

Ponderosa pine

Limber pine

Aspen

Figure 14—Board-foot volume (International 1/4 rule) on nonreserved timberland by 

diameter class and forest type, Bighorn National Forest, 2000.

estimated at 68.7 million square feet, with lodgepole pine 

comprising 45 percent of this total as well.

The net volume of sawtimber trees on nonreserved 

timberland is estimated to be 5.1 billion board feet (or 

1.1 billion cubic feet). This includes all growing stock trees 

9 inches diameter and greater for softwoods and 11 inches 

diameter and greater for hardwoods. Figure 14 illustrates 

the sawtimber volume on nonreserved timberland by diameter class and forest type. The 9-10.9 inch diameter class 

contributes the most volume with 24 percent of the total, 

followed by the 11-12.9 inch class (20 percent) and the 

13-14.9 inch class (17 percent). The lodgepole pine forest 

type holds the majority of each of these diameter classes.

Stand Density Index—Over 73 percent of nonreserved 

timberland on the Bighorn National Forest is at or above 

the 35 percent SDI maximum threshold, which classifies 

them as fully occupied. The spruce-fir forest type has the 

highest percentage of its nonreserved area fully occupied at 

86.3 percent, followed by the Douglas-fir type at 74.9 percent.

Components of change: growth and mortality—Gross 

annual growth of growing-stock trees on nonreserved 

timberland on the Bighorn is estimated to be 25.1 million 

cubic feet, while mortality is estimated at 3.7 million cubic 

feet. This calculates to a net annual growth of 21.4 million 

cubic feet. Lodgepole pine accounts for 51 percent of this 

total. Figure 15 compares gross growth and mortality of 

growing-stock trees by species on nonreserved timberlands on the Bighorn National Forest.

Other inventory highlights of the Bighorn National 

Forest—It is interesting to compare the attributes of 

reserved and nonreserved forests to see if there are differences in structure and composition between them. For instance, using the three high volume forest types found on 

both reserved and nonreserved forest lands (Engelmann 

spruce, lodgepole pine, and spruce-fir), nonreserved forests produce almost twice as much volume per acre of 

these species (1700 cubic feet per acre) than do reserved 

forest lands (890 cubic feet per acre). When biomass is 

compared, there is little difference between lodgepole 

pine and subalpine fir in terms of tons per acre produced 

on reserved and nonreserved lands. However, Engelmann 

spruce contributes nearly twice as much biomass per acre 

on nonreserved forest lands as they do on reserved forests 

(10.1 tons per acre and 5.6 tons per acre respectively).

Standing and down dead trees provide habitat to numerous organisms in forest lands. These include nest cavities 

for birds, incubation sites for invertebrates, hibernacula 

for an assortment of mammals, reptiles, and amphibians, 
11

roost sites for bats, microsites for germination and growth 

of some plants, as well as providing cover and forage 

opportunities for many other species. Figure 16 illustrates 

how dead tree biomass differs between the two major forest types found on both reserved and nonreserved forest 

lands. On the Bighorn National Forest, spruce-fir forest 

types on nonreserved lands provide the bulk of the dead 

tree habitat and structure.

0 2 4 6 8 10 12 14

Species

Million cubic feet

Gross growth

Mortality

Subalpine fir

Ponderosa pine

Lodgepole pine

Limber pine

Engelmann spruce

Douglas-fir

Aspen

Figure 15—Gross annual growth of growingstock trees 5.0” diameter and greater compared 

to mortality on nonreserved timberland by 

species, Bighorn National Forest, 1999.

Inventory methods

Forest Inventory and Analysis (FIA) provides a statisticallybased sample of forest resources across all ownerships 

that can be used for planning and analyses at local, state, 

regional, and national levels. IWFIA uses a two-phase 

sampling procedure for all inventories. Phase one is based 

on a grid of sample points systematically located every 

1,000 meters across all lands in a state. Phase one points 

are assigned ownership and vegetative cover attributes 

using maps and remotely sensed imagery. Field crews 

conduct phase two of the inventory on the sub-sample of 

phase one points that occur on forest land. The sampling 

intensity is one field plot every 5,000 meters, or about every 

3 miles. Phase two plots are stratified based on phase one 

ownership and vegetation information, while weights 

are assigned to each stratum based on the proportion of 

phase one points in that stratum.

Phase two plots were sampled using the mapped-plot 

design (see next section). There were 183 field plots on the 

Bighorn National Forest. One hundred ten plots sampled 

only forest conditions, 51 plots sampled only non-forest 

conditions, 21 sampled both forest and non-forest conditions, and 1 sampled both forest and water conditions 

(rivers, reservoirs, lakes, etc. at least 30 feet wide or 1 acre 

in area). A total of 142 forest conditions were sampled on 

132 plots that contain 123.6 forest, and 8.4 non-forest condition proportions.

About the mapped-plot design—The mapped-plot design 

was adopted by FIA nationwide by 1995. Its predetermined 

subplot layout uses boundary delineation, when necessary, 

0

5

10

15

20

25

30

Lodgepole pine Spruce-fir

Forest type

Reserved 

Nonreserved

Tons per acre

Figure 16—Biomass of dead trees 1.0” diameter and greater by 

forest type and reserve status, Bighorn National Forest, 2000.
12

Table 4—Percent standard errors for area, volume, growth, and mortality estimates 

on total forest land and nonreserved timberland, Bighorn National Forest, 

1999 and 2000.

Land class or Percent

Type of trees Attribute Area or volume standard error

Total forest land (acres) Area 735,714 ±3.2

Total forest land Volume 1,649,479,733 ±6.5

 (all trees cubic feet) Growth 22,334,632 ±7.6

Mortality 4,822,697 ±22.3

Total nonreserved timberland (acres) Area 636,945 ±3.7

Nonreserved timberland Volume 1,491,758,338 ±6.9

 (growing-stock cubic feet) Growth 21,379,268 ±7.8

Mortality 3,721,360 ±25.5

to classify differing conditions. Most plots sample one forest condition, therefore delineating conditions is often not 

required.

Conditions were separated, or mapped, based on differences in any of five attributes: forest/nonforest, forest type, 

stand-size class, stand origin, and stand density. The condition proportion is the fraction of plot area sampled in each 

condition. The sum of all condition proportions for any 

given plot equals 1.00. Therefore, the number and relative 

size of plot conditions determines the weighted area used 

for sample expansion.

Standard errors—The sample was designed to meet 

national standards for precision in state and regional 

estimates of forest attributes. Standard errors, which 

denote the precision of an estimate, are usually higher 

for smaller subsets of the data. Forest-level estimates 

and percent standard errors by land class or type of 

trees for various attributes are presented in table 4. 

Standard errors for other estimates are available upon 

request (see “For further information” section on the 

inside back cover).
13

Federal Recycling Program Printed on Recycled Paper

For further information ________________

Interior West Forest Inventory and Analysis Program (IWFIA)

Rocky Mountain Research Station

c/o Program Manager

507 25th Street, Ogden, UT 84401

Phone: 801-625-5388

FAX: 801-625-5723

World Wide Web: http://www.fs.fed.us/rm/ogden

USDA Forest Service

Bighorn National Forest

2013 Eastside 2nd Street

Sheridan, Wyoming 82801

Phone: 307-674-2600

World Wide Web: http://www.fs.fed.us/r2/bighorn/

Selected data for this National Forest are part of a national data base that 

houses information for much of the forest land in the United States. This data 

base can be accessed on the Internet at the following web site:

http://fia.fs.fed.us/tools-data/

Documentation _______________

Long, James N. 1985. A Practical Appoach to Density Management. The Forest Chronicle 61:23-27.

Long, James N.; Daniel, Theodore W. 1990. Assessment of 

growing-stock in uneven-aged stands. Western Journal 

of Applied Forestry 5(3):93-96.

Reineke, L.H. 1933. Perfecting a stand density index for 

even-aged forests. J. Agric. Res. 46:627-638.

Shaw, John D. 2000. Application of Stand Density Index to 

Irregularly Structured Stand. Western Journal of Applied 

Forestry 15 (1): 40-42.

U.S. Department of Agriculture, Forest Service. 1999. Forest 

Survey field procedures, 2000. Ogden, UT: USDA Forest 

Service, Rocky Mountain Research Station.

U.S. Department of Agriculture, Forest Service. 2000. Land 

Areas of the National Forest System. FS-383. As of September 1999.

U.S. Department of Agriculture, Forest Service. 2002. Periodic 

mapped-plot design inventory terminology. [Online]. Available: http://www.fs.fed.us/rm/ogden/publications/wyoming.shtml

(also available on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 

Ogden, UT).

Van Dyck, M.G. 2002. Keyword reference guide to the 

Forest Vegetation Simulator. WO-Forest Management 

Service Center, Fort Collins, CO.
14

The Rocky Mountain Research Station develops scientific information and technology to improve management, protection, 

and use of the forests and rangelands. Research is designed to meet the needs of National Forest managers, Federal and State 

agencies, public and private organizations, academic institutions, industry, and individuals.

Studies accelerate solutions to problems involving ecosystems, range, forests, water, recreation, fire, resource inventory, 

land reclamation, community sustainability, forest engineering technology, multiple use economics, wildlife and fish habitat, 

and forest insects and diseases. Studies are conducted cooperatively, and applications may be found worldwide.

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

color, national origin, sex, 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, 1400 

Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964 (voice or TDD). USDA is an equal opportunity 

provider and employer.