Casper's Urban Forest

United States

Department of 

Agriculture

Forest Service

Northern 

Research Station

Resource Bulletin NRS-4

Assessing Urban Forest 

Effects and Values
Published by: Available online only at:

USDA FOREST SERVICE www.nrs.fs.fed.us/pubs/ 

11 CAMPUS BLVD SUITE 200

NEWTOWN SQUARE PA 19073-3294 

December 2006

Visit our homepage at: http://www.nrs.fs.fed.us/

An analysis of trees in Casper, WY reveals that this city has about 123,000 trees with canopies that cover 8.9 percent of the area. The most common tree species are plains cottonwood, blue spruce, and American elm. The urban forest currently store about 37,000 tons of 

carbon valued at $689,000. In addition, these trees remove about 1,200 tons of carbon per 

year ($22,000 per year) and about 50 tons of air pollution per year ($249,000 per year). The 

structural, or compensatory, value is estimated at $243 million. Information on the structure 

and functions of the urban forest can be used to improve and augment support for urban 

forest management programs and to integrate urban forests within plans to improve environmental quality in the Casper area.

DAVID J. NOWAK is a research forester and project leader, ROBERT E. HOEHN III, is a biological sciences technician, DANIEL E. CRANE is an information technology specialist, JACK 

C. STEVENS is a forester, and JEFFREY T. WALTON is a research forester with the USDA 

Forest Service’s Northern Research Station at Syracuse, NY.

The authors thank the following contributors for project assistance:

Mark Hughes, Wyoming State Forestry Division

Jim Gerhart, City of Casper Parks Division

Scott Maco, Davey Tree Expert Company

Michael Binkley, Davey Tree Expert Company

Donna Cuin, University of Wyoming Cooperative Extension and Natrona County Master 

Gardener

Tim Meyer, Natrona County Master Gardener and Volunteer

Sylvia Kirby, Natrona County Master Gardener and Volunteer

Cover Photo courtesy of the Department of Parks, Casper, WY

Abstract

The Authors

Acknowledgments
Feature	 	 	 	 Measure

Number of trees	 	 	 123,000

Tree cover	 	 	 	 8.9%

Most common species	 	 plains cottonwood, blue spruce,

	 	 	 	 	 American elm

Percentage of trees < 6-inches	 57.5%

diameter

Pollution removal	 	 	 50 tons/year ($249,000/year)

Carbon storage	 	 	 37,000 tons ($689,000)

Carbon sequestration		 	 1,200 tons/year ($22,000/year)

Building energy increase	 	 $26,700/year

Carbon emissions increase	 	 $700 / year

Structural value	 	 	 $243 million

Ton – short ton (U.S.) (2,000 lbs)

Feature	 	 	 	 Measure

Number of trees	 	 	 123,000

Tree cover	 	 	 	 8.9%

Most common species	 	 plains cottonwood, blue spruce,

	 	 	 	 	 American elm

Percentage of trees < 6-inches	 57.5%

diameter

Pollution removal	 	 	 50 tons/year ($249,000/year)

Carbon storage	 	 	 37,000 tons ($689,000)

Carbon sequestration		 	 1,200 tons/year ($22,000/year)

Building energy increase	 	 $26,700/year

Carbon emissions increase	 	 $700 / year

Structural value	 	 	 $243 million

Ton – short ton (U.S.) (2,000 lbs)

Casper’s Urban Forest Summary



Trees in cities can contribute significantly to human health and environmental quality. Unfortunately, little is known about 

the urban forest resource and what it contributes to the local and regional society and economy. To better understand 

the urban forest resource and its numerous values, the USDA Forest Service, Northern Research Station, developed the 

Urban Forest Effects (UFORE) model. Results from this model are used to advance the understanding of the urban forest 

resource, improve urban forest policies, planning and management, provide data for the potential inclusion of trees within 

environmental regulations, and determine how trees affect the environment and consequently enhance human health and 

environmental quality in urban areas. 

Forest structure is a measure of various physical attributes of the vegetation, such as tree species composition, number of 

trees, tree density, tree health, leaf area, biomass, and species diversity. Forest functions, which are determined by forest 

structure, include a wide range of environmental and ecosystem services such as air pollution removal and cooler air 

temperatures. Forest values are an estimate of the economic worth of the various forest functions.

To help determine the vegetation structure, functions, and values of the urban forest in Casper, a vegetation assessment was 

conducted during the summer of 2006. For this assessment, one-tenth acre field plots were sampled and analyzed using the 

UFORE model. This report summarizes results and values of:

• Forest structure

• Potential risk to forest from insects or diseases

• Air pollution removal

• Carbon storage

• Annual carbon removal (sequestration)

• Changes in building energy use

Executive Summary
Though urban forests have many functions and values, currently only a few of these attributes can be assessed. To help assess 

the city’s urban forest, data from 234 field plots located throughout the city were analyzed using the Forest Service’s Urban 

Forest Effects (UFORE) model1.

UFORE is designed to use standardized field data from randomly located plots and local hourly air pollution and 

meteorological data to quantify urban forest structure and its numerous effects, including: 

• Urban forest structure (e.g., species composition, tree density, tree health, leaf area, leaf and tree biomass, species 

diversity, etc.).

• Amount of pollution removed hourly by the urban forest, and its associated percent air 

 quality improvement throughout a year. Pollution removal is calculated for ozone, sulfur dioxide, 

nitrogen dioxide, carbon monoxide and particulate matter (<10 microns).

• Total carbon stored and net carbon annually sequestered by the urban forest.

• Effects of trees on building energy use and consequent effects on carbon dioxide emissions from 

power plants.

• Compensatory value of the forest, as well as the value of air pollution removal and carbon 

storage and sequestration.

• Potential impact of infestations by Asian longhorned beetles, emerald ash borers, gypsy moth, or 

Dutch elm disease.

In the field, one-tenth acre plots were located in the City of Casper and then post stratified by land use. Land uses were 

used to divide the analysis into smaller zones. The plots were divided among the following land uses: Business/Commercial/

Industrial (51 plots), Institutional (8 plots), Not Zoned (20 plots), Park Historic (40 plots), Planned Development (18 

plots), Residential (97 plots). This distribution allows for comparison among land uses.

Field data were collected for the Forest Service by City of Casper personnel; data collection took place during the leafon season to properly assess tree canopies. Within each plot, data included land-use, ground and tree cover, shrub 

characteristics, and individual tree attributes of species, stem-diameter at breast height (d.b.h.; measured at 4.5 ft.), tree 

height, height to base of live crown, crown width, percentage crown canopy missing and dieback, and distance and direction 

to residential buildings2.

Urban Forest Effects Model and Field Measurements
To calculate current carbon storage, biomass for each tree was calculated using equations from the literature and measured 

tree data. Open-grown, maintained trees tend to have less biomass than predicted by forest-derived biomass equations3. To 

adjust for this difference, biomass results for open-grown urban trees are multiplied by 0.8.3 No adjustment is made for trees 

found in natural stand conditions. Tree dry-weight biomass was converted to stored carbon by multiplying by 0.5.

To estimate the gross amount of carbon sequestered annually, average diameter growth from the appropriate genera and 

diameter class and tree condition was added to the existing tree diameter (year x) to estimate tree diameter and carbon 

storage in year x+1.

Air pollution removal estimates are derived from calculated hourly tree-canopy resistances for ozone, and sulfur and nitrogen 

dioxides based on a hybrid of big-leaf and multi-layer canopy deposition models.4, 5 As the removal of carbon monoxide 

and particulate matter by vegetation is not directly related to transpiration, removal rates (deposition velocities) for these 

pollutants were based on average measured values from the literature6, 7 that were adjusted depending on leaf phenology and 

leaf area. Particulate removal incorporated a 50 percent resuspension rate of particles back to the atmosphere.8

Seasonal effects of trees on residential building energy use was calculated based on procedures described the literature9 using 

distance and direction of trees from residential structures, tree height and tree condition data.

Compensatory values were based on valuation procedures of the Council of Tree and Landscape Appraisers10, which uses tree 

species, diameter, condition and location information10.

To learn more about UFORE methods11 visit:

http://www.nrs.fs.fed.us/UFORE/data/ or www.ufore.org
0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

Residential

Park Historic

Bus./Comm./Ind.

Institutional

Planned Develop

Not Zoned

Number of Trees

0

2

4

6

8

10

12

14

16

18

20

Trees per acre

Total Trees

Per acre

blue spruce

honeylocust crabapple

European 

crabapple quaking aspen

green ash

Rocky Mountain 

juniper

American elm

common 

chokecherry

plains 

cottonwood

other species

The urban forest of Casper has an estimated 123,000 trees with a tree cover of 8.9 percent. Trees that have diameters less 

than 6 inches account for 57.5 percent of the population. The three most common species in the urban forest are plains 

cottonwood (17.5 percent), blue spruce (8.1 percent), and American elm (6.7 percent). The 10 most common species 

account for 68.3 percent of all trees; their relative abundance is illustrated below.

The highest density of trees occurs in the Residential (18.6 trees/acre), followed by the Park Historic (5.5 trees/acre) and 

the Business/Commercial/Industrial (5.3 trees/acre). The overall tree density in Casper is 9.1 trees/acre, which is lower 

than other city tree densities (Appendix I), which range from 14.4 to 119.2 trees/acre. However, most of these cities are in 

forested ecotypes, unlike Casper, which is located in a grassland ecotype. Cities in grassland areas tend to have lower tree 

cover than cities found in forested regions.

Tree Characteristics of the Urban Forest
*North America+ refers to tree species that are native to North America and one other continent

**Americas+ refers to tree species that are native to North and South America and one other continent

0

5

10

15

20

25

30

35

1-34-67-9

10-1213-1516-1819-2122-2425-2728-30

30+

d.b.h. class

Percent

0

10

20

30

40

50

60

70

Wyoming

North America

Eurasia

Asia

Europe

Unknown

Americas

North America +*

Americas +**

Percent

Urban forests are a mix of native tree species that existed prior to the development of the city and exotic species that were 

introduced by residents or other means. Thus, urban forests often have a tree diversity that is higher than surrounding 

native landscapes. An increased tree diversity can minimize the overall impact or destruction by a species-specific insect or 

disease, but the increase in the number of exotic plants can also pose a risk to native plants if some of the exotics species 

are invasive plants that can potentially out-compete and displace native species. In Casper, about 43 percent of the trees are 

from species native to Wyoming. Trees with a native origin outside of North America are mostly from Eurasia (12.2 percent 

of the species).
0

5

10

15

20

25

plains cottonwood

blue spruce

American elm

green ash

honeylocust

European crabapple

spruce

white poplar

Rocky Mountain juniper

crabapple

Percent

% of total leaf area

% of all trees

0 10 20 30 40 50 60 70 80 90 100

Bus./Comm./Ind.

Institutional

Not Zoned

Park Historic

Planned Develop

Residential

Casper

Percent bare soil

duff/mulch cover

herbaceous

impervious surfaces

(excluding buildings)

buildings

Common Name %

Popa

%

LAb IVc

plains cottonwood 17.5 32.2 49.7

blue spruce 8.1 16.1 24.2

American elm 6.7 10.9 17.6

honeylocust 6.3 3.7 10.0

Rocky Mountain 

juniper 6.4 3.0 9.4

green ash 4.6 4.3 8.9

European crabapple 5.0 3.6 8.6

crabapple 5.6 2.7 8.3

quaking aspen 5.0 0.5 5.5

white ash 3.1 1.7 4.8

a

 percent of population

b

 percent of leaf area

c

 %Pop + %LA

Trees cover about 8.9 percent of Casper, 

shrubs cover 2.0 percent of the city. 

Dominant ground cover types include 

herbaceous (e.g., grass, gardens) (52.5 

percent), impervious surfaces (excluding 

buildings) (e.g., driveways, sidewalks, 

parking lots) (23.3 percent), and bare soil 

(15.7 percent).

Many tree benefits are linked directly to 

the amount of healthy leaf surface area of 

the plant. In Casper trees that dominate in 

terms of leaf area are plains cottonwood, 

blue spruce, and American elm.

Tree species contributing a relatively large 

amount of leaf area per tree (typically 

larger trees) are spruce, white poplar, blue 

spruce. Species contributing a relatively 

small amount of leaf area per tree (typically 

smaller trees) are pine, common juniper, 

and quaking aspen.

The importance values (IV) are calculated 

using a formula that takes into account the 

relative leaf area and relative abundance. 

The most important species in the urban 

forest, according to calculated IVs, are 

plains cottonwood, blue spruce, and 

American elm.

Urban Forest Cover and Leaf Area
0

5

10

15

20

25

30

Pollution removed (tons)

0

20

40

60

80

100

120

140

160

180

Value (thousands dollars)

Pollution Removed

Value (U.S. Dollars)

CO NO2 SO2 O3 PM10



Poor air quality is a common problem in many urban areas. It can lead to human health problems, damage to landscape 

materials and ecosystem processes, and reduced visibility. The urban forest can help improve air quality by reducing air 

temperature, directly removing pollutants from the air, and reducing energy consumption in buildings, which consequently 

reduce air pollutant emissions from power plants. Trees also emit volatile organic compounds that can contribute to ozone 

formation. However, integrative studies have revealed that an increase in tree cover leads to reduced ozone formation.12

Pollution removal by trees and shrubs in Casper was estimated using the UFORE model in conjunction with field data and 

hourly pollution and weather data for the year 2000. Pollution removal was greatest for ozone (O3), followed by particulate 

matter less than ten microns (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO). It is 

estimated that trees and shrubs remove 50 tons of air pollution (CO, NO2, O3, PM10, SO2) per year with an associated value 

of $249,000 (based on estimated national median externality costs associated with pollutants13). Trees remove about 3.1 

times more air pollution than shrubs in Casper.

Due to limited pollutant monitors in Casper, pollution concentration data for CO, NO2, and SO2 were derived from the 

nearest city in the Casper region with monitors.

Air Pollution Removal by Urban Trees
0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

0 - 33 - 66 - 9

9 - 12

12 - 1515 - 1818 - 2121 - 2424 - 2727 - 30

30+

d.b.h. class

Storage (tons)

0

50

100

150

200

250

300

350

400

Sequestration (tons)

Carbon Storage 

Carbon Sequestration

0

50

100

150

200

250

300

350

400

450

500

plains cottonwood

American elm

blue spruce

honeylocust

crabapple

European crabapple

white poplar

paper mulberry

eastern cottonwood

Engelmann spruce

Carbon Sequestration (t)

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

Value (U.S. Dollars)

Carbon Sequestration

Value (U.S. Dollars)

Climate change is an issue of global concern. Urban trees can help mitigate climate change by sequestering atmospheric 

carbon (from carbon dioxide) in tissue and by reducing energy use in buildings, and consequently reducing carbon dioxide 

emissions from fossil-fuel based power plants.14

Trees reduce the amount of carbon in the atmosphere by sequestering carbon in new tissue growth every year. The amount 

of carbon annually sequestered is increased with healthier trees and larger diameter trees. Gross sequestration by trees in 

Casper is about 1,200 tons of carbon per year with an associated value of $22,000. Net carbon sequestration in the Casper 

urban forest is about 640 tons.

Carbon storage by trees is another way trees can influence global climate change. As trees grow, they store more carbon 

by holding it in their accumulated tissue. As trees die and decay, they release much of the stored carbon back to the 

atmosphere. Thus, carbon storage is an indication of the amount of carbon that can be lost if trees are allowed to die and 

decompose. Trees in Casper are estimated to store 37,000 tons of carbon ($689,000). Of all the species sampled, plains 

cottonwood stores and sequesters the most carbon (approximately 50.9 percent of the total carbon stored and 38.2 percent 

of all sequestered carbon).

Carbon Storage and Sequestration
Annual energy use change due to trees near residential buildings

Heating Cooling Total

MBTUa -8,600 n/a -8,600

MWHb -100 500 400

Carbon avoided (t) -220 110 -110

Change in annual residential energy expendituresc (U.S. $) during heating

and cooling seasons

Heating Cooling Total

MBTUa -55,000 n/a -55,000

MWHb -6,000 34,000 29,000

Carbon avoided (t) -3,200 2,500 -700

a

Million British Thermal Units

b

Megawatt-hour

c

Based on state-wide energy costs

a

Million British Thermal Units

b

Megawatt-hour

Trees affect energy consumption by shading buildings, providing evaporative cooling, and blocking winter winds. Trees tend 

to reduce building energy consumption in the summer months and can either increase or decrease building energy use in 

the winter months, depending on the location of trees around the building. Estimates of tree effects on energy use are based 

on field measurements of tree distance and direction to space-conditioned residential buildings9.

Based on average energy costs in 2002 dollars, trees in Casper are estimated to increase energy costs from residential 

buildings by $26,700 annually. Trees also cost an additional $700 in value per year by increasing the amount of carbon 

released by fossil-fuel based power plants (an increase of 100 tons of carbon emissions).

Better placement of trees around residential buildings to avoid shading structures during the winter or to increase 

windbreaks could increase energy savings from trees in Casper.

Trees Affect Energy Use in Buildings
0

10

20

30

40

50

60

70

80

90

100

plains cottonwood

blue spruce

American elm

honeylocust

crabapple

white poplar

European crabapple

Rocky Mountain juniper

Engelmann spruce

paper mulberry

Structural value (millions of dollars)

10

Urban forests have a structural value based on the tree itself (e.g., the cost of having to replace the tree with a similar tree). 

The structural value10 of the urban forest in Casper is about $243 million. The structural value of an urban forest tends to 

increase with a rise in the number and size of healthy trees. 

Urban forests also have functional values (either positive or negative) based on the functions the tree performs. Annual 

functional values also tend to increase with increased number and size of healthy trees, and are usually on the order of 

several million dollars per year. There are many other functional values of the urban forest, though they are not quantified 

here (e.g., reduction in air temperatures and ultra-violet radiation, improvements in water quality). Through proper and 

management, urban forest values can be increased. However, the values and benefits also can decrease as the amount of 

healthy tree cover declines.

Structural values:

• Structural value: $243 million

• Carbon storage: $689,000

Annual functional values:

• Carbon sequestration: $22,000

• Pollution removal: $249,000

• Increased energy costs and carbon emissions: $-26,700

More detailed information on the urban forest in Casper can be found at http://www.nrs.fs.fed.us/UFORE/data. 

Additionally, information on other urban forest values can be found in Appendix I and information comparing tree benefits 

to estimates of average carbons emissions in the city, average automobile emissions, and average household emissions can be 

found in Appendix III.

Structural and Functional Values
0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

ALB GM EAB DED

Number of trees

0

20

40

60

80

100

120

140

160

180

Compensatory value ($ millions)

Population at risk

Compensatory Value

Kenneth R. Law 

USDA APHIS PPQ 

(www.invasive.org)

David Cappaert

Michigan State University

(www.invasive.org)

USDA Forest Service Archives 

(www.invasive.org)

11

Various insects and diseases can infest urban forests, potentially killing trees and reducing the health, value and sustainability 

of the urban forest. As various pests have differing tree hosts, the potential damage or risk of each pest will differ. Four exotic 

pests were analyzed for their potential impact: Asian longhorned beetle, gypsy moth, emerald ash borer, and Dutch elm 

disease.

The Asian longhorned beetle (ALB)15 is an insect that bores into and kills a wide range of hardwood species. ALB represents 

a potential loss to the Casper urban forest of $163 million in structural value (64.1 percent of the population).

The gypsy moth (GM)16 is a defoliator that feeds on many species causing widespread defoliation and tree death if outbreak 

conditions last several years. This pest could potentially result in damage to or a loss of $116 million in structural value 

(40.2 percent of the population).

Emerald ash borer (EAB)17 has killed thousands of ash trees in Michigan, Ohio, and Indiana. EAB has the potential to 

affect 7.7 percent of the population ($8 million in structural value).

American elm, one of the most important street trees in the 20th century, has been devastated by the Dutch elm disease 

(DED). Since first reported in the 1930s, it has killed over 50 percent of the native elm population in the United States.18

Although some elm species have shown varying degrees of resistance, Casper possibly could lose 7.0 percent of its trees to 

this disease ($24 million in structural value).

Potential Insect and Disease Impacts
12

Appendix I. Comparison of Urban Forests

A commonly asked question is, “How does this city compare to other cities?” Although comparison among cities should 

be made with caution as there are many attributes of a city that affect urban forest structure and functions, summary data 

are provided from other cities analyzed using the UFORE model.

I. City totals, trees only

City % Tree 

cover

Number of 

trees

Carbon 

storage 

(tons)

Carbon 

sequestration 

(tons/yr)

Pollution 

removal 

(tons/yr)

Pollution 

value U.S. $

Calgary, Canadaa 7.2 11,889,000 445,000 21,400 326 1,611,000

Atlanta, GAb 36.7 9,415,000 1,344,000 46,400 1,663 8,321,000

Toronto, Canadac 20.5 7,542,000 992,000 40,300 1,212 6,105,000

New York, NYb 20.9 5,212,000 1,350,000 42,300 1,677 8,071,000

Baltimore, MDd 21.0 2,627,000 597,000 16,200 430 2,129,000

Philadelphia, PAb 15.7 2,113,000 530,000 16,100 576 2,826,000

Washington, DCe 28.6 1,928,000 526,000 16,200 418 1,956,000

Boston, MAb 22.3 1,183,000 319,000 10,500 284 1,426,000

Woodbridge, NJf 29.5 986,000 160,000 5,560 210 1,037,000

Minneapolis, MNg 26.4 979,000 250,000 8,900 306 1,527,000

Syracuse, NYd 23.1 876,000 173,000 5,420 109 568,000

San Francisco, CAa 11.9 668,000 194,000 5,100 141 693,000

Morgantown, WVh 35.5 658,000 93,000 2,890 72 333,000

Moorestown, NJf 28.0 583,000 117,000 3,760 118 576,000

Jersey City, NJf 11.5 136,000 21,000 890 41 196,000

Casper, WYa 8.9 123,000 37,000 1,200 50 249,000

Freehold, NJf 34.4 48,000 20,000 545 22 110,000

II. Per acre values of tree effects

City No. of 

trees

Carbon 

Storage 

(tons)

Carbon 

sequestration 

(tons/yr)

Pollution 

removal 

(lbs/yr)

Pollution 

value U.S. $

Calgary, Canadaa 66.7 2.5 0.12 3.7 9.0

Atlanta, GAb 111.6 15.9 0.55 39.4 98.6

Toronto, Canadac 48.3 6.4 0.26 15.5 39.1

New York, NYb 26.4 6.8 0.21 17.0 40.9

Baltimore, MDd 50.8 11.6 0.31 16.6 41.2

Philadelphia, PAb 25.1 6.3 0.19 13.6 33.5

Washington, DCe 49.0 13.4 0.41 21.3 49.7

Boston, MAb 33.5 9.1 0.30 16.1 40.4

Woodbridge, NJf 66.5 10.8 0.38 28.4 70.0

Minneapolis, MNg 26.2 6.7 0.24 16.4 40.9

Syracuse, NYd 54.5 10.8 0.34 13.5 35.4

San Francisco, CAa 22.5 6.6 0.17 9.5 23.4

Morgantown, WVh 119.2 16.8 0.52 26.0 60.3

Moorestown, NJf 62.1 12.4 0.40 25.1 61.3

Jersey City, NJf 14.4 2.2 0.09 8.6 20.7

Casper, WYa 9.1 2.8 0.09 5.6 13.9

Freehold, NJf 38.3 16.0 0.44 34.9 88.2

Data collection group

a

 City personnel e Casey Trees Endowment Fund b

 ACRT, Inc. f New Jersey Department of Environmental Protection c

 University of Toronto g Davey Resource Group

d U.S. Forest Service h West Virginia University
13

Appendix II. General Recommendations for Air Quality Improvement

Urban vegetation can directly and indirectly affect local and regional air quality by altering the urban atmospheric 

environment. Four main ways that urban trees affect air quality are:

Temperature reduction and other microclimatic effects

Removal of air pollutants

Emission of volatile organic compounds (VOC) and tree maintenance emissions

Energy conservation on buildings and consequent power plant emissions

The cumulative and interactive effects of trees on climate, pollution removal, and VOC and power plant emissions 

determine the overall impact of trees on air pollution. Cumulative studies involving urban tree impacts on ozone 

have revealed that increased urban canopy cover, particularly with low VOC emitting species, leads to reduced ozone 

concentrations in cities. Local urban forest management decisions also can help improve air quality.

Urban forest management strategies to help improve air quality include:

Strategy Reason

Increase the number of healthy trees Increase pollution removal

Sustain existing tree cover Maintain pollution removal levels

Maximize use of low VOC-emitting trees Reduces ozone and carbon monoxide formation

Sustain large, healthy trees Large trees have greatest per-tree effects

Use long-lived trees Reduce long-term pollutant emissions from planting and removal

Use low maintenance trees Reduce pollutants emissions from maintenance activities

Reduce fossil fuel use in maintaining vegetation Reduce pollutant emissions

Plant trees in energy conserving locations Reduce pollutant emissions from power plants

Plant trees to shade parked cars Reduce vehicular VOC emissions

Supply ample water to vegetation Enhance pollution removal and temperature reduction

Plant trees in polluted or heavily populated areas Maximizes tree air quality benefits

Avoid pollutant-sensitive species Improve tree health

Utilize evergreen trees for particulate matter Year-round removal of particles
14

Appendix III. Relative Tree Effects

The urban forest in Casper provides benefits that include carbon storage and sequestration, and air pollutant removal. To 

estimate a relative value of these benefits, tree benefits were compared to estimates of average carbon emissions in the city19, 

average passenger automobile emissions20, and average household emissions21.

General tree information:

Average tree diameter (d.b.h.) = 9.3 in.

Median tree diameter (d.b.h.) = 4.9 in.

Average number of trees per person = 2.4

Number of trees sampled = 235

Number of species sampled = 47

Average tree effects by tree diameter:

Pollution

Carbon storage Carbon sequestration removal

D.b.h.

Class (inch) (lbs) ($) (miles) a (lbs/yr) ($/yr) (miles) a (lbs) ($)

1-3 6 0.06 20 1.8 0.02 7 0.1 0.21

3-6 41 0.38 150 7.1 0.07 26 0.3 0.67

6-9 143 1.31 520 14.1 0.13 51 0.6 1.48

9-12 310 2.85 1,130 19.5 0.18 71 0.9 2.12

12-15 506 4.66 1,850 21.6 0.20 79 0.7 1.72

15-18 854 7.87 3,130 35.2 0.32 129 0.9 2.17

18-21 1,185 10.92 4,340 36.9 0.34 135 1.7 4.13

21-24 1,815 16.72 6,650 44.8 0.41 164 1.7 4.21

24-27 2,408 22.18 8,820 58.4 0.54 214 1.6 4.03

27-30 2,780 25.61 10,180 76.6 0.71 281 1.2 2.99

30+ 5,347 49.25 19,580 106.5 0.98 390 2.8 7.06

a miles = number of automobile miles driven that produces emissions equivalent to tree effect

The Casper urban forest provides:

Carbon storage equivalent to:

Amount of carbon (C) emitted in city in 44 days or

Annual carbon emissions from 22,000 automobiles or 

Annual C emissions from 11,300 single family houses 

Carbon monoxide removal equivalent to:

Annual carbon monoxide emissions from 4 automobiles or

Annual carbon monoxide emissions from 15 single family 

houses 

Nitrogen dioxide removal equivalent to:

Annual nitrogen dioxide emissions from 160 automobiles or

Annual nitrogen dioxide emissions from 110 single family 

houses 

Sulfur dioxide removal equivalent to:

Annual sulfur dioxide emissions from 5,700 automobiles or

Annual sulfur dioxide emissions from 90 single family houses 

Particulate matter less than 10 micron (PM10

) removal 

equivalent to:

Annual PM10 emissions from 28,200 automobiles or

Annual PM10 emissions from 2,700 single family houses 

Annual C sequestration equivalent to:

Amount of C emitted in city in 1.4 days or

Annual C emissions from 700 automobiles or

Annual C emissions from 400 single family homes
15

Percent Percent Potential Pestb

Genus Species Common Name Population Leaf Area IVa ALB GM EAB DED

Acer ginnala Amur maple 0.4 0.2 0.6 o

Broussonetia papyrifera paper mulberry 0.6 0.7 1.3

Cornus florida flowering dogwood 0.4 0 0.4

Elaeagnus angustifolia Russian olive 2.2 0.6 2.8 o

Fraxinus americana white ash 3.1 1.7 4.8 o o

Fraxinus pennsylvanica green ash 4.6 4.3 8.9 o o

Gleditsia triacanthos honeylocust 6.3 3.7 10

Gymnocladus dioicus Kentucky coffeetree 0.4 0.1 0.5

Juniperus communis common juniper 2.1 0.1 2.2

Juniperus scopulorum Rocky Mountain juniper 6.4 3 9.4

Malus prunifolia plumleaf crabapple 0.4 0.7 1.1

Malus pumila apple 0.4 0 0.4 o o

Malus species crabapple 5.6 2.7 8.3 o o

Malus sylvestris European crabapple 5 3.6 8.6

Picea engelmannii Engelmann spruce 0.4 1.5 1.9

Picea mariana black spruce 0.8 2.2 3

Picea pungens blue spruce 8.1 16.1 24.2

Picea species spruce 1 3.6 4.6

Pinus edulis pinyon pine 0.4 0.1 0.5

Pinus maritima pinus maritima 0.5 0.5 1

Pinus nigra Austrian pine 1.5 0.2 1.7

Pinus species pine 1.5 0 1.5

Pinus sylvestris scotch pine 0.4 0.1 0.5

Poitea punicea caracol illo 0.5 0 0.5

Populus alba white poplar 1.3 3.5 4.8 o

Populus angustifolia narrowleaf cottonwood 1.5 0.5 2 o o

Populus deltoides eastern cottonwood 0.4 2.6 3 o

Populus nigra black poplar 0.4 0.2 0.6 o

Populus sargentii plains cottonwood 17.5 32.2 49.7 o o

Continued

Appendix IV. Tree Species Sampled in Casper
16

Percent Percent Potential Pestb

Genus Species Common Name Population Leaf Area IVa ALB GM EAB DED

Populus tremuloides quaking aspen 5 0.5 5.5 o o

Prestoea montana Sierra palm 0.4 0.1 0.5

Prunus species cherry 0.4 0.1 0.5 o

Prunus virginiana common chokecherry 3.1 0.6 3.7 o

Purshia pinkavea Pinkava’s cliffrose 2.3 0.2 2.5

Quercus alba white oak 0.6 0 0.6 o

Quercus macrocarpa bur oak 0.4 0 0.4 o

Salix alba white willow 0.4 0 0.4 o o

Salix babylonica weeping willow 0.4 0.2 0.6 o o

Salix matsudana corkscrew willow 0.8 0.9 1.7 o

Salix species willow 0.4 0 0.4 o o

Sassafras albidum sassafras 0.4 0.4 0.8

Syringa persica Persian lilac 0.6 0 0.6

Tilia americana American basswood 0.4 0 0.4 o o

Tilia cordata littleleaf linden 0.8 0.1 0.9 o o

Tilia europaea European linden 0.4 0.5 0.9 o o

Ulmus americana American elm 6.7 10.9 17.6 o o

Ulmus pumila Siberian elm 2.5 0.6 3.1 o

Ulmus serotina September elm 0.4 0.1 0.5 o o

a

 IV = importance value (% population + % leaf area)

b

 ALB = Asian longhorned bettel; GM = gypsy moth; EAB = emerald ash borer; DED = Dutch elm disease

Appendix IV continued
17

References

1 Nowak, D.J.; Crane, D.E. 2000. The Urban Forest

Effects (UFORE) Model: quantifying urban forest

structure and functions. In: Hansen, M.; Burk, T., 

eds. Integrated tools for natural resources inventories 

in the 21st century. Proceedings of IUFRO 

conference. Gen. Tech. Rep. NC-212. St. Paul, MN: 

U.S. Department of Agriculture, Forest Service, 

North Central Research Station: 714-720.

2 Nowak, D.J.; Crane, D.E.; Stevens, J.C.; Hoehn, R.E. 

2005. The urban forest effects (UFORE) model:

field data collection manual. V1b. [Newtown 

Square, PA]: U.S. Department of Agriculture, 

Forest Service, Northeastern Research Station. 34 p. 

http://www.fs.fed.us/ne/syracuse/Tools/downloads/

UFORE_Manual.pdf

3 Nowak, D.J. 1994. Atmospheric carbon dioxide

reduction by Chicago’s urban forest. In: 

McPherson, E.G.; Nowak, D.J.; Rowntree, R.A., 

eds. Chicago’s urban forest ecosystem: results of 

the Chicago Urban Forest Climate Project. Gen. 

Tech. Rep. NE-186. Radnor, PA: U.S. Department 

of Agriculture, Forest Service, Northeastern Forest 

Experiment Station: 83-94.

4 Baldocchi, D. 1988. A multi-layer model for

estimating sulfur dioxide deposition to a

deciduous oak forest canopy. Atmospheric 

Environment. 22: 869-884.

5 Baldocchi, D.D.; Hicks, B.B.; Camara, P. 1987. A

canopy stomatal resistance model for gaseous

deposition to vegetated surfaces. Atmospheric 

Environment. 21: 91-101.

6 Bidwell, R.G.S.; Fraser, D.E. 1972. Carbon

monoxide uptake and metabolism by leaves.

Canadian Journal of Botany. 50: 1435-1439.

7 Lovett, G.M. 1994. Atmospheric deposition of

nutrients and pollutants in North America: an

ecological perspective. Ecological Applications. 

4:629-650.

8 Zinke, P.J. 1967. Forest interception studies in the

United States. In: Sopper, W.E.; Lull, H.W., eds. 

Forest hydrology. Oxford, UK: Pergamon Press: 137-

161.

9 McPherson, E.G.; Simpson, J.R. 1999. Carbon

dioxide reduction through urban forestry:

guidelines for professional and volunteer tree

planters. Gen. Tech. Rep. PSW-171. Albany, CA: 

U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 237 p. Can 

be accessed through http://www.fs.fed.us/psw/

publications/gtrs.shtml

10 Nowak, D.J.; Crane, D.E.; Dwyer, J.F. 2002. 

Compensatory value of urban trees in the United

States. Journal of Arboriculture. 28(4): 194-199.

11 Nowak, D.J.; Crane, D.E.; Stevens, J.C.; Ibarra, 

M. 2002. Brooklyn’s urban forest. Gen. Tech. Rep. 

NE-290. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Research 

Station. 107 p.

12 Nowak D.J.; Dwyer, J.F. 2000. Understanding the

benefits and costs of urban forest ecosystems.

In: Kuser, John E., ed. Handbook of urban and 

community forestry in the northeast. New York: 

Kluwer Academics/Plenum: 11-22.

13 Murray, F.J.; Marsh L.; Bradford, P.A. 1994. New

York state energy plan, vol. II: issue reports.

Albany, NY: New York State Energy Office. 

14 Abdollahi, K.K.; Ning, Z.H.; Appeaning, A., eds. 

2000. Global climate change and the urban forest.

Baton Rouge, LA: GCRCC and Franklin Press. 77 p.

15 Northeastern Area State and Private Forestry. 2005. 

Asian Longhorned Beetle. Newtown Square, PA: 

U.S. Department of Agriculture, Northeastern Area 

State and Private Forestry. http://www.na.fs.fed.

us/fhp/alb/

16 Northeastern Area State and Private Forestry. 

2005. Gypsy moth digest. Newtown Square, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Area State and Private Forestry. http://

www.na.fs.fed.us/fhp/gm/
18

17 Northeastern Area State and Private Forestry. 2005. 

Forest health protection emerald ash borer

home. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Area State 

and Private Forestry. http://www.na.fs.fed.us/fhp/eab

18 Stack, R.W.; McBride, D.K.; Lamey, H.A. 1996. 

Dutch elm disease. PP-324 (revised). Fargo, ND: 

North Dakota State University, Cooperative Extension 

Service. http://www.ext.nodak.edu/extpubs/plantsci/

trees/pp324w.htm
19

Explanation of Calculations of Appendix III and IV

19 Total city carbon emissions were based on 2003 

U.S. per capita carbon emissions, calculated as 

total U.S. carbon emissions (Energy Information 

Administration, 2003, Emissions of Greenhouse 

Gases in the United States 2003. http://www.eia.

doe.gov/oiaf/1605/1605aold.html ) divided by 

2003 total U.S. population (www.census.gov). Per 

capita emissions were multiplied by Minneapolis 

population to estimate total city carbon emissions.

20 Average passenger automobile emissions per 

mile were based on dividing total 2002 pollutant 

emissions from light-duty gas vehicles (National 

Emission Trends http://www.epa.gov/ttn/chief/

trends/index.html) by total miles driven in 2002 by 

passenger cars (National Transportation Statistics 

http://www.bts.gov/publications/national_

transportation_statistics/2004/).

Average annual passenger automobile emissions 

per vehicle were based on dividing total 2002 

pollutant emissions from light-duty gas vehicles 

by total number of passenger cars in 2002 

(National Transportation Statistics http://www.

bts.gov/publications/national_transportation_

statistics/2004/).

Carbon dioxide emissions from automobiles 

assumed 6 pounds of carbon per gallon of gasoline 

with energy costs of refi nement and transportation 

included (Graham, R.L.; Wright, L.L.; Turhollow, 

A.F. 1992. The potential for short-rotation woody 

crops to reduce U.S. CO2 emissions. Climatic 

Change. 22:223-238.)

21 Average household emissions based on average 

electricity kWh usage, natural gas Btu usage, fuel oil 

Btu usage, kerosene Btu usage, LPG Btu usage, and 

wood Btu usage per household from: 

Energy Information Administration. Total Energy 

Consumption in U.S. Households by Type of 

Housing Unit, 2001 www.eia.doe.gov/emeu/recs/

recs2001/detailcetbls.html. 

CO2, SO2, and NOx power plant emission per 

KWh from: 

U.S. Environmental Protection Agency. U.S. 

power plant emissions total by year www.epa.

gov/cleanenergy/egrid/samples.htm.

CO emission per kWh assumes one-third of 1

percent of C emissions is CO based on:

Energy Information Administration. 1994. 

Energy use and carbon emissions: non-OECD 

countries. DOE/EIA-0579(94). Washington, 

DC: epartment of Energy, Energy Information 

Administration. http://tonto.eia.doe.gov/

bookshelf

PM10 emission per kWh from: 

Layton, M. 2004. 2005 Electricity environmental 

performance report: electricity generation and 

air emissions. Sacramento, CA: California 

Energy Commission. http://www.energy.

ca.gov/2005_energypolicy/documents/2004-11-

15_workshop/2004-11-15_03- A_LAYTON.PDF

CO2, NOx, SO2, PM10, and CO emission per 

Btu for natural gas, propane and butane (average 

used to represent LPG), Fuel #4 and #6 (average 

used to represent fuel oil and kerosene) from:

Abraxas energy consulting. http://www. 

abraxasenergy.com/emissions/

CO2 and fine particle emissions per Btu of wood 

from:

Houck, J.E.; Tiegs, P.E.; McCrillis, R.C.; 

Keithley, C.; Crouch, J. 1998. Air emissions from 

residential heating: the wood heating option put 

into environmental perspective. In: Proceedings 

of U.S. EPA and Air and Waste Management 

Association conference: living in a global 

environment, V.1: 373-384.
20

CO, NOx and SOx emission per Btu of wood 

based on total emissions from wood burning 

(tonnes) from:

Residential Wood Burning Emissions in British 

Columbia. 2005. http://www.env.gov.bc.ca/air/

airquality/pdfs/wood_emissions.pdf.

Emissions per dry tonne of wood converted to 

emissions per Btu based on average dry weight per 

cord of wood and average Btu per cord from:

Kuhns, M.; Schmidt, T. 1988. Heating with wood: 

species characteristics and volumes I. NebGuide 

G-88-881-A. Lincoln, NE: University of Nebraska, 

Institute of Agriculture and Natural Resources, 

Cooperative Extension.
Nowak, David J.; Hoehn, Robert E. III; Crane, Daniel E.; Stevens, Jack, C.; Walton, 

Jeffrey T. 2006 Assessing urban forest effects and values, Casper’s urban 

forest. Resour. Bull. NRS-4. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. 20 p.

An analysis of trees in Casper, WY reveals that this city has about 123,000 trees with 

canopies that cover 8.9 percent of the area. The most common tree species are plains 

cottonwood, blue spruce, and American elm. The urban forest currently store about 

37,000 tons of carbon valued at $689,000. In addition, these trees remove about 

1,200 tons of carbon per year ($22,000 per year) and about 50 tons of air pollution per 

year ($249,000 per year). The structural, or compensatory, value is estimated at $243 

million. Information on the structure and functions of the urban forest can be used to 

improve and augment support for urban forest management programs and to integrate 

urban forests within plans to improve environmental quality in the Casper area.

KEY WORDS: urban forestry; ecosystem services, air pollution removal; carbon

sequestration; tree value
www.nrs.fs.fed.us

Capitalizing on the strengths of existing science capacity in the 

Northeast and Midwest to attain a more integrated, cohesive, 

landscape-scale research program

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