Memorandum
Date:
May 7, 2007
To:
Mary Roby and Darin Crew, HRWA
Rebecca Feldman, Baltimore City Forestry
Halle Vandergaag and Suzanne Greene, JFWA
Christine Dunham, CBT
From:
Paul Sturm and Julie Tasillo
Re:
A summary of the “Benefits of Urban Trees” for submittal and discussion
with MDE
8390 Main Street, 2nd Floor
Ellicott City, MD 21043
410.461.8323
FAX 410.461.8324
www.cwp.org
www.stormwatercenter.net
Benefits of Trees
A Summary of Existing Research in Support of the Baltimore Urban Tree Canopy
compiled for Herring Run Watershed Association, Baltimore City Forestry Department,
Jones Falls Watershed Association and Chesapeake Bay Trust
Introduction
Trees are a vital part of our environment and provide numerous benefits including;
protection of water quality, habitat for fish and wildlife, improvement of air quality and
quality of life, and encourage recreation and benefit the economy. This letter summarizes
the benefits trees provide to air quality and water quality.
Air Quality Benefits
Trees are often called the “lungs of our cities” because of their ability to remove
contaminants from the air. Trees remove pollutants from the air, reduce air temperatures,
lower concentrations of ozone, and indirectly provide utility savings. The major air
pollutants and their sources that affect our environment include:
 Carbon dioxide: burning oil, coal, natural gas for energy.
 Sulfur dioxide: burning coal to generate electricity.
 Ozone: chemical reactions of sunlight on automobile exhaust gases. Ozone is a
major pollutant in smog.
 Methane: Burning fossil fuels, livestock waste, landfills and rice production.
 Nitrous oxides: burning fossil fuels and automobile exhausts.
 Chloroflorocarbons: air conditioners, refrigerators, industrial foam.
Trees remove pollution from the air through the absorption of gaseous pollutants (such as
ozone, nitrogen oxides, and sulfur dioxide) through leaf surfaces and the interception of
particulate matter (such as dust, ash, pollen and smoke) on plant surfaces. Trees take up
carbon dioxide and use it for growth. Urban tree canopies in Baltimore, Maryland, and
Washington, D.C., absorbed and intercepted more than 500 and nearly 400 metric tons of
air pollution respectively in 2000 (Conservation Fund, 2006). In Sacramento, California,
a study found that the city’s six million trees removed approximately 335 thousand tons
of atmospheric carbon dioxide annually, with an implied value of $3.3 million
(McPherson, 1998). As air temperatures increase, the formation of ozone increases. By
cooling the air and shading impervious surfaces, trees can reduce ozone concentrations.
(Arborist News, 2004). In Chicago, the regions 50.8 million trees were estimated to
remove 2.34 tons of Particulate Matter, 210 tons of ozone, 93 tons of sulfur dioxide, and
17 tons of carbon monoxide annually. These environmental services were valued at $9.2
million (Nowak, 1994).
Trees reduce the urban heat island effect by cooling the air through evapotranspiration
and shading impervious surfaces. Air temperatures can be 4 to 8°F cooler in well-shaded
parking lots than in unshaded parking lots (McPherson, 1998). Similarly, air
temperatures in neighborhoods with mature canopy were 3 to 6°F lower in daytime than
in newer neighborhoods with no trees (Akbari et. al., 1992). Other studies have found
that trees reduce surface asphalt temperatures by up to 36 °F, and vehicle cabin
temperatures by 47°F (CUFR, 2001).
Trees ability to cool the air translates into energy savings. Trees shade buildings in the
summer and act as wind blocks in the winter. When buildings use less energy, pollutant
emissions form power plants also are reduced. McPherson and Simpson (2003) found
that planting 50 million more shade trees in California cities would provide savings
equivalent to seven 100-megawatt power plants. The urban tree canopy in Washington,
D.C. saves residents approximately $2.6 million dollars per year (Conservation Fund,
2006). Trees planted as windbreaks can provide annual heating savings of 10 to 12
percent (Heisler,1986).
Table 1 shows a summary of cost savings provided by an acre of trees in the BaltimoreWashington region. This data is based on the 1997 tree cover value (American Forests,
1999). The dollar value of these pollutants are calculated by multiplying the number of
tons of pollutants by an “externality cost,” or costs to society that are not reflected in
marketplace activity.
Table 1. Baltimore-Washington Air Quality Benefits (1997 annual benefits)
Cost Savings
Pollutants
(per metric ton/acre of forest)
Carbon Monoxide (CO)
$3.33
Sulfur Dioxide (SO2)
$5.77
Nitrogen Dioxide (NO2) and Ozone (O3)
$23.62
Particulate Matter 10 microns or less (PM10)
$15.75
American Forests, 1999
Stormwater Benefits
Trees provide natural stormwater management by intercepting rainfall and slowing the
rate at which it runs over the land. This results in a reduction of stormwater which
decreases downstream flooding and stream degradation. Reducing the volume of
stormwater and its peak flow reduces the size and cost of stormwater structures. By
incorporating trees into a city’s infrastructure, managers can build a smaller, less
expensive stormwater management system. Guidance for planting trees in stormwater
treatment practices are presented in Cappiella et al., 2006.
Trees are also natural filters of pollution, such as metals, pesticides, and organic
compounds, and use nutrients like nitrogen, phosphorus, and potassium thus reducing
their presence in the environment. It was found that one sugar maple (1 foot in diameter)
along a roadway was shown to retain 60 milligrams (mg) cadmium, 140 mg chromium,
820 mg nickel and 5,200 mg lead from the environment in one growing season (Coder,
1996). Table 2 summarizes data on rainfall interception, evapotranspiration, air pollution
and nutrient uptake for a single tree. The data shows a reduction of 760 gallons of
stormwater runoff per year along with a nitrogen reduction of 0.05 pounds per year per
tree. It should be noted that the numbers shown are examples from specific studies of
mature trees, and are somewhat oversimplified. The actual per tree benefit will vary
greatly with tree species, age, climate, condition, etc., and additional research may be
needed to define the credit that can be attributed to an individual of a specific tree or acre
of forest.
Table 2. Hydrologic and Water Quality Benefits of Trees
Per Tree Annual
Benefit
Source and Description
Quantification of Benefit
Rainfall Interception
500 - 760 gallons of
Annual rainfall interception
water per tree per year
by a large deciduous front
yard tree (CUFR, 2001)
Evapotranspiration
100 gallons of water per
Transpiration rate of poplar
tree per year
trees for one growing season
(EPA, 1998)
Nitrogen Uptake
0.05 pounds nitrogen per Based on daily rate of
tree per year
nitrogen uptake by poplar
trees (Licht, 1990)
NOx (from air)
1 lb per year
Component of acid rain and
nutrient runoff (CUFR, 2001)
O3 (Ozone)
4 lbs per year
Ground level ozone hazardous
to human health (CUFR,
2001)
Particulates
3 lbs per year
Pollutants are linked to
respiratory problems (asthma
and diseases) (CUFR, 2001)
CO2
48 lbs per year
Increase is root cause in
climate change (CUFR, 2001)
Carbon
13 lbs per year
Carbon sequestration by tree
incorporation (Coder, 1996)
References
Akbari, H., S. Davis, S. Dorsano, J. Huang, and S. Winnett, eds., 1992. Cooling our
communities: a guidebook on tree planting and light-colored surfacing. 22P-2001.
Washington, DC: U.S. Environmental Protection Agency. 217p.
American Forests, 1999. Final Report. Regional Ecosystem Analysis Chesapeake Bay
Region and the Baltimore-Washington Corridor.
https://www.americanforests.org/downloads/rea/AF_Chesapeake.pdf.
Arborist News, 2004. Continuing Education Unit. www.isa-arbor.com. December 2004.
Cappiella, K., T. Schueler, and T. Wright. 2005. Urban Watershed Forestry Manual.
Part 2: Conserving and Planting Trees at Development Sites. USDA Forest Service,
Newtown, PA.
Center for Urban Forest Research. 2001. Benefits of the urban forest: Fact Sheet #1.
Davis, CA: USDA Forest Service, Pacific Southwest Research Station.
Coder, R. 1996. Identified Benefits of Community Trees and Forests. University of
Georgia Cooperative Extension Service Forest Resources Unit Publication FOR96-39;
October 1996.
The Conservation Fund. 2006. The State of Chesapeake Forests. E. Sprague, D. Burke,
S. Claggett, and A. Todd (Eds.).
EPA, 1998, A Citizen's Guide to Phytoremediation, U.S. Environmental Protection
Agency, Office of Solid Waste and Emergency Response, EPA 542-F-98-011, August.
Heisler, G.M. 1986. Energy savings with trees. Journal of Arboriculture 12(5):113-125.
Licht, L.A. (1990) Deep-rooted Popular Trees Grown in the Riparian Zone for Biomass
Production and Non-point Source Pollution Control. Ph.D. Thesis, University of Iowa.
McPherson, E.G., 1998. Shade trees and parking lots. Arid Zone Times. February: 1-2.
McPherson, E.G., and J.R. Simpson. 2003. Potential energy savings in buildings by an
urban tree planting program in California. Urban Forestry & Urban Greening 2:73-86.
Nowak, D.J. 1994. Air pollution removal by Chicago’s urban forest, pp 63-82. In
McPherson, E.G., D.J. Nowak, and R.A. Rowntree (Eds.). Chicago’s Urban Forest
Ecosystem: Results of the Chicago Urban Forest Climate Project. GRE NE-186, USDA
Forest Service, Northeastern Forest Experiment Station, Radnor, PA.