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Understanding the Benefits and Costs of Urban Forest Ecosystems

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Abstract

One of the first considerations in developing a strong and comprehensive urban forestry program is determining the desired outcomes from managing vegetation in cities. Urban trees can provide a wide range of benefits to the urban environment and well-being of people. However, there are also a wide range of potential costs and as with all ecosystems, numerous interactions that must be understood if one is to optimize the net benefits from urban vegetation. Inadequate understanding of the wide range of benefits, costs, and expected outcomes of urban vegetation management options, as well as interactions among them, may drastically reduce the contribution of vegetation toward improving urban environments and quality of life. By altering the type and arrangement of trees in a city (i.e., the urban forest structure), one can affect the city’s physical, biological, and socioeconomic environments. Management plans can be developed and implemented to address specific problems within cities. Although trees can provide multiple benefits at one site, not all benefits can necessarily be realized in each location. Individual management plans should focus on optimizing, in a particular area, the mix of benefits that are most important. 2. Urban Land in the United States The importance of urban forests and their benefits in the United States is increasing because of the expansion of urban land. The percentage of the coterminous land in United States, classified as urban, increased from 2.5% in 1990 to 3.1% in 2000, an area about the size of Vermont and New Hampshire combined. The states with the
1. Introduction
One of the first considerations in developing a strong and comprehensive urban
forestry program is determining the desired outcomes from managing vegetation in
cities. Urban trees can provide a wide range of benefits to the urban environment and
well-being of people. However, there are also a wide range of potential costs and as
with all ecosystems, numerous interactions that must be understood if one is to opti-
mize the net benefits from urban vegetation. Inadequate understanding of the wide
range of benefits, costs, and expected outcomes of urban vegetation management
options, as well as interactions among them, may drastically reduce the contribution
of vegetation toward improving urban environments and quality of life.
By altering the type and arrangement of trees in a city (i.e., the urban forest struc-
ture), one can affect the city’s physical, biological, and socioeconomic environments.
Management plans can be developed and implemented to address specific problems
within cities. Although trees can provide multiple benefits at one site, not all benefits
can necessarily be realized in each location. Individual management plans should focus
on optimizing, in a particular area, the mix of benefits that are most important.
2. Urban Land in the United States
The importance of urban forests and their benefits in the United States is increas-
ing because of the expansion of urban land. The percentage of the coterminous land
in United States, classified as urban, increased from 2.5% in 1990 to 3.1% in 2000, an
area about the size of Vermont and New Hampshire combined. The states with the
25
Urban and Community Forestry in the Northeast, 2nd ed., edited by, J. E. Kuser.
© 2007 Springer.
Chapter 2
Understanding the Benefits and Costs
of Urban Forest Ecosystems
David J. Nowak1and John F. Dwyer2
David J. Nowak USDA Forest Service, Northeastern Research Station, Syracuse, New York
John F. Dwyer The Morton Arboretum, Lisle, Illinois
highest percentage of urban land are New Jersey (36.2%), Rhode Island (35.9 %),
Connecticut (35.5%), and Massachusetts (34.2%) with 7 of the top 10, most urban-
ized states in the Northeast United States (Fig. 1) (Nowak et al., 2005).
The most urbanized regions of the United States are the Northeast (9.7%) and the
Southeast (7.5%), with these regions also exhibiting the greatest increase in percentage
of urban land between 1990 and 2000 (1.5% and 1.8%, respectively). States with the
greatest increase in percentage of urban land between 1990 and 2000 were Rhode Island
(5.7%), New Jersey (5.1%), Connecticut (5.0%), Massachusetts (5.0%), Delaware
(4.1%), Maryland (3.0%), and Florida (2.5%) (Nowak et al., 2005). Nationally, urban
tree cover in the United States averages 27.1%. However, urban tree cover tends to be
highest in forested regions (34.4% urban tree cover), followed by grasslands (17.8%),
and deserts (9.3%) (Dwyer et al., 2000; Nowak et al., 2001).
Patterns of urban growth reveal that increased growth rates are likely in the future
(Nowak et al., 2005). As the Northeast is the most urbanized region of the country and
is likely to have some of the greatest increases in urban land growth over the next sev-
eral decades, understanding the benefits and costs of urban vegetation is paramount
to sustain human health and environmental quality in this region.
3. Physical/Biological Benefits and Costs of Urban Vegetation
Through proper planning, design, and management, urban trees can mitigate many
of the environmental impacts of urban development by moderating climate, reducing
building energy-use and atmospheric carbon dioxide (CO2), improving air quality,
26 David J. Nowak and John F. Dwyer
FIGURE 1. Urban areas in coterminous United States (2000) based on the US Census Bureau Definition
of Urban Land.
lowering rainfall runoff and flooding, and reducing noise levels. However, inappropri-
ate landscape designs, tree selection, and tree maintenance can increase environmental
costs, such as pollen production and chemical emissions from trees and maintenance
activities that contribute to air pollution, and also increase building energy-use, waste
disposal, infrastructure repair, and water consumption. These potential costs must be
weighed against the environmental benefits in developing management programs.
3.1. Urban Atmosphere
Trees influence the urban atmosphere in the following four general, interactive
ways that can be remembered by using the word TREE (Nowak, 1995): (1) Temperature
and microclimatic effects, (2) Removal of air pollutants, (3) Emission of volatile organ-
ic compounds by trees and emissions due to tree maintenance, and (4) Energy conser-
vation in buildings and consequent effects on emissions from power plants. The cumu-
lative effect of these four factors determines the overall impact of urban trees on the
urban atmosphere and particularly air pollution.
3.1.1. Temperature and Microclimatic Modifications
Trees influence climate at a range of scales, from an individual tree to a forest
covering an entire metropolitan area. By transpiring water, altering windspeeds, shad-
ing surfaces, and modifying the storage and exchanges of heat among urban surfaces,
trees affect local climate and thereby influence thermal comfort and air quality. Often,
one or more of these microclimatic influences of trees produces an important benefit,
while other influences can reduce benefits or increase costs (Heisler et al., 1995).
Trees alter windspeed and direction. Dense tree crowns have a significant impact
on wind, but for isolated trees, their influence nearly disappears within a few crown
diameters downwind (Heisler et al., 1995). Several trees on a residential lot, in con-
junction with trees throughout the neighborhood, reduce windspeed significantly. In
a residential neighborhood in central Pennsylvania with 67% tree cover, windspeeds
at 2 m above ground level were reduced by 60% in winter and 67% in summer com-
pared to windspeeds in a comparable neighborhood with no trees (Heisler, 1990a).
Trees also have a dramatic influence on incoming solar radiation, and can reduce
it by 90% or more (Heisler, 1986). Some of the radiation absorbed by tree canopies
leads to the evaporation and transpiration of water from leaves. This evapotranspira-
tion cools tree leaves and the air. Despite large amounts of energy used for evapo-
transpiration on sunny days, air movement rapidly disperses cooled air, thereby dis-
persing the overall cooling effect. Under individual and small groups of trees, air tem-
perature at 1.5 m above the ground is usually within 1°C of the air temperatures in an
open area (Souch and Souch, 1993). Along with transpirational cooling, tree shade
can help cool the local environment by reducing the solar heating of some below-
canopy artificial surfaces (e.g., buildings, parking lots). Together these effects can
reduce air temperatures by as much as 5°C (Akbari et al., 1992).
Although trees usually contribute to cooler summer air temperatures, their pres-
ence can increase air temperatures in some instances (Myrup et al., 1991). In areas
with scattered tree canopies, radiation can reach and heat ground surfaces; at the same
time, the canopy may reduce atmospheric mixing such that cooler air is prevented
Benefits and Costs of Urban Forest Ecosystems 27
from reaching the area. In this case, tree shade and transpiration may not compensate
for the increased air temperatures due to reduced mixing (Heisler et al., 1995). Thus,
it is important to recognize that it is the combined effects of trees on radiation, wind,
and transpirational cooling that affect local air temperatures and climate.
Besides providing for transpirational cooling, the physical mass and thermal/
radiative properties of trees can affect other aspects of local meteorology and micro-
climate, such as ultraviolet radiation loads, relative humidity, turbulence, albedo, and
boundary-layer heights (i.e., the height of the layer of atmosphere that, because of
turbulence, interacts with the earth’s surface on a time scale of a few hours or less
(Lenschow, 1986)).
3.1.2. Removal of Air Pollutants
Trees remove gaseous air pollution primarily by uptake through leaf stomata,
though some gases are removed by the plant surface (Smith, 1990). Once inside the
leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form
acids or react with the inner surfaces of leaves. Trees also remove pollution by inter-
cepting airborne particles. Some particles can be absorbed into the tree (Ziegler, 1973;
Rolfe, 1974), though most intercepted particles are retained on the plant surface.
Often vegetation is only a temporary retention site for atmospheric particles as the
intercepted particles may be resuspended to the atmosphere, washed off by rain, or
dropped to the ground with leaf and twig fall (Smith, 1990).
Pollution removal by trees in a city varies throughout the year (Fig. 2).
28 David J. Nowak and John F. Dwyer
0
10
20
30
40
50
60
70
JFMAMJJASOND
Month
Removal (t)
O3
PM10
NO2
SO2
CO
FIGURE 2. Monthly pollution removal by trees (metric tons) in Philadelphia, PA (1994). PM10 = partic-
ulate matter <10 microns; O3= ozone; NO2= nitrogen dioxide; SO2= sulfur dioxide; CO = carbon monoxide.
PM10 removal assumes 50% resuspension of particles. City area = 350 km2; tree cover = 21.6%.
Factors that affect pollution removal by trees include the amount of healthy leaf-
surface area, concentrations of local pollutants, and local meteorology. Computer
simulations using the Urban Forest Effects Model (Nowak and Crane, 2000, Nowak
et al., 2002b) with local field data reveal that pollution removal by urban trees in var-
ious cities range from 19 metric tons per year in Freehold, New Jersey to over 1500
metric tons per year in Atlanta and New York (Table 1). Pollution removal was typi-
cally greatest for ozone, followed by particulate matter less than 10 microns, nitrogen
dioxide, sulfur dioxide, and carbon monoxide. Value of pollution removal, based on
national median externality values for each pollutant (Murray et al., 1994), ranged
from $109,000 in Freehold to $8.3 million in Atlanta.
Average annual pollution removal per square meter of canopy cover was 9.3 g,
but ranged between 6.6 g/m2in Syracuse and 12.0 g/m2in Atlanta (Table 1). The aver-
age annual dollar value per hectare of tree cover was $500, but ranged between
$378/ha cover in Syracuse and $663/ha cover in Atlanta. As existing canopy cover in
cities remove significant amounts of air pollution, increasing tree cover in urban areas
will lead to greater pollution removal, as well as reduced air temperatures that can
help improve urban air quality.
Average improvement in air quality from pollution removal by trees during the
daytime of the in-leaf season among 14 cities (Table 1) was 0.62% for particulate mat-
ter less than 10 microns (PM10), 0.61% for ozone (O3), 0.60% for sulfur dioxide (SO2),
0.39% for nitrogen dioxide (NO2), and 0.002% for carbon monoxide (CO). Air quali-
ty improvement increases with increased percentage of tree cover and decreased
boundary-layer heights. In urban areas (Table 1) with 100% tree cover (i.e., contigu-
ous forest stands), short-term improvements (1 h) in air quality due to pollution
removal from trees were as high as 14.9% for SO2, 14.8% for O3, 13.6% for PM10,
8.3% for NO2, and 0.05% for CO. In Chicago in 1991, large, healthy trees—those >77
cm in diameter at breast height (dbh)—removed an estimated 1.4 kg of pollution,
about 70 times more pollution than small (<7 cm dbh) trees (Nowak, 1994a).
Trees can also reduce atmospheric CO2by directly storing carbon (C) from CO2
as they grow. Large trees store approximately 3 metric tons of carbon (tC) or 1000
times more carbon than stored by small trees (Nowak, 1994b). Healthy trees contin-
ue to sequester additional carbon each year; large, healthy trees sequester about 93 kg
C/year vs 1 kg C/year for small trees. Net annual sequestration by trees in the Chicago
area (140,600 tC) equals the amount of carbon emitted from transportation in the
Chicago area in about 1 week (Nowak, 1994b).
Urban trees in the coterminous United States currently store 700 million metric
tons of carbon (335 to 980 million tC; $14,300 million value) with a gross carbon seques-
tration rate of 22.8 million tC/year (13.7 to 25.9 million tC/year; $460 million/year)
(Nowak and Crane, 2002). These results correspond with previous analyses that
estimated national carbon storage by urban trees as between 350–750 million tC
(Nowak, 1993a) and 600–900 million tC (Nowak, 1994b). Carbon storage by urban trees
nationally is only 4.4% of the estimated 15,900 million tC stored in trees in US nonurban
forest ecosystems (Birdsey and Heath, 1995). The estimated carbon storage by urban
trees in United States is equivalent to the amount of carbon emitted by the US popula-
tion in about 5.5 months. National annual carbon sequestration by urban trees is equiv-
alent to the US population emissions over a 5-day period (Nowak and Crane, 2002).
Benefits and Costs of Urban Forest Ecosystems 29
30 David J. Nowak and John F. Dwyer
Table 1. Total Estimated Pollution Removal (Metric Tons) by Trees During Nonprecipitation Periods (Dry Deposition), and Associated Monetary
Value for Various Cities (Pollutant Year = 2000)
City Pollution removed
CO (t) NO2(t) O3(t) PM10 (t) SO2(t) Total (t) Range (t) g/m2covera$ $/ha coverb
New York, NY 67 364 536 354 199 1,521 (619–2,185) 9.1 8,071,000 482
Atlanta, GA 39 181 672 528 89 1,508 (538–2,101) 12.0 8,321,000 663
Baltimore, MD 9 94 223 142 55 522 (183–725) 9.9 2,876,000 545
Philadelphia, PA 10 93 185 194 41 522 (203–742) 9.7 2,826,000 527
San Juan, PR 56 55 161 153 86 511 (222–768) 11.2 2,342,000 511
Washington, DC 18 50 152 107 51 379 (150–568) 8.3 1,956,000 429
Boston, MA 6 48 108 73 23 257 (94–346) 8.1 1,426,000 447
Woodbridge, NJ 6 42 66 62 15 191 (72–267) 10.8 1,037,000 586
San Francisco, CA 7 25 47 42 7 128 (51–195) 9.0 693,000 486
Moorestown, NJ 2 14 43 38 9 107 (41–157) 10.1 576,000 541
Syracuse, NY 2 12 55 23 7 99 (37–134) 6.6 568,000 378
Morgantown, WV 1 5 26 18 9 60 (22–98) 7.5 311,000 387
Jersey City, NJ 2 9 13 9 5 37 (16–56) 8.4 196,000 445
Freehold, NJ 1 3 9 6 1 20 (7–27) 11.4 110,000 632
Estimates are for particulate matter less than 10 microns (PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2) and carbon monoxide (CO). Pollution removal model methods are described
in Nowak et al. (1998). Monetary value of pollution removal by trees was estimated using the median externality values for United States for each pollutant. Externality values are: NO2= $6750 t1,
PM10 = $4500 t1,SO
2= $1650 t1, and CO = $950 t1(Murray et al., 1994). Externality values for O3were set to equal the value for NO2.
aAverage grams of pollution removal per year per square meter of canopy cover.
bAverage dollar value of pollution removal per year per hectare of canopy cover.
Carbon storage within the cities ranges from 1.2 million tC in New York City and
Atlanta to 19,300 tC in Jersey City, New Jersey (Table 2).
Urban trees in the North Central, Northeast, South Central and Southeast
regions of the United States store and sequester the most amount of carbon, with
average carbon storage per hectare greatest in Southeast (31.1 tC/ha), North Central
(30.7 tC/ha), Northeast (30.5 tC/ha), and Pacific Northwest (30.2 tC/ha) regions,
respectively. The national average urban forest carbon storage density is 25.1 tC/ha as
compared to 53.5 tC/ha in forest stands (Nowak and Crane, 2002).
3.1.3. Emission of Volatile Organic Compounds and Tree Maintenance Emissions
Some trees emit into the atmosphere volatile organic compounds (VOCs) such as
isoprene and monoterpenes. These compounds are natural chemicals that make up
essential oils, resins, and other plant products and may be useful to the tree in attract-
ing pollinators or repelling predators (Kramer and Kozlowski, 1979). Isoprene is also
believed to provide thermal protection to plants by helping prevent irreversible leaf
damage at high temperatures (Sharkey and Singsaas, 1995). The VOC emissions by
trees vary with species, air temperature, and other environmental factors (Tingey
et al., 1991; Guenther et al., 1994).
Volatile organic compounds can contribute to the formation of O3and CO
(Brasseur and Chatfield, 1991). Because the VOC emissions are temperature dependent
and trees generally lower air temperatures, it is believed that increased tree cover lowers
overall VOC emissions and, consequently, reduces O3levels in urban areas. A comput-
er simulation of June 4, 1984 ozone conditions in Atlanta, Georgia revealed that a 20%
loss in the area’s forest could lead to a 14% increase in O3concentrations. Although
there were fewer trees to emit VOCs, an increase in Atlanta’s air temperatures due to
Benefits and Costs of Urban Forest Ecosystems 31
Table 2. Estimated Carbon Storage, Gross and Net Annual Sequestration, Number of Trees, and
Percent Tree Cover for 10 US Cities (Nowak and Crane, 2002)
City Storage Annual sequestration No. of trees Tree cover
(tC) Gross (tC/yr) Net (tC/yr) (×103) (percent)
Total SE Total SE Total SE Total SE % SE
New York, NY 1,225,200 150,500 38,400 4,300 20,800 4,500 5,212 719 20.9 2.0
Atlanta, GA 1,220,200 91,900 42,100 2,800 32,200 4,500 9,415 749 36.7 2.0
Sacramento, CAa1,107,300 532,600 20,200 4,400 na na 1,733 350 13.0 na
Chicago, ILb854,800 129,100 40,100 4,900 na na 4,128 634 11.0 0.2
Baltimore, MD 528,700 66,100 14,800 1,700 10,800 1,500 2,835 605 25.2 2.2
Philadelphia, PA 481,000 48,400 14,600 1,500 10,700 1,300 2,113 211 15.7 1.3
Boston, MA 289,800 36,700 9,500 900 6,900 900 1,183 109 22.3 1.8
Syracuse, NY 148,300 16,200 4,700 400 3,500 400 891 125 24.4 2.0
Oakland, CAc145,800 4,900 na na na na 1,588 51 21.0 0.2
Jersey City, NJ 19,300 2,600 800 90 600 100 136 22 11.5 1.2
aMcPherson (1998).
bNowak (1994b).
cNowak (1993c).
SE = standard error.
na = not analyzed.
the urban heat island, which occurred concomitantly with the tree loss, increased VOC
emissions from the remaining trees and anthropogenic sources and altered O3photo-
chemistry such that concentrations of O3increased (Cardelino and Chameides, 1990).
A simulation of California’s South Coast Air Basin suggested that the impact on
air quality from increased urban tree cover might be locally positive or negative. The
net basinwide effect of increased urban vegetation is a decrease in O3concentrations
if the additional trees are low-VOC emitters (Taha, 1996). Examples of low-VOC
emitting genera include Fraxinus spp., Ilex spp., Malus spp., Prunus spp., Pyrus spp.,
and Ulmus spp.; high-VOC emitters include Eucalyptus spp., Quercus spp., Platanus
spp., Populus spp., Rhamnus spp., and Salix spp. (Benjamin et al., 1996).
Tree management and maintenance also affects pollutant emissions. The equip-
ment used in many maintenance activities emits pollutants and global gases such as
VOCs, CO, CO2, nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter
(US EPA, 1991). Thus, while evaluating the overall net change in air quality due to
trees, managers and planners must consider the amount of pollution that results from
tree maintenance and management activities. The greater the use of fossil fuels (e.g.,
from vehicles, chain saws, augers, and chippers) in establishing and maintaining a cer-
tain vegetation structure, the longer the trees must live and function to offset the pol-
lutant emissions from vegetation maintenance.
While considering the net effect of tree growth on atmospheric CO2, managers
must also consider that nearly all of the carbon sequestered by trees will be converted
back to CO2due to decomposition after the tree dies. Hence, the benefits of carbon
sequestration will be relatively short-lived if vegetation structure is not sustained.
However, if carbon (via fossil-fuel combustion) is used to maintain vegetation structure
and health, urban forest ecosystems will eventually become net emitters of carbon
unless secondary carbon reductions (e.g., energy conservation) or limiting decomposi-
tion via long-term carbon storage (e.g., wood products, landfills) can be accomplished
to offset the carbon emissions during maintenance (Nowak et al., 2002c).
Trees in parking lots can also help reduce VOCs emissions by shading parked
cars and thereby reducing evaporative emissions from vehicles. Increasing parking lot
tree cover from 8% to 50% could reduce Sacramento County, California, light duty
vehicle VOC evaporative emission rates by 2% and nitrogen oxide start emissions by
<1% (Scott et al., 1999).
3.1.4. Net Effects on Ozone
Besides the studies by Cardelino and Chameides (1990) and Taha (1996), other
studies reveal that increased urban tree cover can lead to reduced ozone concentra-
tions. Modeling the effects of increased urban tree cover on ozone concentrations
from Washington, DC to central Massachusetts revealed that urban trees generally
reduce ozone concentrations in cities. Interactions of the effects of trees on the phys-
ical and chemical environment demonstrate that trees can cause changes in pollution
removal rates and meteorology, particularly air temperatures, wind fields, and mixing-
layer heights, which, in turn, affect ozone concentrations. Changes in urban tree species
composition had no detectable effect on ozone concentrations (Nowak et al., 2000).
Modeling of the New York City metropolitan area also revealed that increasing tree
32 David J. Nowak and John F. Dwyer
cover by 10% within urban areas could reduce maximum ozone levels by about 4 ppb,
which is about 37% of the amount needed for attainment of the National Ambient
Air Quality Standard (Luley and Bond, 2002).
Based on the various research on urban tree effects on ozone, the US
Environmental Protection Agency (US EPA) released a guidance document that
details how new measures, including “strategic tree planting,” can be incorporated in
State Implementation Plans as a means to help states meet National Ambient Air
Quality Standards (US EPA, 2004).
3.1.5. Energy Conservation
Trees can reduce building heating and cooling energy needs, as well as conse-
quent emissions of air pollutants and CO2by power plants, by shading buildings and
reducing air temperatures in the summer, and by blocking winds in winter. However,
trees that shade buildings in winter can also increase heating needs. Energy conserva-
tion from trees varies by regional climate, the size and amount of tree foliage, and the
location of trees around buildings. Tree arrangements that save energy provide shade
primarily on east and west walls and roofs, and wind protection from the direction of
prevailing winter winds. However, wind reduction in the summer can lead to increased
energy use for air conditioning, but wind and shade effects combined lead to reduced
summer energy use for cooling (Akbari et al., 1992; Heisler, 1990b). Energy use in a
house with trees can be 20% to 25% lower per year than that for the same house in an
open area (Heisler, 1986). It has been estimated that establishing 100 million mature
trees around residences in the United States could save about $2 billion annually in
reduced energy costs (Akbari et al., 1988).
Proper tree placement near buildings is critical to maximize energy conservation.
For example, it has been estimated that annual costs of air conditioning and heating
for a typical residence in Madison, Wisconsin, would increase from $671 for an ener-
gy-efficient planting design to $700 for no trees and $769 for trees planted in locations
that block winter sunlight and provide little summer shade (McPherson, 1987). In this
instance, average annual energy savings with properly placed trees were about 4%
more than with no trees and 13% more than with improperly placed trees.
3.2. Urban Hydrology
By intercepting and retaining or slowing the flow of precipitation reaching the
ground, trees (in conjunction with soils) can play an important role in urban hydro-
logic processes. They can reduce the rate and volume of stormwater runoff, flood-
ing damage, stormwater treatment costs, and other problems related to water qual-
ity. Estimates of runoff for an intensive storm in Dayton, Ohio, showed that the
existing tree canopy (22%) reduced potential runoff by 7% and that a modest
increase in canopy cover (to 29%) would reduce runoff by nearly 12% (Sanders,
1986). A study of the Gwynns Falls watershed in Baltimore indicated that heavily
forested areas can reduce total runoff by as much as 26% and increase low-flow
runoff by up to 13% compared with nontree areas in existing land cover and land-
use conditions (Neville, 1996). Further, tree cover over pervious surfaces reduced
Benefits and Costs of Urban Forest Ecosystems 33
total runoff by as much as 40%; while tree canopy cover over impervious surfaces
had a limited effect on runoff.
In reducing runoff, trees function like retention/detention structures. In many
communities, reduced runoff due to rainfall interception can also reduce costs
of treating stormwater by decreasing the volume of water handled during periods of
peak runoff (Sanders, 1986).
There may also be hydrologic costs associated with urban vegetation, particular-
ly in arid environments where water is increasingly scarce or on reactive clay soils
where water uptake by roots may cause localized-soil drying, shrinkage, and cracking.
Increased water use in desert regions could alter the local water balance and various
ecosystem functions that are tied to the desert water cycle. In addition, annual costs
of water for sustaining vegetation can be twice as high as energy savings from shade
for tree species that use large amounts of water, e.g., mulberry (McPherson and
Dougherty, 1989). However, in Tucson, Arizona, 16% of the annual irrigation require-
ment of trees was offset by the amount of water conserved at power plants due to
energy savings from trees (Dwyer et al., 1992).
3.3. Urban Noise
Field tests have shown that properly designed plantings of trees and shrubs can
significantly reduce noise. Leaves and stems reduce transmitted sound primarily by
scattering it, while the ground absorbs sound (Aylor, 1972). For optimum noise reduc-
tion, trees and shrubs should be planted close to the noise source rather than the
receptor area (Cook and Van Haverbeke, 1971). Wide belts (30 m) of tall, dense trees
combined with soft ground surfaces can reduce apparent loudness by 50% or more
(6 to 10 decibels) (Cook, 1978). For narrow planting spaces (<3 m wide), reductions
of 3 to 5 decibels can be achieved with dense belts of vegetation, i.e., one row of
shrubs along the road and one row of trees behind it (Reethof and McDaniel, 1978).
Buffer plantings in these circumstances typically are more effective in screening views
than in reducing noise.
Vegetation can also mask sounds by generating its own noise as wind moves tree
leaves or as birds sing in the tree canopy. These sounds may make individuals less
aware of offensive noises because people are able to filter unwanted noise while con-
centrating on more desirable sounds (Robinette, 1972). The perception of sounds by
humans is also important. By visually blocking the sound source, vegetation can
reduce individuals’ perceptions of the amount of noise they actually hear (Anderson
et al., 1984). The ultimate effectiveness of plants in moderating noise is determined by
the sound itself, the planting configuration used, the proximity of the sound source,
receiver, and vegetation, as well as climatic conditions.
3.4. Urban Wildlife and Biodiversity
There are many additional benefits associated with urban vegetation that con-
tribute to the long-term functioning of urban ecosystems and the well-being of urban
residents. These include wildlife habitat and enhanced biodiversity. Urban wildlife can
34 David J. Nowak and John F. Dwyer
provide numerous benefits but also have detrimental effects (VanDruff et al., 1995).
Urban wildlife can serve as biological indicators of changes in the health of the envi-
ronment (e.g., the decline of certain bird populations was traced to pesticides), and
can provide economic benefit to individuals and society (VanDruff et al., 1995). For
example, bird feeding supports a $170 to $517 million American industry (DeGraff
and Payne, 1975; Lyons, 1982).
Surveys have shown that most city dwellers enjoy and appreciate wildlife in their
day-to-day lives (Shaw et al., 1985). Among New York State’s metropolitan residents,
73% showed an interest in attracting wildlife to their backyard (Brown et al., 1979).
Feelings of personal satisfaction from helping wildlife were the most frequently
reported reason for feeding wildlife in backyards across America (Yeomans and
Barclay, 1981). Detrimental wildlife effects include damage to plants and structures,
droppings, threats to pets, annoyance to humans, animal bites, and transmission of
diseases (VanDruff et al., 1995).
Urbanization can sometimes lead to the creation and enhancement of animal
and plant habitats, which, in turn, usually increases biodiversity. For example, tree
species diversity and richness in Oakland, California, increased from an index value
of about 1.9 (Shannon–Weiner diversity index value) and 10 species in 1850 to 5.1 and
>350 species in 1988 (Nowak, 1993c). However, the introduction of new plant species
into urban areas can lead to problems for managers in maintaining native plant struc-
ture, as exotic plants can invade and displace native species in forest stands. One
example of exotic plant invasion in some areas of the northeastern United States is
that by Norway maple (Acer platanoides L.) (Nowak and Rowntree, 1990). Also, alter-
ing vegetation structure in urban areas can change the prevalence of certain tree
insects and diseases (Nowak and McBride, 1992) and could increase the potential for
urban wildfires (East Bay Hills Vegetation Management Consortium, 1995).
Urban forests can act as reservoirs for endangered species. For example, 20
threatened or endangered faunal species and 130 plant species are listed for Cook
County, the most populated county of the Chicago Metropolitan Area (Howenstine,
1993). In addition, urbanites are increasingly preserving, cultivating, and restoring
rare and native species and ecosystems (Howenstine, 1993). A notable example of the
involvement of a wide range of individuals and groups in the restoration and man-
agement of urban natural areas is the work of the Chicago Region Biodiversity
Council, often known as Chicago Wilderness (2005).
Because of increased environmental awareness and concerns about quality of life
and sustainability of natural systems, ecological benefits of the urban forest are like-
ly to increase in significance over time (Dwyer et al., 1992).
3.5 Phytoremediation
Trees and other plants show significant potential for remediating brownfields,
landfills, and other contaminated sites by absorbing, transforming, and containing a
number of contaminants (Westphal and Isebrands, 2001). More information about
brownfields and the issues and opportunities that they present can be obtained from
USEPA (2000) and De Sousa (2003).
Benefits and Costs of Urban Forest Ecosystems 35
4. Social and Economic Benefits and Costs of Urban Vegetation
In conjunction with the many effects of urban trees on the physical/biological
environment, trees and associated forest resources can significantly influence the
social and economic environment of a city. These influences range from altered aes-
thetic surroundings and increased enjoyment with everyday life to improved health
and a greater sense of meaningful connection between people and the natural envi-
ronment. The benefits and costs associated with these influences are highly variable
within and among urban areas and often difficult to measure. Nevertheless, they
reflect important contributions of trees and forests to the quality of life for urban
dwellers.
4.1. Benefits to Individuals
Urban forest environments provide aesthetic surroundings and are among the
most important features contributing to the aesthetic quality of residential streets and
community parks (Schroeder, 1989). Perceptions of aesthetic quality and personal
safety are related to features of the urban forest such as number of trees per acre and
viewing distance (Schroeder and Anderson, 1984). Urban trees and forests provide
significant emotional and spiritual experiences that are important in people’s lives and
can foster a strong attachment to particular places and trees (Chenoweth and Gobster,
1990; Dwyer et al., 1991; Schroeder, 1991, 2002, 2004). A wide range of individual
benefits has been associated with volunteer tree planting and care (Westphal, 1993).
Volunteers continue to play an increasingly important role in urban forestry efforts,
such as inventory (Bloniarz and Ryan, 1996), and Sommer (2003) encourages explo-
ration of expanding opportunities for resident involvement in tree planting and care.
Nearby nature, even when viewed from an office window, can provide substantial
psychological benefits that affect job satisfaction and a person’s well-being (Kaplan,
1993). Reduced stress and improved physical health for urban residents have been asso-
ciated with the presence of urban trees and forests in a number of environments. Living
in a green environment has been associated with a wide range of individual benefits,
including improved learning and behavior by children in urban areas (Taylor, Kuo, and
Sullivan, 2001a, b; Wells, 2000). Experiences in urban parks have been shown to change
moods and reduce stress (Hull, 1992a; Kaplan and Kaplan, 1989), and to provide pri-
vacy refuges (Hammitt, 2002). Hospital patients with window views of trees have been
shown to recover significantly faster and with fewer complications than the patients
without such views (Ulrich, 1984). In addition, tree shade reduces ultraviolet radiation
and thus can help reduce health problems associated with increased ultraviolet radia-
tion exposure, e.g., cataracts, skin cancer (Heisler et al., 1995).
Many of the benefits associated with urban trees contribute to improved human
health in a wide variety of ways, ranging from improved air quality to reduction of
stress and interpersonal conflict. With increased concern over obesity and the need for
changing lifestyles (e.g., more exercise) to reduce obesity, trees and forests are receiv-
ing increased attention as contributing to a solution. This solution ranges from pro-
viding environments that encourage exercise (e.g., playing in well-landscaped parks or
walking/running along tree-lined streets and trails) to the actual exercise experienced
36 David J. Nowak and John F. Dwyer
by the many volunteers who work with trees and associated landscapes (Librett et al.,
2005). A comprehensive overview of the relationship of urban design to human health
and condition concluded, “There are strong public health arguments for the incorpo-
ration of greenery, natural light, and visual and physical access to open space in
homes and other buildings (Jackson, 2003).”
Along with the human health benefits, such as those outlined above in this sec-
tion, some decreases in well-being and increases in health care costs may be associat-
ed with urban vegetation. This negative side to urban trees is associated with allergic
reactions to plants, pollen, or associated animal and insects, diseases such as Lyme
disease that are carried by wildlife, injuries from branch or tree failures, and a fear of
trees, forests, and associated environments.
4.2. Benefits to Communities
Urban forests can make important contributions to the economic vitality and
character of a city, neighborhood, or subdivision. It is no accident that many cities,
towns, and subdivisions are named after trees (e.g., Oakland, Elmhurst, Oak Acres)
and that many cities strive to be a “Tree City USA.” Often, trees and forests on pub-
lic lands–and on private lands to some extent–are significant “common property”
resources that contribute to the economic vitality of an entire area (Dwyer et al.,
1992). The substantial efforts that many communities undertake to develop and
enforce local tree ordinances and manage their urban forest resource attest to the sub-
stantial return that they expect from these investments.
A stronger sense of community and empowerment of inner-city residents to
improve neighborhood conditions can be attributed to involvement in urban forestry
efforts (Feldman and Westphal, 1999; Westphal, 1999, 2003). Active involvement in
tree-planting programs has been shown to enhance a community’s sense of social
identity, self-esteem, and territoriality; it teaches residents that they can work togeth-
er to choose and control the condition of their environment. Planting programs also
can project a visible sign of change and provide the impetus for other community
renewal and action programs (Feldman and Westphal, 1999; Westphal, 1999, 2003).
Several studies have shown that participation in tree-planting programs influences
individuals’ perceptions of their community (Sommer et al., 1994a, 1994b, 1995,
2003). Conversely, a loss of trees within a community can have a significant psycho-
logical effect on residents (Hull, 1992b). A useful framework for considering social
benefits of urban and community forestry projects has been developed and illustrat-
ed with community examples (Westphal, 2003).
Urban trees and forests can help alleviate some of the hardships of inner-city liv-
ing, especially for low-income groups (Dwyer et al., 1992). Extensive research in inner-
city areas of Chicago suggests that urban trees and forests contribute to stronger ties
among neighbors, greater sense of safety and adjustment, more supervision of children
in outdoor places, healthier patterns of children’s play, more use of neighborhood
common spaces, fewer incivilities, fewer property crimes, and fewer violent crimes
(Kuo, 2003; Kuo et al., 1998; Kuo and Sullivan, 2001a,b; Sullivan and Kuo, 1996).
While there is sometimes concern over the influence of trees and other vegeta-
tion in urban areas on the incidence of crime, research has provided management
Benefits and Costs of Urban Forest Ecosystems 37
guidelines that can reduce the fear of crime in urban forest areas (Schroeder and
Anderson, 1984; Michael and Hull, 1995).
Consumer behavior has been found to be positively correlated with streetscape
greening, suggesting important benefits to commercial establishments and a basis for
partnerships with the business community in urban forest planning and management
(Wolf, 2003a, 2004). However, improper landscaping of business areas can have a neg-
ative impact by blocking business signs and/or reducing the attractiveness of the area.
Regardless of the community benefits derived from urban trees, tree planting and
maintenance programs might be perceived by some people as an inappropriate use of
resources because of the perception that funds for such efforts could be used to
address what they see as more critical urban community problems.
4.3. Real Estate Values
The sales value of real estate reflects the benefits that buyers attach to attributes
of the property, including vegetation on and near the property. A survey of sales of
single-family homes in Athens, Georgia indicated that landscaping with trees was asso-
ciated with an increase in sales prices of 3.5% to 4.5% (Anderson and Cordell, 1988).
Builders have estimated that homes on wooded lots sell on an average for 7% more than
equivalent houses on unwooded lots (Selia and Anderson, 1982, 1984). Research in
Baton Rouge, Louisiana indicates that mature trees contributed about 2% of the home
market (Dombrow et al., 2000). A recent study in Athens, Georgia indicates that an
additional percentage increase in relative tree cover is associated with an increase of
$296 in residential value (Sydor et al., 2005). A study of small, urban-wildland inter-
face properties in the Lake Tahoe Basin indicates that forest density and health char-
acteristics contributed between 5% and 20% to property values (Thompson et al.,
1999). Shopping centers often landscape their surroundings to attract shoppers, there-
by increasing the value of the business and shopping center (Dwyer et al., 1992).
Parks and greenways have been associated with increases in nearby residential
property values (Corrill et al., 1978; More et al., 1988; Crompton, 2004). Some of
these increased values have been substantial, and it appears that parks with “open
space character” add the most to nearby property values. Part of the contribution to
the value of residential property is associated with the view from that property.
A study of the value of a view in the single-family housing market suggests that a
good view adds 8% to the value of a single-family house (Rodriquez and Sirmans,
1994). A premium of 5% to 12% in housing prices in the Netherlands was associated
with an attractive landscape view from the property (Luttik, 2000). Although this
remains to be investigated, parks also may have a negative impact on local property
values if these are perceived as unmaintained or a place where undesirable/criminal
activities are concentrated.
Increased real estate values generated by trees also produce direct economic gains
to the local community through property taxes. A conservative estimate of a 5%
increase in residential property values due to trees converts to $25/year on a tax bill of
$500 and is equivalent to $1.5 billion/year based on 62 million single-family homes in
the United States (Dwyer et al., 1992). However, from a homeowner’s perspective,
increased tax expense due to trees is an additional annual cost of owning a home.
38 David J. Nowak and John F. Dwyer
4.4. Tree Value Formulas
Various approaches and formulas are used to estimate the value of individual
trees (see Chapter 19). One of the most widely used is the Council of Tree and
Landscape Appraisers’ (2000), Guide for Plant Appraisal, which estimates the com-
pensation that landowners should receive for the loss of a tree on their property. For
smaller trees, the value is the replacement cost. For larger trees, the formula calculates
tree value from measured tree variables and tree assessments by professionals. The
species, diameter, location, and condition of the tree are an integral part of the
assessment. Because the values estimated with the tree valuation formula are not nec-
essarily tied to the functions that trees perform in the urban environment, they do not
relate directly to the values associated with the environmental, social, and economic
benefits from trees. An exception is a single study that suggested that the formula pro-
duced values that were similar to a tree’s contribution to residential property values
(Morales et al., 1983).
Compensatory values represent compensation to owners for the loss of an indi-
vidual tree and can be viewed as the value of the tree as a structural asset.
Compensatory value is based on the structure in place as an asset, while the functional
value is an annual value based on the various functions of the particular structure.
Trees can have both positive (e.g., air pollution removal) and negative functional val-
ues (e.g., trees can increase annual building energy use in certain locations). Trees also
have various maintenance costs, which are essential for maintaining tree health,
human safety, and overall tree functional benefits. Management of urban forests is
needed to enhance functional values and improve human health and well-being, and
environmental quality in cities. Maximizing net functional benefits of the urban for-
est will lead to the greatest value to society (Nowak et al., 2002a).
Based on the data from eight cities, overall citywide compensatory values ranged
between $23 and $64/m2($2.1–$5.9/feet2) of tree cover. However, 75% of the city val-
ues were between $27 and $39 m2($2.5–$3.6/feet2) of cover. The total compensatory
value for the urban forests of the 48 adjacent US states is estimated at $2.4 trillion or
$630/tree (Nowak et al., 2002a).
Urban forest compensatory values can be used to estimate actual or potential
loss due to catastrophic agents. For example, the loss to the urban forest in Oakland,
California, due to a large fire in 1991 was estimated at $26.5 million (Nowak 1993b).
Compensatory value of potential loss due to Asian longhorned beetle (Anoplophora
glabripennis) infestation in various US cities ranges between $72 million (Jersey City,
New Jersey) and $2.3 billion (New York, New York). The estimated maximum poten-
tial national urban impact of infestations by A. glabripennis is $669 billion (Nowak
et al., 2001b).
4.5. Other Benefits and Costs of Urban Trees and Forests
The presence of urban trees and forests can make the urban environment a
more pleasant place to live, work, and spend leisure time. A study of urbanites that
use parks and forest preserves indicated that they were willing to pay extra to have
trees and forests in recreation areas (Dwyer et al., 1989). For example, they would
Benefits and Costs of Urban Forest Ecosystems 39
be willing to pay an addition $1.60/visit to have a site that was “mostly wooded,
some open grassy areas under trees” rather than “mowed grass, very few trees any-
where.” The total contribution of trees in urban park and recreation areas to the
value of the outdoor leisure and recreation experiences in the United States may
exceed $2 billion/year (Dwyer, 1991).
A national survey indicated that drivers prefer trees as a screen of commercial
developments along highways (Wolf, 2003b). Reduced driver aggression (Cackowski
and Nasar, 2003) and stress recovery (Parsons, et al., 1998) have also been associated
with treed thoroughfares. These findings provide the basis for opportunities to incor-
porate urban forestry into the planning of high-speed urban transportation corridors
(Wolf, 2003b).
Urban trees and forests often figure prominently in urban environmental educa-
tion programs. The high visibility, variability, and complexity of urban forest ecosys-
tems make an outstanding laboratory for environmental education. The lessons
learned about forest ecosystems have implications for the management of public and
private forest resources far beyond the city boundary (Dwyer and Schroeder, 1994).
Because trees and forests can increase the quality of the urban environment and
make spending leisure time there more attractive, there can be a substantial saving in
the amount of automobile fuel used because people do not need to drive long dis-
tances to reach recreation sites.
At the same time there are direct economic costs associated with urban trees.
These include costs of planting, maintenance, management, and removal, as well as
costs of damage from falling tree limbs and cracked sidewalks due to tree roots
(Dwyer, 1995). However, these costs can be offset by economic benefits generated by
trees. For example, homeowners may pay for tree care and driveway repair due to root
damage, but receive savings on their utility bill from the energy conserving effects of
the trees. At a larger scale, a municipality paying for street and park tree maintenance
and management may receive increased tax revenues due to the contribution of trees
to property values, and also may achieve savings in storm water management costs
due to the influence of trees. Net benefits or costs need to be considered when devel-
oping urban vegetation designs or management plans.
5. Benefit–Cost Analyses
The wide range of important benefits and costs that may be associated with man-
aging the urban forest and the significant interactions between the processes that pro-
duce important outcomes complicate the analysis of options available to urban forest
resource managers. This complexity makes it difficult to predict the influence of trees
on the urban environment for various vegetation designs and management options. In
many instances, the location of trees with respect to other resources can make a sub-
stantial difference in the benefits that they provide, such as with building heating and
cooling costs and the management of rights-of-way where improperly placed trees can
greatly increase costs. Not all of the benefits are easily translated into monetary terms,
and even when they are, it often is difficult to assess the incidence of benefits and
costs, i.e., who pays and who gains? Trees planted on a residential property may
40 David J. Nowak and John F. Dwyer
provide benefits to others in the neighborhood and across the city in terms of aes-
thetics, reduced air temperatures, and improved air quality. Yet these very trees may
present problems for one’s neighbor by blocking solar heating through windows in the
winter and making it difficult to grow flowers or a vegetable garden in the summer.
The management of trees in public areas and rights-of-way often is intertwined with
that of other resources, such as park and recreation facilities and programs, streets
and roads, utilities, and other aspects of the urban infrastructure. When attempting
benefit–cost analyses, one must be aware of these various interconnections, as well as
the limitations of the information used in the analyses.
6. Implications for Planning, Design, and Management
It is clear that careful planning and design are critical to increasing the net ben-
efits of trees and forests in urban environments. A change in species or location of
trees with respect to each other or buildings and other components of the urban infra-
structure can have a major impact on benefits and costs. Similarly, maintenance activ-
ities can greatly influence benefits and costs. It often is critical that forest resources are
managed in the context of other aspects of the urban structure; including people,
buildings, roads and streets, utility rights-of-way, recreation areas, and other open
spaces.
Management plans must consider the potential of vegetation to improve indi-
vidual site conditions or alleviate local problems (e.g., poor air quality, neighborhood
revitalization) and design appropriate vegetation structure at the site with considera-
tion of how individual sites interact across the landscape (i.e., the benefits at one site
might lead to costs and benefits at other site). Determining the benefits and costs over
the urban environment is a complex task that often calls for approaching problems at
the landscape level (and sometimes extending beyond the urban system), particularly
with respect to aesthetics, meteorology, pest problems, risk of fire, and air quality.
Urban landscape designs and management plans must take account of these numer-
ous interactions and the myriad of potential benefits and costs to implement appro-
priate strategies to maximize the net environmental, social, economic, and human
health benefits of urban vegetation. In addition, careful attention must be given to the
question of who gains and who pays as a result of forest management efforts across
the urban landscape.
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46 David J. Nowak and John F. Dwyer
... Tree canopies intercept and reflect up to 90 percent of incoming solar radiation. Shade makes heat more tolerable and can protect people from excessive sun exposure during travel, work, or leisure (Nowak and Dwyer 2007). Trees that shade buildings can reduce surface temperatures in a wide variety of contexts (Wang et al. 2014); for example, surface temperatures were reduced by 11°C-25°C in Sacramento, California (Akbari et al. 1997); by 5°C-7°C in Akure, Nigeria (Morakinyo et al. 2013); and by 9°C in Melbourne, Australia (Berry et al. 2013). ...
... Noise, for instance, is both a nuisance and an environmental stressor that can interfere with communication, alter behavior, and impair work performance (Stansfeld and Matheson 2003). The foliage of urban forests can create a physical barrier that absorbs the energy of sound waves and reduces noise overall (Nowak and Dwyer 2007;Wang et al. 2014;Säumel et al. 2016). ...
... ▪ Direct participation in tree-planting programs or other green space stewardship activities may stimulate a sense of community identity and ownership (Higgs 2003;Nowak and Dwyer 2007;Jennings et al. 2016). ...
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