An
urban forest is a
forest
or a collection of trees that grow within a
city,
town or a
suburb. In a wider sense it may include any kind of
woody plant vegetation growing in and around human settlements. In
a narrower sense (also called
forest park) it
describes areas whose
ecosystems are
inherited from wilderness leftovers or remnants. Care and
management of urban forests is called
urban forestry.
Urban forests play an important role in
ecology of human habitats in many ways: they filter
air, water, sunlight, provide shelter to animals and recreational
area for people. They moderate local climate, slowing wind and
stormwater, and shading homes and businesses to conserve energy.
They are critical in cooling the urban
heat
island effect, thus potentially reducing the number of
unhealthful
ozone days that plague major
cities in peak summer months.
In many countries there is a growing understanding of the
importance of the natural ecology in urban forests. There are
numerous projects underway aimed at restoration and preservation of
ecosystems, ranging from simple elimination of leaf-raking and
elimination of invasive plants to full-blown reintroduction of
original species and
riparian
ecosystems.
Benefits
The benefits of urban trees are many, including beautification,
reduction of the
urban heat island
effect, reduction of stormwater runoff, reduction of
air pollution, reduction of energy costs
through increased shade over buildings, enhancement of property
values, improved wildlife habitat, and mitigation of overall urban
environmental impact.
Social, psychological, recreational, wildlife
The presence of trees reduces stress, and trees have long been seen
to benefit the health of urban dwellers. The shade of trees and
other urban green spaces make place for people to meet and
socialize and play. Proper planning and community involvement are
important for the positive results to be realized.
Trees provide nesting sites and food for birds and other animals.
People appreciate watching, feeding, photographing, painting urban
trees, and wildlife. Urban trees and wildlife help people maintain
their connection with nature.
Economic benefits
The economic benefits of trees have been understood for a long
time. Recently, more of these benefits are becoming quantified.
Quantification of the economic benefits of trees helps justify
public and private expenditures to maintain them. One of the most
obvious examples of economic utility is the deciduous tree planted
on the south and west of a building. The shade shelters and cools
the building during the summer, but allows the sun to warm it in
the winter after the leaves fall. The USDA 'Guide" notes on page 17
that "Businesses flourish, people linger and shop longer,
apartments and office space rent quicker, tenants stay longer,
property values increase, new business and industry is attracted"
by trees. The physical effects of trees--the shade (solar
regulation), humidity control, wind control, erosion control,
evaporative cooling, sound and visual screening, traffic control,
pollution absorption and precipitation--all have economic
benefits
Air pollution reduction
As cities struggle to comply with air quality standards, the ways
that trees can help to clean the air should not be overlooked. The
most serious pollutants in the urban atmosphere are ozone, nitrogen
oxides (NOx), sulfuric oxides (SOx) and particulate pollution.
Ground-level ozone, or smog, is created by chemical reactions
between NOx and volatile organic compounds (VOCs) in the presence
of sunlight. High temperatures increase the rate of this reaction.
Vehicle emissions, emissions from industrial facilities, gasoline
vapors, and chemical solvents are the major sources of NOx and
VOCs. Particulate pollution, or particulate matter (PM10 and PM25),
is made up of microscopic solids or liquid droplets that can be
inhaled and retained in lung tissue causing serious health
problems. Most particulate pollution begins as smoke or diesel soot
and can cause serious health risk to people with heart and lung
diseases and irritation to healthy citizens. Trees are an
important, cost-effective solution to reducing pollution and
improving air quality.
- Trees reduce temperatures and smog
With an extensive and healthy urban forest air quality can be
drastically improved. Trees help to lower air temperatures and the
urban heat island affect in urban areas (see: 'Trees are energy
savers' for more information on this process). This reduction of
temperature not only lowers energy use, it also improves air
quality, as the formation of ozone is dependent on temperature.
- As temperatures climb, the formation of ozone increases.
- Healthy urban forests decrease temperatures, and reduce the
formation of ozone.
- Large shade trees can reduce local ambient temperatures by 3 to
5 °C
- Maximum mid-day temperature reductions due to trees range from
0.04 °C to 0.2 °C per 1% canopy cover increase.
- In
Sacramento
County, California
, it was estimated that doubling the canopy cover to
five million trees would reduce summer temperatures by 3 degrees
. This reduction in temperature would reduce peak ozone
levels by as much as 7% and smoggy days by 50%.
- Lower temperatures reduce emissions in parking lots
Temperature reduction from shade trees in parking lots lowers the
amount of evaporative emissions from parked cars. Unshaded parking
lots can be viewed as miniature heat islands, where temperatures
can be even higher than surrounding areas. Tree canopies will
reduce air temperatures significantly. Although the bulk of
hydrocarbon emissions come from tailpipe exhaust, 16% of
hydrocarbon emissions are from evaporative emissions that occur
when the fuel delivery systems of parked vehicles are heated. These
evaporative emissions and the exhaust emissions of the first few
minutes of engine operation are sensitive to local microclimate. If
cars are shaded in parking lots, evaporative emissions from fuel
and volatilized plastics will be greatly reduced.
- Cars parked in parking lots with 50% canopy cover emit 8% less
through evaporative emissions than cars parked in parking lots with
only 8% canopy cover.
- Due to
the positive effects trees have on reducing temperatures and
evaporative emissions in parking lots, cities like Davis,
California
, have
established parking lot ordinances that mandate 50% canopy cover
over paved areas.
- "Cold Start" emissions
The volatile components of asphalt pavement evaporate more slowly
in shaded parking lots and streets. The shade not only reduces
emissions, but reduces shrinking and cracking so that maintenance
intervals can be lengthened. Less maintenance means less hot
asphalt (fumes) and less heavy equipment (exhaust). The same
principle applies to asphalt-based roofing.
- Active pollutant removal
Trees also reduce pollution by actively removing it from the
atmosphere. Leaf
stomata, the pores on the
leaf surface, take in polluting gases which are then absorbed by
water inside the leaf. Some species of trees are more susceptible
to the uptake of pollution, which can negatively affect plant
growth. Ideally, trees should be selected that take in higher
quantities of polluting gases and are resistant to the negative
affects they can cause.
A study
across the Chicago
region
determined that trees removed approximately 17 tonnes of carbon monoxide (CO), 93 tonnes of sulfur dioxide (SO2), 98 tonnes of nitrogen dioxide (NO2), and 210
tonnes of ozone (O3) in 1991.
- Carbon sequestration
Urban forest managers are sometimes interested in the amount of
carbon removed from the air and stored in their forest as wood in
relation to the amount of carbon dioxide released into the
atmosphere while running tree maintenance equipment powered by
fossil fuels.
- Interception of particulate matter
In addition to the uptake of harmful gases, trees also act as
filters intercepting airborne particles and reducing the amount of
harmful particulate matter. The particles are captured by the
surface area of the tree and its foliage. These particles
temporarily rest on the surface of the tree, as they can be washed
off by rainwater, blown off by high winds, or fall to the ground
with a dropped leaf. Although trees are only a temporary host to
particulate matter, if they did not exist, the temporarily-housed
particulate matter would remain airborne and harmful to humans.
Increased tree cover will increase the amount of particulate matter
intercepted from the air.
- Large evergreen trees with dense foliage collect the most
particulate matter.
- The Chicago study determined that trees removed approximately
234 tonnes of particulate matter less than 10 micrometres (PM10) in
1991.
- Large healthy trees greater than 75 cm in trunk diameter
remove approximately 70 times more air pollution annually
(1.4 kg/yr) than small healthy trees less than 10 cm in
diameter (0.02 kg/yr).
Biogenic volatile organic compounds
One important thing to consider when assessing the urban forest's
effect on
air quality is that trees emit
some biogenic volatile organic compounds (BVOCs). These are the
chemicals (primarily
isoprene and
monoterpenes) that make up the
essential oils,
resins,
and other organic compounds that plants use to attract pollinators
and repel predators. As mentioned above, VOCs react with nitrogen
oxides (NOx) to form ozone. BVOCs account for less than 10% of the
total amount of VOCs and BVOCs emitted in urban areas. This means
that BVOC emissions from trees can contribute to the formation of
ozone. Although their contribution may be small compared with other
sources, BVOC emissions could exacerbate a smog problem.
Not all species of trees, however, emit high quantities of BVOCs.
The tree species with the highest isoprene emission rates should be
planted with caution:
Trees that are well adapted to and thrive in certain environments
should not be replaced just because they may be high BVOC emitters.
The amount of emissions spent on maintaining a tree that may emit
low amounts of BVOCs, but is not well suited to an area, could be
considerable and outweigh any possible benefits of low BVOC
emission rates.
Trees should not be labeled as polluters because their total
benefits on air quality and emissions reduction far outweigh the
possible consequences of BVOC emissions on ozone concentrations.
Emission of BVOCs increase exponentially with temperature.
Therefore, higher emissions will occur at higher temperatures. In
desert climates, locally native trees
adapted to drought conditions emit significantly less BVOCs than
plants native to wet regions. As discussed above, the formation of
ozone is also temperature dependent. Thus, the best way to slow the
production of ozone and emission of BVOCs is to reduce urban
temperatures and the effect of the urban heat island. As suggested
earlier, the most effective way to lower temperatures is with an
increased canopy cover.
These effects of the urban forest on ozone production have only
recently been discovered by the scientific community, so extensive
and conclusive research has not yet been conducted. There have been
some studies quantifying the effect of BVOC emissions on the
formation of ozone, but none have conclusively measured the affect
of the urban forest. Important questions remain unanswered. For
instance, it is unknown if there are enough chemical reactions
between BVOC emissions and NOx to produce harmful amounts of ozone
in urban environments. It is therefore, important for cities to be
aware that this research is still continuing and conclusions should
not be drawn before proper evidence has been collected. New
research may resolve these issues.
See also
References
- Nowak, D. (2000). Tree Species Selection, Design, and
Management to Improve Air Quality Construction Technology.
Annual meeting proceedings of the American Society of Landscape
Architects (available online, pdf file).
- Nowak, D. The Effects of Urban Trees on Air Quality
USDA Forest Service (available online, pdf file).
- Nowak, D. (1995). Trees Pollute? A "Tree Explains It All".
Proceedings of the 7th National Urban Forest Conference
(available online, pdf file).
- Nowak, D. (1993). Plant Chemical Emissions. Miniature
Roseworld 10 (1) (available online, pdf file).
- Nowak, D. & Wheeler, J. Program Assistant, ICLEI. February
2006.
- McPherson, E. G. & Simpson, J. R. (2000). Reducing Air
Pollution Through Urban Forestry. Proceedings of the 48th
meeting of California Pest Council (available online, pdf file).
- McPherson, E. G., Simpson, J. R. & Scott, K. (2002).
Actualizing Microclimate and Air Quality Benefits with Parking Lot
Shade Ordinances. Wetter und Leben 4: 98 (available
online, pdf file).
- Hanson, Michael L.(1990). Urban & Community Forestry, a
Guide for the Interior Western United States, USDA Forest Service,
Intermountain Region, Ogden, Utah.
External links