Urban forest
Encyclopedia
An urban forest is a forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 or a collection of trees that grow within a city
City
A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.For example, in the U.S...

, town
Town
A town is a human settlement larger than a village but smaller than a city. The size a settlement must be in order to be called a "town" varies considerably in different parts of the world, so that, for example, many American "small towns" seem to British people to be no more than villages, while...

 or a suburb
Suburb
The word suburb mostly refers to a residential area, either existing as part of a city or as a separate residential community within commuting distance of a city . Some suburbs have a degree of administrative autonomy, and most have lower population density than inner city neighborhoods...

. 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 ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s are inherited from wilderness leftovers or remnants. Care and management of urban forests is called urban forestry
Urban forestry
Urban forestry is the careful care and management of urban forests, i.e., tree populations in urban settings for the purpose of improving the urban environment. Urban forestry advocates the role of trees as a critical part of the urban infrastructure...

.

Urban forests play an important role in ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

 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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 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 and shrubs are many, including beautification, reduction of the urban heat island
Urban heat island
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...

 effect, reduction of stormwater runoff, reduction of air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

, reduction of energy costs through increased shade over buildings, enhancement of property values, improved wildlife habitat, and mitigation of overall urban environmental impact (Wilson 2011).

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. The Biophilia hypothesis
Biophilia Hypothesis
The biophilia hypothesis suggests that there is an instinctive bond between human beings and other living systems. Edward O. Wilson introduced and popularized the hypothesis in his book entitled Biophilia.- Love of living systems :...

 argues that people are instinctively drawn to nature, while Attention Restoration Theory
Attention Restoration Theory
Attention Restoration Theory asserts that people can concentrate better after spending time in nature, or even looking at scenes of nature. Natural environments abound with "soft fascinations" which a person can reflect upon in "effortless attention", such as clouds moving across the sky, leaves...

 goes on to demonstrate tangible improvements in medical, academic and other outcomes, from access to nature. Proper planning and community involvement are important for the positive results to be realized.

Trees and shrubs provide nesting sites and food for birds and other animals. People appreciate watching, feeding, photographing, and painting urban wildlife and the environment they live in. Urban trees, shrubs and wildlife help people maintain their connection with nature.

Economic benefits

The economic benefits of trees and various other plants 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 example of the deciduous tree planted on the south and west of a building (in the Northern Hemisphere), or north and east (in the Southern Hemisphere). 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
    Sacramento County, California
    Sacramento County is a county in the U.S. state of California. Its county seat is Sacramento, which is also the state capital. As of 2010 the county had a population of 1,418,788....

    , 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
    Davis, California
    Davis is a city in Yolo County, California, United States. It is part of the Sacramento–Arden-Arcade–Roseville Metropolitan Statistical Area...

    , 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 stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

ta, 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
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...

 region determined that trees removed approximately 17 tonnes of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 (CO), 93 tonnes of sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 (SO2), 98 tonnes of nitrogen dioxide
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...

 (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
Isoprene
Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

 and monoterpene
Monoterpene
Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear or contain rings...

s) that make up the essential oil
Essential oil
An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...

s, resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

s, 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 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:
  • Casuarina
    Casuarina
    Casuarina is a genus of 17 species in the family Casuarinaceae, native to Australasia, southeast Asia, and islands of the western Pacific Ocean. It was once treated as the sole genus in the family, but has been split into three genera .They are evergreen shrubs and trees growing to 35 m tall...

    (Beefwood)
  • Eucalyptus
    Eucalyptus
    Eucalyptus is a diverse genus of flowering trees in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia...

  • Liquidambar (Sweetgum)
  • Nyssa
    Tupelo
    The tupelo , black gum, or pepperidge tree, genus Nyssa , is a small genus of about 9 to 11 species of trees with alternate, simple leaves...

    (Tupelo or Black gum)
  • Platanus
    Platanus
    Platanus is a small genus of trees native to the Northern Hemisphere. They are the sole living members of the family Platanaceae....

    (Plane)
  • Populus
    Poplar
    Populus is a genus of 25–35 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar , aspen, and cottonwood....

    (Poplar)
  • Quercus
    Oak
    An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...

    (Oak)
  • Robinia
    Black locust
    Robinia pseudoacacia, commonly known as the Black Locust, is a tree in the subfamily Faboideae of the pea family Fabaceae. It is native to the southeastern United States, but has been widely planted and naturalized elsewhere in temperate North America, Europe, Southern Africa and Asia and is...

    (Black locust)
  • Salix
    Willow
    Willows, sallows, and osiers form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere...

    (Willow)


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 climate
Desert climate
A desert climate , also known as an arid climate, is a climate that does not meet the criteria to be classified as a polar climate, and in which precipitation is too low to sustain any vegetation at all, or at most a very scanty scrub.An area that features this climate usually experiences less than...

s, 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 effect 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

  • Atlanta — known as "the city in a forest."
  • Forest Park — one of the largest urban forests in the United States
    United States
    The United States of America is a federal constitutional republic comprising fifty states and a federal district...

     located in Portland, Oregon
    Portland, Oregon
    Portland is a city located in the Pacific Northwest, near the confluence of the Willamette and Columbia rivers in the U.S. state of Oregon. As of the 2010 Census, it had a population of 583,776, making it the 29th most populous city in the United States...

    .
  • Green belt
    Green belt
    A green belt or greenbelt is a policy and land use designation used in land use planning to retain areas of largely undeveloped, wild, or agricultural land surrounding or neighbouring urban areas. Similar concepts are greenways or green wedges which have a linear character and may run through an...

  • Jefferson Memorial Forest
    Jefferson Memorial Forest
    The Jefferson Memorial Forest is a forest located in southwest Louisville, Kentucky , in the knobs region of Kentucky. At , it is the largest municipal urban forest in the United States...

     — largest municipal urban forest in the United States
  • Kosutnjak
    Košutnjak
    Košutnjak is a park-forest and urban neighborhood of Belgrade, the capital of Serbia. It is divided between in the municipalities of Čukarica and Rakovica .-Location:...

     — large urban forest in Belgrade
    Belgrade
    Belgrade is the capital and largest city of Serbia. It is located at the confluence of the Sava and Danube rivers, where the Pannonian Plain meets the Balkans. According to official results of Census 2011, the city has a population of 1,639,121. It is one of the 15 largest cities in Europe...

    , Serbia
    Serbia
    Serbia , officially the Republic of Serbia , is a landlocked country located at the crossroads of Central and Southeast Europe, covering the southern part of the Carpathian basin and the central part of the Balkans...

  • Banjica Forest
    Banjica Forest
    Banjička šuma or Banjica forest is a forest in Belgrade, the capital of Serbia. It is located in the Belgrade's municipality of Voždovac.- Location :...

     — urban forest in Belgrade
    Belgrade
    Belgrade is the capital and largest city of Serbia. It is located at the confluence of the Sava and Danube rivers, where the Pannonian Plain meets the Balkans. According to official results of Census 2011, the city has a population of 1,639,121. It is one of the 15 largest cities in Europe...

    , Serbia
    Serbia
    Serbia , officially the Republic of Serbia , is a landlocked country located at the crossroads of Central and Southeast Europe, covering the southern part of the Carpathian basin and the central part of the Balkans...

    , 41.6 ha. Protected due to diversity of bird species.
  • Million Tree Initiative
  • Ottawa Forests and Greenspace Advisory Committee
    Ottawa Forests and Greenspace Advisory Committee
    Established in 2001, the Ottawa Forests and Greenspace Advisory Committee provides a forum for interested citizens and City Councillors to engage on current forest and greenspace issues in the City of Ottawa...

  • Sanjay Gandhi National Park
    Sanjay Gandhi National Park
    Sanjay Gandhi National Park , previously Borivali National Park, is a large protected area in the northern part of suburban Mumbai city in Maharashtra State in India...

     in Mumbai
    Mumbai
    Mumbai , formerly known as Bombay in English, is the capital of the Indian state of Maharashtra. It is the most populous city in India, and the fourth most populous city in the world, with a total metropolitan area population of approximately 20.5 million...

    , India
    India
    India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

    ; the largest national park in the world located within city limits.
  • Tijuca Forest
    Tijuca Forest
    The Tijuca Forest is a mountainous hand-planted rainforest in the city of Rio de Janeiro, Brazil. It is the world's largest urban forest, covering some 32 km²...

     — the largest urban forest in the world, in Rio de Janeiro
    Rio de Janeiro
    Rio de Janeiro , commonly referred to simply as Rio, is the capital city of the State of Rio de Janeiro, the second largest city of Brazil, and the third largest metropolitan area and agglomeration in South America, boasting approximately 6.3 million people within the city proper, making it the 6th...

    , Brazil
    Brazil
    Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

  • Toronto ravine system
    Toronto ravine system
    The Toronto ravine system is one of the most distinctive features of the geography of Toronto, Ontario, Canada. It is a network of deep ravines that form a large urban forest that runs throughout much of the city...

  • Urban forestry
    Urban forestry
    Urban forestry is the careful care and management of urban forests, i.e., tree populations in urban settings for the purpose of improving the urban environment. Urban forestry advocates the role of trees as a critical part of the urban infrastructure...


External links

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