Urban forest
An urban forest is a forest, or a grove 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.
As opposed to a forest park, whose ecosystems are also inherited from wilderness leftovers, urban forests often lack amenities like public bathrooms, paved paths, or sometimes clear borders which are distinct features of parks. Care and management of urban forests is called urban forestry. Urban forests can be privately and publicly owned. Some municipal forests may be located outside of the town or city to which they belong.
Examples
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.Some sources claim that the largest man-made urban forest in the world is located in Johannesburg in South Africa. The city is located in the highveld, a grassland biome typically lacking large numbers of trees, yet Johannesburg is still a very densely wooded city with reportedly 10 million artificially introduced trees and is rated as the city with the eighth highest tree coverage in the world.
File:Tidal Basin blossoms by Matthew Bisanz.JPG|left|thumb|220x220px|Cherry Blossoms lining the Tidal Basin in Washington, D.C.
Rio de Janeiro is also home to two of the vastest urban forests in the world, one of which is considered by some sources to be the largest one. Tijuca Forest is the most famous. It began as a restoration policy in 1844 to conserve the natural remnants of forest and replant in areas previously cleared for sugar and coffee. Despite the worldwide recognition of Tijuca Forest, another forest in the same city encompasses roughly three times the size of its more prominent neighbor: Pedra Branca State Park occupies 12,500 hectares of city land, against Tijuca's 3,953 hectares. The larger metropolitan area encircles the forests which moderate the humid climate and provide sources of recreation for urban dwellers. Along with seven other smaller full protection conservation units in the city, they form an extensive natural area that contains the Transcarioca Trail, a 180-km footpath.
Sanjay Gandhi National Park in Mumbai, Maharashtra, India is also an example of an urban forest. It covers roughly around 20% area of the city. The forest is filled with many animals freely roaming around. It also has an important cultural site of ancient history situated in it known as the Kanheri caves.
Nebraska National Forest is the largest man-made forest in the United States located in the state of Nebraska. It lies in several counties within the state and is a popular destination for campers year-round.
Several cities within the United States have also taken initiative investing in their urban forests to improve the well-being and economies of their communities. Some notable cities among them are Austin, Atlanta, Nashville, New York, Seattle, and Washington, D.C. New York, for example, has taken initiative to combat climate change by planting millions of trees around the city. In Austin, private companies are funding tree-planting campaigns for environmental and energy-saving purposes.
Nashville has an organization known as the Alliance to Conserve Nashville's Highland Rim Forest serving as a catalyst to pool action from across numerous conservation nonprofits that include the Cumberland River Compact, Friends of Beaman Park, and the Tennessee Environmental Council. Nashville contains the largest urban forest within the city limits of any city in the world with a population exceeding 500,000.
Environmental impact
Urban forests play an important role in benefitting the environmental conditions of their respective cities. They moderate local climate, slowing wind and stormwater, and filter air and sunlight. 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.Air pollution reduction
As cities struggle to comply with air quality standards, trees can help to clean the air. The most serious pollutants in the urban atmosphere are ozone, nitrogen oxides, sulfuric oxides and particulate pollution. Ground-level ozone, or smog, is created by chemical reactions between NOx and volatile organic compounds in the presence of sunlight. High temperatures increase the rate of this reaction. Vehicle emissions, and emissions from industrial facilities are the major sources of NOx. Vehicle emissions, industrial emissions, gasoline vapors, chemical solvents, trees and other plants are the major sources of VOCs. Particulate pollution, or particulate matter, 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 effect in urban areas. This reduction of temperature not only lowers energy use, it also improves air quality, as the formation of ozone is dependent on temperature. Trees reduce temperature not only by directly shading: when there is a large number of trees it create a difference in temperatures between the area when they are located and the neighbor area. This creates a difference in atmospheric pressure between the two areas, which creates wind. This phenomenon is called urban breeze cycle if the forest is near the city and park breeze cycle if the forest is in the city. That wind helps to lower temperature in the city.
- 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%.
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
;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 effects they can cause.
A study across the Chicago region determined that trees removed approximately 17 tonnes of carbon monoxide, 93 tonnes of sulfur dioxide, 98 tonnes of nitrogen dioxide, and 210 tonnes of ozone 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 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 in 1991.
- Large healthy trees greater than 75 cm in trunk diameter remove approximately 70 times more air pollution annually than small healthy trees less than 10 cm in diameter.