Urban Forests

What Urban Forests Do

Urban forests improve air quality through the absorption of pollutants by the canopy cover. Among the most serious urban pollutants are ozone, nitrogen oxides, sulfuric oxides, and particulate matter. Emissions from motor vehicles and industrial facilities are the main sources of nitrous oxides, sulfuric oxides, and volatile organic compounds. Ground-level ozone, or smog, is formed by chemical reactions between nitrous oxides and volatile organic compounds in the presence of sunlight. High temperatures increase the rate of ozone formation, which aggravates asthma and other respiratory illnesses such as emphysema and bronchitis. Urban trees and forests help to reduce air temperatures, reducing ozone formation and providing cleaner air for urban residents.

Urban forests help to protect watersheds. As cities grow larger, the demand for clean drinking water increases and cities become dependent on peri-urban forests for municipal drinking water. These forests have a great effect on the movement and flow of water. Falling rain is intercepted and absorbed by the tree canopy and, through evapotranspiration, is returned to the atmosphere. Water that does reach the ground is absorbed by the forest floor, where it percolates through the soil and becomes ground or surface water. In addition, healthy forests stabilize slopes and streambanks, minimizing erosion and reducing sediment inputs to urban streams, rivers, and ponds. Unlike impervious surfaces, urban forests act like a sponge and store water, which helps to reduce the load on drainage systems. In storing water, the soil, leaf litter, and vegetation of an urban forest act to remove stormwater pollutants acquired from roadways, sidewalks, yards, and buildings, thereby lessening the amount of potentially harmful substances leaching into surface water. By providing shade, urban forests reduce the temperature of urban waterways, thereby increasing dissolved oxygen and reducing stress for aquatic life.

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