Urban Forests – Why We Care

Urban forests help purify the air we breathe.

Urban trees in the lower U.S. have been found to remove nearly 800,000 tons of air pollution from the atmosphere every year.[1] In the modern day of bustling factories and countless cars on the road, this service of air purification has become more necessary than ever. As mentioned in a U.S. Forest Service report,“Urban forests freshen the air we breathe by releasing oxygen as a byproduct of photosynthesis. Net annual oxygen production differs depending on tree species, size, health and location. A healthy tree, for example, a 32-foot-tall ash, produces about 260 pounds of net oxygen annually. A typical person consumes 386 pounds of oxygen per year. Therefore, two medium-sized, healthy trees can supply the oxygen required for a single person over the course of a year. In colder climates, oxygen release will be less than in areas with longer growing seasons.”[2]

Credit: Perpetual Sunset/Flickr

Urban forests help manage a city’s water.

A single front-yard tree can intercept 760 gallons of rainwater in its crown, reducing runoff and flooding on your property.[3]

Because a city has so many impermeable surfaces, rainwater often builds up rather than being absorbed into the ground. This means that even a small rainstorm can cause flooding, as most of the water overflows into the stormwater system rather than into the ground. As the rainwater flows over the pavement, it becomes contaminated with pollutants and may eventually end up in our urban streams and waterways — and even our faucets.

Urban forests help a city’s water in several ways:

  • They intercept rainfall, allowing the water to be absorbed into the tree, roots and soil. This often results in cities not having to build as many artificial stormwater controls, saving the city and its citizens money.
  • They purify the water on its way into the ground by removing the pollutants collected.
  • The water retained by the urban forest also helps to sustain the growth of the urban trees, parks and vegetation.

These services, provided naturally instead of artificially, can save a city billions of dollars each year.

Urban forests help reduce energy demand.

When planted in the right place, urban forests provide shade to homes, businesses, roads and parking lots. Just three large trees around your home — two on the west side and one on the east — can provide enough shade to reduce your air-conditioning costs by 30 percent in the summer. And, when placed properly to reduce wind exposure, they can reduce heating bills in the winter by two to eight percent.[4]

Urban forests help improve the urban experience for its residents.

The strategic planting of urban trees reduce the noise of the city considerably. Tall, dense trees with soft ground surfaces can reduce noise by 50 percent or more.[5] And, it has been shown that urban trees help provide an overall feeling of community well-being. For example, a stronger sense of community and empowerment to improve neighborhood conditions in inner cities has been attributed to involvement in urban forestry efforts.[6]

For more information on the social benefits of urban forests, check out the resources at the College of Environment, University of Washington, on Human Dimensions of Urban Forestry and Urban Greening.

These are only a few of the many benefits that urban forests can provide if kept healthy and if they are cared for properly. They also provide bird and wildlife habitat and recreational opportunities, improve soil quality and reduce erosion, add to your home’s property value and more.

Urban forests can help mitigate climate change.

Urban Forests can help to mitigate climate change two ways: through direct carbon uptake and sequestration and by reducing the amount of carbon and other greenhouse gases emitted.

Shrubs and trees store carbon in woody tissue, leaves and roots for years — even decades to centuries — taking carbon dioxide out of our atmosphere, reducing the greenhouse gas effect and slowing the rate of climate change. In fact, urban trees in the lower 48 U.S. states store 770 million tons of carbon, valued at $14.3 billion.[7]

Trees in urban areas, through shade in the summer and as windbreaks in winter, help limit the need for cooling and heating and reduce the amount of greenhouse gases emitted by power plants, commercial and residential buildings. According to the U.S. Forest Service, “reduced emissions from trees can be substantial, especially in regions with large numbers of air conditioned buildings, long cooling seasons and where coal is the primary fuel for electric power generation.” For more information, check out this article on “Urban Tree Planting and Greenhouse Gas Reductions.”

Urban forests can help communities adapt to climate change.

Urban trees can be strategically placed to better prepare communities for the impacts of climate change. For example, specific species of trees in urban areas can be planted to provide shade, thereby reducing the Urban Heat Island Effect and creating more comfortable outdoor space in hot summers. Urban trees can be placed to create soil stability in areas that are prone to erosion from severe weather events and placed to provide protection from strong winds. For more information, check out the document called “Urban forests: A Climate Adaptation Guide” and this paper called “Climate Change Mitigation and Adaptation in Urban Forests: A Framework for Sustainable Urban Forest Management.”

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[1] Nowak, D.J.; Crane, D.E.; and Stevens, J.C. Air Pollution Removal by Urban Trees and Shrubs in the United States. Urban Forestry and Urban Greening. 2006, 4, 115-123.
[2] U.S. Department of Agriculture. U.S. Forest Service. Pacific Southwest Research Station. Northeast Community Tree Guide: Benefits, Costs and Strategic Planting. (accessed Oct. 15, 2012).
[3] U.S. Department of Agriculture. U.S. Forest Service. Pacific Southwest Research Station. Center for Urban Forest Research. Urban Forest Research March 2001. (accessed Oct. 15, 2012).
[4] Simpson, J.R. and McPherson, E.G. Potential of Tree Shade for Reducing Residential Energy Use in California. Journal of Arboriculture. 1996, 22(1), 10-18.
[5] Cook, D.I. In Proceedings: National Urban Forest Conference, Trees, Solid Barriers and Combinations: Alternatives for Noise Control, Syracuse, New York, 1978; Hopkins, G. Ed.; SUNY College of Environmental Science and Forestry.
[6] Kuo, F. The Role of Arboriculture in a Healthy Social Ecology. Journal of Arboriculture. 2003, 29(3), 148-155.
[7] Nowak, D.J. and Crane, D.E. Carbon Storage and Sequestration by Urban Trees in the USA. Environmental Pollution. 2002, 116, 381-389.

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