Benefits of Urban Vegetation

Trees and vegetation benefit not only the individual properties on which they are planted, but also the community at large. What’s more, they are aesthetically pleasing and help to beautify cities and towns.

The benefits of urban vegetation can be divided into two categories:
- Cooling and energy reduction
- Community-wide benefits


urban vegetation and energy savings

Urban vegetation can save energy in homes in a variety of ways, all of which reduce power plant emissions. Photo credit: Mike Thomas, International Society of Arboriculture (Click image to enlarge)

Cooling and Energy

One reason to increase trees and other vegetation in your community is their ability to cool buildings and neighborhoods, which can reduce building energy use and heating and cooling costs.


students planting a tree in Maryland

Students planting a tree in Maryland. Photo credit: Keith Weller, US Forest Service

Cooler communities

Here is some evidence that trees can help cool communities:

  • Trees can lower outdoor air temperatures by as much as 9°F (5°C) through evapotranspiration.[1]
  • In Davis and Sacramento, CA, neighborhoods with mature tree canopies enjoyed daytime temperatures that were 3 to 6°F (1.6 to 3.3°C) lower than in areas without canopies.[2]
  • Trees in large urban parks help create a cool “oasis” that affects local wind patterns. Changes in the density of cooler air in parks and warmer air in the urban areas generate cooling breezes, which can extend to the surrounding neighborhoods.[3]
  • A computer simulation found that a tree coverage increase of 25 percent in Phoenix, AZ, and Sacramento, CA—regions that are relatively hot and sunny—can result in temperature reductions of 6- 10°F (3.3-5.5°C) at midday in July.[4]

Cooler buildings & reduced building energy use and cost

Trees and vegetation can help cool buildings by providing shade from direct sunlight. This in turn reduces the need for air-conditioning, resulting in large energy savings and cost savings in hot and sunny climates.

  • Planting trees around two houses in Sacramento resulted in average energy savings of 30 percent for both houses from June through October.[5] The biggest energy savings came when trees were planted to the west and southwest of buildings.
  • In a computer simulation, two trees planted west and southwest of a building, forming a canopy that shaded 20 percent of the house, could reduce cooling bills by 8 to 18 percent and reduce heating bills by 2 to 8 percent.[6]
  • At the city scale, a study estimated that if the Los Angeles region increased vegetation in residential neighborhoods, near-surface air temperatures would drop by 1-2°C (1.8-3.6°F), which could save $20 million annually in air conditioning costs for the region.[7]

Community Benefits

In addition to their energy-use and cooling benefits, trees and plants provide many other community benefits.


interaction between trees and carbon dioxide

This illustration depicts the interaction between trees and carbon dioxide (CO2). Photo credit: Mike Thomas, International Society of Arboriculture (Click image to enlarge)

Reduced greenhouse gas emissions

Trees and plants are a natural carbon sink. In order to grow, plants absorb carbon dioxide from the air and turn it into oxygen and sugars through photosynthesis. This absorbed carbon is stored in the vegetation and in the soil. In recognition of trees’ ability to absorb carbon, the California Air Resources Board identified urban forests as an additional strategy to achieve greenhouse gas emission reduction targets under California’s Global Warming Solutions Act (AB 32), the landmark bill that commits the state to reducing its greenhouse gas emissions.

Additionally, the cooling effect of increased trees and vegetation can reduce energy consumption used for cooling. If the energy is generated by fossil fuels (such as natural gas, which is typically used in California), a decrease in energy consumption translates to a reduction in greenhouse gas emissions from power plants.

Improved air quality

Plants help to clean the air of pollutants, such as particulate matter. The pores of leaves remove dangerous compounds, such as nitrogen oxide, sulfur dioxide, carbon monoxide, and ozone from the air. A 2006 study found that urban trees remove about 711,000 metric tonnes of air pollutants in the U.S. every year, which confers an estimated $3.8 billion of economic value.[8] Tree shade can also help to reduce the evaporation of volatile organic compounds (VOCs) from parked cars by keeping the gas tanks cooler.

NOTE While plants and trees do improve air quality, they also release VOCs, also referred to as biogenic emissions. The emission of VOCs is affected by water, sunlight and humidity so there is wide variation in the emission rates. To learn more about the varying rates of emissions from trees, visit the CA Air Resources Board website for the Biogenic Working Group.


interaction between trees and air pollutants

This illustration depicts the interaction between trees and air pollutants. Photo credit: Mike Thomas, International Society of Arboriculture (Click image to enlarge)

Decreased stormwater runoff

Trees and vegetation can help reduce stormwater runoff, minimize erosion, and protect water resources. Plants intercept rainfall by collecting it on their surfaces  and can increase rainwater absorption by soil. During summers in Sacramento, evergreens and conifers were found to prevent 35 percent of rainfall from reaching the surface.[9]

Reduced pavement maintenance

Tree shade can help slow down pavement deterioration. One study found that shade trees reduced pavement resurfacing costs by 15 to 60 percent, depending on the type and location of the trees.[10]

Improved health and quality of life

In addition to reducing harmful air pollutants, trees and plants have many other benefits to health and quality of life. Increased shade reduces human exposure to the sun’s ultraviolet rays. Their cooling effects on neighborhoods and on indoor temperatures can help decrease health impacts from summertime heat waves. Trees and vegetation help to beautify communities and create more pleasant surroundings. They can provide habitats for birds, insects and other creatures. A good row of trees can serve as an effective sound barrier between homes and urban noise pollution, such as traffic.[11] Other studies have found that urban trees are associated with reduced crime, increased property values, and other psychological and social benefits that help decrease stress and aggressive behavior.[12],[13],[14] These benefits cannot be easily monetized but are important for creating sustainable communities.  


Footnotes:
 

[1] EPA. 1992. Cooling our Communities: A Guidebook on Tree Planting and Light-Colored Surfacing. US Environmental Protection Agency, Office of Policy Analysis, Climate Change Division. p32.

[2] Ibid.

[3] Spronken-Smith RA and Oke TR. 1999. Scale modeling of nocturnal cooling in urban parks. Boundary Layer Meteorology 93: 287-312. Accessed via American Planning Association. How cities use parks for climate change management. http://www.planning.org/cityparks/briefingpapers/climatechange.htm

[4] EPA. 1992. Cooling our Communities: A Guidebook on Tree Planting and Light-Colored Surfacing. US Environmental Protection Agency, Office of Policy Analysis, Climate Change Division. p32.

[5] Akbari, H., D. Kurn, S. Bretz, and J. Hanford. 1997. Peak power and cooling energy savings of shade trees. Energy and Buildings. 25:139-148. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p5)

[6] McPherson, E.G. and J.R. Simpson. 2000. Carbon Dioxide Reduction through Urban Forestry: Guidelines for Professional and Volunteer Tree Planters. PSW GTQ-171. USDA Forest Service, Pacific Southwest Research Station. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p5)

[7] Kurn DM, Bretz SE, Huang B, and Akbari H. 1994. The Potential for Reducing Urban Air Temperatures and Energy Consumption Through Vegetative Cooling. Lawrence Berkeley National Laboratory Report LBL-35320, Berkeley, CA.

[8] Nowak, D.J., D.E. Crane, and J.C. Stevens. 2006. Air pollution removal by urban trees and shrubs in the United States. Urban Forestry and Urban Greening. 4(2006):115-123. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p6)

[9] Xiao, Q., E.G. McPherson, J.R. Simpson, and S.L. Ustin. 1998. Rainfall Interception by Sacramento’s Urban Forest. Journal of Arboriculture. 24(4):235-244.  (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p8)

[10] McPherson, E.G. and J. Muchnick. 2005. Effects of Street Tree Shade on Asphalt Concrete Pavement Performance. Journal of Arboriculture. 31(6). (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p8)

[11] Nowak, D.J. and J.F. Dwyer. 2007. Understanding the Benefits and Costs of Urban Forest Ecosystems. In: Kuser, J.E. Handbook of Urban and Community Forestry in the Northeast. New York: Kluwer Academic/Plenum Publishers. 25-46. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p9)

[12] Wolf, K. 1998. Urban Nature Benefits: Psycho-Social Dimensions of People and Plants. Center for Urban Horticulture, College of Forest Resources, University of Washington, Fact Sheet #1. Seattle, WA. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p9)

[13] Laverne, R.J. and K. Winson-Geideman. 2003. The Influence of Trees and Landscaping on Rental Rates at Office Buildings. Journal of Arboriculture. 29(5):281-290. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p9)

[14] Kuo, Francis E. and W.C. Sullivan. 2001. Environment and Crime in the Inner City: Does Vegetation Reduce Crime? Environment and Behavior. 33(3):343-367. (Accessed via Reducing Urban Heat Islands: Compendium of Strategies - Trees and Vegetation, p9)

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