Urban Air Quality Briefing

A briefing paper from the National Association of State Foresters prepared by the Urban and Community Forestry Committee

Urban trees can play a significant role in State air quality plans. State air quality authorities are encouraged to adopt tree planting and urban canopy conservation and management as innovative, voluntary measures for improving air quality. NASF has published a primer on assessing urban forest canopy cover emphasizing the value in planting trees and maintaining tree cover as a way to increase the environmental benefits and provide a platform to build community support to increase the environmental health and economic vitality of places where we live, work and play.

Strategic tree planting and tree canopy conservation and management are innovative voluntary measures included in several adopted state plans to improve air quality (SIP-STATE IMPLEMENTATION PLAN). These measures will achieve area-wide expansion of the tree canopy, providing air quality and other benefits including reductions in ground-level ozone in metropolitan nonattainment areas. One of the most dramatic improvements achievable from area-wide comprehensive tree canopy conservation and planting is reducing the negative effects of urban heat islands (the rise in temperatures due to an increased number of buildings and impermeable surface areas retaining heat). Strategic placement of trees around homes, buildings, streets, and parking lots increases shade and evapotranspiration, thereby addressing the heat island effect by lowering summertime air temperatures and surface temperatures of asphalt, concrete, and other impervious areas.

During photosynthesis, trees release secondary metabolic products. Some of these include biogenic volatile organic compounds (VOCs), precursors to the formation of ozone. In most instances, the improvements in air quality gained from trees outweigh the concerns over additional biogenic VOC emissions. Additionally, large trees are considerably more beneficial for air quality than small trees.

To achieve reductions in ground-level ozone, government agencies, volunteer organizations, and private landowners must make long-term commitments to conserving existing canopy and planting significant numbers of trees in strategic locations.

Local Government Involvement
Under such measures local governments would be encouraged to:

  1. Measure Existing Resources and Track Changes - Initiate and/or enhance efforts to measure, track, and enhance existing urban tree canopy and canopy expansion efforts.
  2. Implement Programs to Enhance and Increase Benefits from Trees - Implement urban forestry programs to enhance canopy coverage to reduce summertime air and surface temperatures. Programs include planting trees in strategic locations to cool targeted surfaces and provisions for long-term maintenance. Priority planting sites include locations where buildings, streets, driveways, and parking lots will be shaded by the new plantings.
  3. Initiate Public Outreach -undertake a public outreach program designed to promote tree and canopy conservation and planting. Local governments, counties, states and regional organizations will work with volunteer tree planting organizations, school children, property owners, and stakeholder groups of businesses to support tree conservation and planting, conduct educational outreach regarding the benefits of trees and canopy, species selection, tree planting and establishment, and long-term tree maintenance. Efforts are made to document all conservation and planting efforts including voluntary programs.

State Forestry Agency Involvement
Under such measures state forestry agencies would be encouraged to:

  1. Develop Regional Canopy Management Plan -work with local governments to develop a long range plan to enhance tree conservation and planting, and to establish goals for increasing tree canopy coverage base on current baseline data between now and a 20/30 year horizon that could lead to lower levels of ground- level ozone pollution. Issues to address include coordination of efforts, tracking of progress in centralized databases, continuation and increases of resources from state and federal sources, involvement of private landowners and businesses, and periodic evaluations and reports.
  2. Species Selection -Promote the selection and planting trees of appropriate tree species which provide for large-canopy size and long-term survival based on specific site conditions and adjusted, when possible, for low-VOC emitters. Survival is considered as the most beneficial criteria thus the "Right tree in the Right place" is critical.
  3. Monitor Programs - Assist in monitoring these activities and report periodically within an agreed upon framework.

Related Actions by NASF
The National Association of State Foresters refer to the benefits of trees and forests in maintaining air quality in numerous publications, reports, news releases, letters, and statements, and has adopted the following resolutions and reports highlighting the benefits of trees and forests in maintaining air quality:

Resolution 1989-3 Trees to Reduce Global Warming
Resolution 1989-4 Forestry and Global Warming
Resolution 1990-2 Urban Forestry
Report: Assessing Urban Forest Canopy: A Primer for State Foresters

Commitments and Expectations
NASF affirms that:

  • Urban tree canopy should be considered as a voluntary measure in state and regional air quality plans.
  • Communications to initiate discussion of the issues identified above can best be accomplished by state foresters with their respective state air planning directors and municipal officials.
  • State foresters may be available to assist in developing appropriate baseline tree cover data in each SIP planning area in their state.
  • State foresters may be available to support the development and implementation of local and regional urban tree canopy management plans including setting urban tree canopy goals for communities and identifying strategic tree planting locations.
  • The US Environmental Protection Agency and the US Forest Service should work collaboratively to support development of a framework for tracking and reporting tree planting, tree survival, and growth consistent with standards for adopted voluntary canopy expansion plans incorporated in an air quality plan.
  • State forester involvement will inform the municipal and state decision making process on a range of scales, from prioritizing communities within a state to targeting individual properties for tree plantings by identifying and determining the amount of existing and potential tree canopy.


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