February 2013
Photo: Jacob Creswick
Post by Julia Triman, Biophilic Cities Project Researcher
Julia is a masters candidate in Urban and Environmental Planning at the University of Virginia.
Trees are widely recognized for their benefits to ecosystem health, and more and more cities have plans to increase tree canopy – to beautify, increase human health, assist the environment, and provide economic benefit among other reasons. Here is some of the latest research on urban tree canopy, including studies on the benefits of trees for people and nature in cities and how to measure tree canopy to make sure cities are reaching their goals.
What are the benefits of urban trees?
Recent research suggests many different reasons for increasing tree canopy in our cities. Researchers in Maryland are studying the urban heat island effect, and have released preliminary results agreeing with conventional wisdom that benefits provided by tree canopy and other vegetation can significantly contribute to reducing temperatures in urban areas (Loughner et al.: Roles of Urban Tree Canopy and Buildings in Urban Heat Island Effects: Parameterization and Preliminary Results). An earlier study in London modeled effects of tree canopy in the Greater London Authority, concluding that trees absorb a significant amount of particulate pollution, and planting trees in areas with the highest concentration of pollution will have the greatest effect on improving urban air quality (Tallis et al.: Estimating the removal of atmospheric particulate pollution by the urban tree canopy of London, under current and future environments).
The Million Trees NYC Plan selected neighborhoods for tree planting that have disproportionately large populations with asthma, extending the importance of trees for maintaining high air quality to the benefits for human health: http://www.milliontreesnyc.org/html/million_trees/neighborhoods.shtml. Million Trees NYC also includes several other important reasons for increasing tree canopy, including: noise pollution, promoting habitat density, and reducing the percent of impervious surface in the city, among others.
Measuring Tree Canopy
To advance the objectives identified above, New York City now has a high-resolution land-cover map, developed using object-based image analysis, that includes tree canopy data to the level of detail of individual trees. On the ground, as part of their Urban Forest Plan, San Francisco has conducted a Street Tree Census, gathering information on location, age, species type, and condition of 27,000 of the city’s street trees. Washington, D.C.’s Casey Trees maintains an interactive map of the city’s street trees, derived from GIS data and LiDAR elevational data. Work recently published in the Journal of Forestry addresses the challenges inherent in measuring tree canopy (time, expense, etc.) and offers a case study of urban tree canopy in Winchester, Virginia as a model (McGee et al.: “Using Geospatial Tools to Assess the Urban Tree Canopy: Decision Support for Local Governments”). Finally, Remote Sensing of Environment published an analysis comparing two types of imagery for assessment of urban tree canopy in Tampa, Florida; in the case of this study, WorldView-2 imagery was found to be more effective (Pu: “A comparative analysis of high spatial resolution IKONOS and WorldView-2 imagery for mapping urban tree species”).
Additional Resources
American Forests has recently released Urban Forests Case Studies, detailing tree-planting efforts, successes, and challenges of twelve U.S. cities, including Portland, Oregon, Milwaukee, Wisconsin, and Baltimore, Maryland.
Seattle’s Forest Ecosystem Values is a recent publication detailing a variety of rationale for maintaining and increasing tree canopy, including:
Residential Building Energy Effects (energy savings for homeowners)
Replacement value of $4.99 billion
Ecosystem services including improved human health and well-being
Works Referenced
Loughner, Christopher P., Dale J. Allen, Da-Lin Zhang, Kenneth E. Pickering, Russell R. Dickerson, and Laura Landry. (2012). “Roles of Urban Tree Canopy and Buildings in Urban Heat Island Effects: Parameterization and Preliminary Results.” Journal of Applied Meteorology and Climatology 51: 1775–1793.
McGee, John A., Susan D. Day Randolph H. Wynne, and Barbara M. White. (2012). “Using Geospatial Tools to Assess the Urban Tree Canopy: Decision Support for Local Governments.” Journal of Forestry 110 (5): 275-286.
Pu, Ruiliang and Shawn Landry. (2012). “A Comparative Analysis of High Spatial Resolution IKONOS and WorldView-2 Imagery for Mapping Urban Tree Species.” Remote Sensing of Environment 124: 516-533.
Tallis, Matthew, Gail Taylor, Danielle Sinnett, Peter Freer-Smith. (2011). “Estimating the Removal of Atmospheric Particulate Pollution by the Urban Tree Canopy of London, Under Current and Future Environments.” Landscape and Urban Planning 103 (2): 129-138.