By Ben Ginsberg
Chicago, IL (March 1, 2009)- William Penn’s “greene countrie town” has undergone a significant transformation since he laid out Philadelphia’s walkable street grid in 1682. Dense high-rise office and condominium towers blend with Penn’s original five squares and low-rise residential row houses. One result is that central Philadelphia now has 90,000 residents, the third largest downtown population in the U.S.
Recognizing the importance of a sustainable city in attracting residents and visitors, and of maintaining Penn’s vision, Philadelphia Mayor Michael A. Nutter established the city’s first cabinet-level sustainability position in 2008. Through renewed open space planning efforts, the city is creating baseline inventories of its sustainable amenities.
Among those amenities are street trees, which are integral to the quality of life in urban areas throughout the world. Think of the Champs-Elysees, La Rambla, or Rittenhouse Square. Street trees reduce the urban heat island effect, increase property values, absorb stormwater runoff, and create a calm green aesthetic. A recent Columbia University study found that children living in areas with a critical mass of street trees have a lower prevalence of asthma than children in treeless neighborhoods. According to the U.S. Forest Service, large, healthy trees remove 70 times more air pollution than smaller, recently planted trees. To plan for and maintain street trees, planners should undertake an analysis of the existing tree collection.
In June 2008, Philadelphia’s Fairmount Park Commission, the agency responsible for street trees, contracted with the Center City District, the business improvement organization for downtown Philadelphia, to conduct a street tree census. New York City’s Street Tree Census was used as a model.
From July through October 2008, Central City District planners and landscape architects spent 520 hours canvassing the 2.6-square-mile downtown, which was divided into 12 survey areas. Staffers used Hewlett-Packard iPac handheld computers and the program Arc Pad v.6. A special survey program, developed using ArcView v.9.2, enabled the users to document each tree’s location, species, condition, trunk diameter, estimated canopy diameter, and any special issues.
Using a species identification booklet created by the district, the survey found and identified 10,830 street trees in the city’s central area. Nearly 6,500 trees, or 60 percent of the downtown collection, fell into just a few species, including London planetree, red maple, sugar maple, honey locust, and Callery pear. More than 9,600, or 89 percent, were in fair or good condition. A dozen trees were singled out for immediate attention due to dangerously cracked branches and trunks. The survey also found 737 empty tree pits or stumps.
The Fairmount Park Commission is using the data to set priorities for its collection of street trees, specifically when and where to replace them. Areas with blank canopies or with concentrations of trees in poor condition will be the first to be addressed.
The prescient songwriter Joni Mitchell had it right when she wrote that “you don’t know what you’ve got ’til it’s gone.” This is especially true for trees. A street tree census is integral to sustainability planning and keeping pace with the fast-changing patterns of urban development. Planners cannot establish sustainability goals unless certain parameters, such as tree canopy size, are known. A census also provides a spatial framework in which planners can identify neighborhoods in need and establish priorities for funding and resources.
Philadelphia’s Tree Count