A new way to see our city forests.
What do you see as you walk through your neighborhood? Show three people the same street scene, and chances are you'll get three different answers. A resident might see homes surrounded by bright summer-green shade trees. An urban forester would probably see a rich diversity of tree species and ages, noting which publicly maintained trees need attention. To an ecologist it might be a complex set of living and non-living features operating together to sustain life.
A natural disaster can change all these perspectives overnight; when Hurricane Andrew blew into south Dade County, it destroyed homes, businesses, trees, and familiar street scenes. In much the same way, new ideas and techniques are uprooting the traditional views and practices of managing the urban forest. Studies by AMERICAN FORESTS and others show that the quality and diversity of our urban forests are declining and suggest it is time to rebuild.
And just what is an "urban forest"? Broadly defined, it's an ecosystem of trees and other natural resources within a community. And as we realize the ecological complexity of our urban communities, cities--including those untouched by disaster--are taking advantage of high-tech wizardry to improve how they inventory, plan, and manage their trees. Mapping methods as old as the Civil War are being combined with state-of-the-art computers to create a new image of the urban forest.
This new image puts ecology on equal footing with other factors considered by the designers, engineers, and policymakers who physically shape our communities. In south Dade County, for example, the legacy of Hurricane Andrew--$30 billion in damage, 300,000 homeless, and more than 63,000 homes and 80 percent of Homestead-area businesses destroyed--is a clean slate on which to build a new, more ecologically sound community.
But how do you justify this type of rebuilding in a time of shrinking public dollars when we cannot fully measure the urban forest's value as an environmental, economic, social, and aesthetic resource? How do you balance its worth against the wealth of other public needs? The challenge now is to define its value, measure the costs and benefits, and translate those benefits into urban policy.
Before assigning a value to the urban forest, practioners first must describe exactly what's in it. Urban forest inventories traditionally have accounted for public trees--the ones planted along roads, in parks, and adjacent to public buildings. Some of these surveys are nothing more than drive-by counts; others are tree-by-tree inventories that enter information such as size, condition, and maintenance needs into a computer database. This information helps managers budget money for maintenance and set priorities for their crews.
But that still doesn't put a price tag on the benefits of the entire urban forest. Publically maintained trees typically represent only 10 percent of this resource; the other 90 percent are those trees on private property, in woodlots, and along waterways. Now urban foresters are utilizing sophisticated computer equipment to more easily arrive at a dollar value.
AMERICAN FORESTS is working with the U.S. Forest Service on a new way to map and understand urban forests' diversity and to categorize important ecological elements. Technicians turn aerial photographs and satellite images into digital maps, interpret the data, and produce computerized maps that can be easily modified for planning purposes. They can also be translated into a form planners and policymakers can use to make decisions.
The concept of mapping to better identify and understand our world is nothing new--aerial maps can be traced back to the Civil War. And in 1886, Canada's surveyor general mapped the rugged terrain of the Canadian Rockies using aerial photographs taken from balloons.
Of course, the process has undergone tremendous changes since then. One advance is the technique of layering different sources of information such as natural resources, human impacts, zoning sectors, and census data, first popularized in the 1960s by Ian McHarg, an urban planner at Harvard University. "Zones" were drawn around each resource on a transparent piece of paper and then overlain on a base map. Each resource added a layer of information, and the final product allowed planners to study the way the various environmental, socio-economic, and land-use pieces fit together. As computer technology developed, the layering process expanded dramatically, turning complex data into information-rich images. Today microcomputers can analyze information captured in digitized maps--called Geographic Information Systems (GIS)--allowing even average-sized communities access to powerful images of their urban forest resource.
GIS allows users to enter, store, analyze, and manipulate data in a computer-generated map using aerial photographs or multispectral images. The user adds separate layers of information--trees, buildings, roads, utilities, and waterways--that can be viewed individually or in various combinations.
Aerial photographs use several types of film: black and white panchromatic and true-color film, which are sensitive to reflected light--like film for a 35mm camera; and infrared films, which are sensitive both to visible light and to reflected infrared light, invisible to the human eye. Colors appear as "signatures" to our eyes, and these signatures are even more distinct in infrared, making this film invaluable in evaluating the composition and health of different species.
The type of information derived from these images depends on the quality of the photo, the scale at which it was taken, and the season in which it was taken. It also depends on the experience of the photo interpreter, who views the images through a stereoscope to transform flat photographs into three-dimensional images. Aerial maps can show detail with great accuracy and are excellent indicators of change over time, since they can be compared against historical photos.
Satellite images can also be used in GIS mapping. They too rely on sunlight reflected from different surfaces to produce wavelengths. Commercial and government satellites with sensors orbit the earth and record these wavelength signatures. This method collects data in a digital format--as opposed to a standard photograph--and requires specialized equipment to process and create an image.
In a satellite image, forests, croplands, developed areas, and geologic features all have their own individual signature. These signatures are converted into a numerical or "digital" value and stored in a computer. The electromagnetic spectrum is divided into different color regions that are used for interpreting different natural resources. For example:
Green region (visible to the eye) identifies the condition of vegetation.
Red region (visible to the eye) distinguishes plant species, soil boundaries, and geologic features.
Reflective Infrared region (invisible to the eye) assesses biomass and identifies crops, soil types, and land use.
Once the images are captured, via either an aerial photo or a satellite image, they are scanned into a GIS image-processing system. Several contractors are working with AMERICAN FORESTS to interpret GIS images for use in urban forestry. On a large scale, the entire tree canopy cover--the land area covered by trees' leaf surfaces in relation to a city's built surfaces--can be identified, and from that, environmental benefits calculated. On a small scale--used in site planning--the image can be enlarged to show individual trees, homes, and utilities. Comp-Tron Inc. in Baltimore, Maryland, is under contract with AMERICAN FORESTS to interpret both small- and large-scale GIS images. The idea is to create maps that can be easily combined with the other maps a community uses regularly--maps of underground utilities, zoning, and roads, for example.
"The beauty of using this new GIS technology over the manual mapping method or straight photo interpretation is that GIS maps can be linked with a wealth of information stored in a computer database," explains Chris Daniel of Comp-Tron, Inc. Planners can instantly call up history or specifications on features such as individual trees or buildings, utility lines, or roads.
The information gathered in the GIS maps explain the value of urban trees through four distinct "relationships." Together they give a complete picture of the urban forest.
The "spatial relationship" describes the relationship between natural resources and the "built environment"--roads, buildings, and utilities. On the large scale, development pressures that encroach on the remaining open space in Ann Arbor, Michigan, have prompted the city to propose a Natural Features Preservation Ordinance that provides greater protection of forested woodlots, wetlands, and landmark trees. But since the ordinance can be effective only with a comprehensive inventory of the area's resources, Maine's James W. Sewall Company produced color-infrared aerial photos and GIS maps to identify sensitive land areas for protection.
Small-scale GIS maps are helping with the mammoth task of rebuilding south Dade County. Before the hurricane, AMERICAN FORESTS had designated the county as a Cool Community. Seven localities across the country are participating in the AMERICAN FORESTS/Environmental Protection Agency pilot project to save energy and lower heating and cooling bills through strategic tree planting and lightening of surface colors.
"We have a clean slate to work from, and we are planting for energy conservation," says Nancy Masterson, AMERICAN FORESTS' southeast regional coordinator and a 17-year resident of south Dade County. "With every tree we plant, we also speed the healing process in rebuilding our neighborhoods and our lives."
To begin the process, before-and-after aerial photographs that show houses, roads, and other physical elements are used to locate planting sites for energy-efficient trees.
The "functional relationship" shows how trees are environmentally beneficial. The GIS maps can be used to compare tree-canopy coverage, since many of the benefits trees provide are based on their canopy cover. Leaves intercept rainfall, minimizing erosion; they release water vapor, cooling the surrounding air; and they produce oxygen. In a recently released study by the Forest Service's Chicago Urban Forest Climate Project, maps of cities that measured less than 50 percent canopy cover showed ample planting space for trees, especially in residential yards and unimproved lands. Los Angeles' urban forest had about 15 percent tree cover, and Chicago as a whole had 11 percent.
Dave Nowak, a Forest Service researcher on the project, noted that, "This is one of the first comprehensive analyses of an urban forest area to determine how trees affect temperatures and energy costs." The final report will be released next year.
Once researchers have mapped and analyzed the trees' spatial and functional relationships, they can quantify their costs and benefits, the third "relationship." Researchers with the Forest Service have developed models that can assess an urban forest's dollar value based on its ability to cool homes and cities without using fossil fuels, to retain soil and water, and to reduce pollutants in the air. Four of the Cool Communities are testing the cost/benefit model using the climate, building type, and tree species specific to their area.
Added to the mix of considerations is the fourth relationship--how environmental information can best serve a community's needs. In Baltimore, Yale's Urban Research Institute staff digitized satellite images into GIS maps and then added layers of information on streets, addresses, census tracts, and property records. That information was used to evaluate the relationship between social variables: home ownership, abandoned lots, levels of community involvement, and the health of the urban forest.
"This information will help us understand and focus our community outreach efforts to increase the stewardship of trees while understanding the needs of the citizens we work with," said Sarah Buikema, community activist and research assistant for the Baltimore project.
Everyone working in urban forestry has an important role to play in this emerging science. Researchers are working to understand the complexity of our urban forest ecosystem; computer scientists are developing the tools necessary to map and see the urban forest in this new way. Planners, designers, and engineers are responsible for incorporating the urban forest into the land-use planning process. Urban forest managers must adopt sustainable tree-care practices and understand how their management techniques will affect all the natural resources in their community. Citizen activists, tree commissioners, and environmental groups must promote ecologically sound urban forestry programs and policies, and lobby for the money needed to perpetuate our valuable resource.
In the end, if we are to maintain healthy communities, we must all begin to see the urban forest through new eyes.
Gary Moll is AMERICAN FORESTS' vice president of urban forestry. Cheryl Kollin is director of urban forestry.
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|Date:||Sep 1, 1993|
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