By Gary Moll, American Forests, Washington, D.C.

Urban land cover can be roughly divided into green (trees, shrubs, grass) and gray (buildings, parking lots, sidewalks, streets). When communities grow and develop they lose green space, and recent studies show natural areas declining by 30 percent or more over the last 25 years and urban surfaces increasing by 20 percent.

The City of Salem, Oregon, has taken advantage of satellite images and computer technology to incorporate its green land cover into the City's gray infrastructure. Combining these two pieces of data into one decision making model provides City leaders with the information they need to make better decisions about the future of their community. While building and maintaining a transportation system of roads and sidewalks to connect residential and Business interests is challenge enough for most cities, in Salem they wanted more.

The natural environment should be an asset to the City, so the City's Planning Department put the gray and green issues in context by using the data obtained from remote sensing, which described gray and green land cover in watershed units. With the aid of GIS software, which calculated the engineering work performed by the green land cover in terms of storm water and air quality, the Planning Department was able to provide the decision makers and the public with a comprehensive analysis of the City's structure.

How did the City of Salem create a context for decision making about the green and gray infrastructure? The first step was to determine what area should be considered. Oregon cities such as Salem have established an urban growth boundary that focuses urban development and growth around the city proper. It is the line that surrounds the city limits and marks the area where growth is most likely to occur. Using this boundary as a shapefile, high-resolution satellite imagery provided the data for analyzing land cover in units as small as 14 feet on a side. Using a knowledge-based image analysis technique, the land cover was divided into green and gray components. Next the land use planning shapefiles showing the 12 subwatersheds falling within the urban growth boundary were incorporated into the image by using ArcView and the CITYgreen extension from Esri Business Partner American Forests, Washington, D.C. Finally, calculations were made documenting the effect of the land cover on air quality and storm water management.

The high-resolution imagery provided a description of all of the land cover needed to run scientific and engineering formulas describing how the structure of the land affects its function in terms of air quality and water quality. The findings from the analysis were published on the Web. By visiting www.open.org/~naturalr/Trees/American_forests/Salem_final_report.htm, the residents of the Salem area, as well as anyone else who is interested, can find out detailed information about the effect of tree cover in any of the 12 subwatersheds on air and water.

This Web site encourages the public to evaluate the role of the green infrastructure in their part of the City. Using an ecological planning unit, such as a watershed, provides a community with an accurate way to measure the work that trees do to improve a city's infrastructure and save taxpayers money.

Measuring Successful Growth

Successful growth should not be measured by the miles of roads, number of buildings, or square miles of parking lots. Successful growth should be measured by the effectiveness of a community to blend into the natural landscape, mixing the green and gray elements of the community into one infrastructure.

Trees are indicators of the quality of an urban environment. The benefits they provide in terms of managing storm water and improving air quality can be calculated using formulas such as the model created by the United States Forest Service. While the science behind tree value measurements is complex, GIS technology makes the application of the science easy. With ArcView and the CITYgreen extension, a community's staff can quickly conduct complex calculations describing the effect of trees on the quality and quantity of air and water resources.

In most urban areas trees are considered an afterthought; after the roads and buildings are in place, you look for a place to put in the trees. Three studies by American Forests over the last 15 years show that finding a place for trees in a community after the infrastructure is in place leads to short lives for urban trees. The average urban tree lives only 32 years, and the average downtown tree lives less than 10 years. Over the last couple of years American Forests has used Landsat satellite images to document the loss of tree canopy in a dozen urban areas and found that trees not only provide communities with approximately $400 billion in storm water and air quality benefits, but metropolitan areas have lost 30 percent or more of their tree cover over the last 25 years.

Visit www.americanforests.org for more information.