Salem Tree Canopy Methodology
State Capitol in bloom
Click on the terms below to learn more about the methodology that was used in the tree canopy analysis. If you have questions or would like more information, please contact Water Resources staff at 503-588-6211.
Ikonos Satellite Imagery 
High resolution satellite imagery was obtained from the Ikonos Satellite, purchased through the Space Imaging Corporation. Resolution of this imagery is 4 meter multispectral and 1 meter panchromatic. Multispectral means that data is being collected simultaneously in more than one band of the electromagnetic spectrum. For example, the landsat Thematic Mapper (TM) sensor collects data simultaneously in 7 bands: blue, green, red, near Infrared, mid-Infrared, Thermal Infrared, and another Mid-Infrared. Panchromatic technically means that it is sensitive to all colors. Operationally, however, it means black and white. Panchromatic sensors are sensitive to all visible colors, but they assign them to shades of grey in the image.
Full pixel classifications were conducted on the 4 meter multispectral images producing detailed land cover classifications for Salem. Ecological Benefits analyses were conducted based on High-Resolution Satellite imagery using an advanced version of CITYgreen software capable of handling raster data sets. The City of Salem provided boundary data for the UGB, watersheds and riparian corridors; once land cover compositions were classified in the Ikonos Imagery, American Forests estimated the ecological and economic benefits. Top
Trees and soil function together to reduce stormwater runoff. Trees reduce stormwater flow by intercepting rainwater on leaves, branches, and trunks. Some of the intercepted water evaporates into the atmosphere and some soaks into the ground, decreasing peak flows and the total amount of runoff that must be managed in urban areas. Trees also slow storm flow, reducing the volume of water that must be managed at once.
Communities that use increased tree cover to help manage stormwater can reduce the cost of constructing stormwater control infrastructure. The value of trees for stormwater management has been calculated based on avoided costs of handling stormwater runoff. Local costs are multiplied by the total volume of avoided storage to determine dollars saved by trees. For example, in the Salem Urban Growth Boundary study area, the existing tree canopy reduces the need for stormwater management by almost 50 million cubic feet. Using a conservative estimate of $2.00/cu ft. stormwater management cost, trees currently save the City of Salem nearly 100 million dollars in one-time construction costs. Top
Trees provide air quality benefits by removing pollutants such as nitrogen dioxide, carbon monoxide, sulfur dioxide, ozone, and particulate matter less than 10 microns in size. To calculate the value for these pollutants, economists multiply the number of tons of pollutants removed by "externality costs," or costs to society not reflected in marketplace activity, as established by state public service commissions. This figure represents costs that society would have paid in areas such as health care, if trees did not remove these pollutants. Top
This study also analyzed the amount of carbon stored and sequestered per year in the region’s trees. Carbon accounts for about half the dry weight of most trees. The carbon related function of trees is measured in two ways: storage, or the amount currently stored in tree biomass, and sequestration, the rate of absorption per year. Top
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