A Pixel Worth 1,000 Words
Satellite images reveal startling tree loss in American cities
As some of the largest living things on the planet, trees should be impossible to miss. But apparently they can be invisible. Urban forests are fast disappearing, and nobody has noticed. Victims of air pollution, Dutch elm disease, and development, many of the trees that had made American cities cool and green are gone. Like the aging of a familiar face, the decline in city trees has been so gradual as to be almost imperceptible.
City arborists noticed they were losing their charges but had no way of quantifying the damage, particularly since only 10 percent of urban trees are owned by cities. The loss isn't purely aesthetic. Tree leaves filter tons of pollutants from the air, their roots absorb storm-water runoff, their canopies insulate houses and shelter pedestrians, and their tissues serve as carbon sinks that help limit global warming. But without a scientific method to measure tree loss and its economic consequences, tree advocates were hard pressed to make a case for preservation.
Help came from an unexpected quarter: outer space. NASA's Earth-observing Landsat satellites have been photographing the planet since 1972. The color pixels in Landsat images represent the wavelengths of light reflected from water, vegetation, and other features on the Earth's surface. Gary Moll, a forester and vice president at American Forests, a tree conservation organization, decided to merge Landsat data with a computer mapping technique called GIS, for geographic information systems, to measure changes in tree cover over time. In 1992, Moll began comparing decades-old Landsat images with more recent ones, fine-tuning the computer analysis over time to gauge the tree cover density within the 30-meter width represented by each pixel.
The system worked so well that by 1998, Moll and his American Forests colleagues felt ready to tackle an analysis of the 3.9-million-acre Puget Sound watershed that surrounds Seattle. Population in the area had doubled between 1962 and 1998 as high-technology and other service-sector jobs drew migrants from other areas. The extent of the tree loss revealed in the maps startled even the foresters. In Puget Sound, the amount of land with less than 20 percent tree-canopy coverage more than doubled over that time, from 25 to 57 percent. At the same time, the amount of land with more than 50 percent canopy coverage dropped by 37 percent, from 42 to 27 percent.
Moll then programmed the computer to plug the mapping results into standard engineering formulas capable of ascribing dollar values to the services the lost trees once provided. By Moll's calculations, Puget Sound lost much more than green space when it felled its trees. Peak storm-water runoff, which would have been absorbed by trees and soil, increased by 1.2 billion cubic feet, or 29 percent. Containing that flood in overflow reservoirs costs communities an estimated $2.4 billion. And even if residents don't miss the trees, their lungs surely do. The vanished trees could have filtered 35 million pounds of pollutants from the air, removing greenhouse gases such as ozone and carbon monoxide and capturing particulates that irritate airways. Filtering that volume of air with industrial techniques would have cost $95 million.
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