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.
Counting trees. Impressed with the system's capabilities, planners in the prosperous suburb of Bellevue, just across Lake Washington from Seattle, decided to conduct a more detailed study of the their own town's tree cover, neighborhood by neighborhood. To do this, they bought a scaled-down version of the American Forests mapping software, called CITYgreen. The $800 package is designed to help cities manage existing trees and minimize future environmental damage by calculating the effects of proposed development.
Aerial photos of Bellevue were taken with a digital mapping camera. From the data, planners outlined representative plots of different types of tree cover. A local leadership group and student volunteers then went out on foot to assess the size, health and species of all trees on the sample plots.
Bellevue just completed its local analysis this spring, but planners already credit the project with at least two tree-preservation victories--stopping one effort by property owners to cut down trees to enlarge their view, and adding more land to Bellevue's park system. "It's a marketing tool to get the information out in an easily digestible manner," says Dan DeWald, Bellevue's natural resources manager. After being shown the dollar benefits of open space, DeWald says, policy makers "didn't blink an eye at the cost" of the land.
Smart growth. The Pacific Northwest is not alone in finding that a booming economy and robust development mean fewer trees. American Forests has found significant tree loss from the early 1970s to the mid-1990s in every area it has surveyed. Metropolitan Atlanta, once famed for its trees, lost more than half of its heavy tree cover (50 percent canopy coverage) between 1974 and 1998, a drop from 57 percent to 30 percent. Heavy tree cover in the Canton-Akron area of Ohio shrank from 55 percent to 38 percent; in Roanoke, Va., it decreased from 40 to 35 percent.
In Atlanta, sprawl-induced air quality problems have grown so severe that last year the federal government banned new highway construction until the area can come up with a better plan to manage growth. "Trees are bulldozed every day, and I get calls every day from people asking 'what can I do?' " says Marcia Bansley, director of the nonprofit group Trees Atlanta. "People are very, very upset. If you're going to have smart growth, you're going to have to have some way to preserve green spaces so people don't feel like they're living in a desert."
Last month, Georgia Gov. Roy Barnes appointed Joel Cowan, an environmentalist and developer, to lead the Georgia Regional Transportation Authority, a commission formed to mitigate the region's transportation woes. Cowan has been meeting with American Forests to investigate using CITYgreen and tree planting to improve air quality.
The decline in the ranks of urban forests comes at a time when rural areas are becoming more wooded than they were at the turn of the century, due to a decrease in farming. But today, most people in the United States live in cities and the suburbs being built around them. "We're not against development," says Moll, but "people developing right now are not thinking it's important to keep trees." Moll and other foresters say it's possible to preserve existing trees by building around them, and they encourage homeowners to plant new trees in barren subdivision yards and redeveloped city blocks. With these efforts, the foresters say, the decades-long loss of urban trees can be reversed, and the "invisible" forest will stand strong once again.
This story appears in the July 19, 1999 print edition of U.S. News & World Report.