Study's goal: Finding out how much Seattle's trees are worth

By LARRY LANGE

SPECIAL TO SEATTLEPI.COM

August 8, 2010

Standing on a steep incline surrounded by tall alders, cottonwoods and maple trees, the two men in orange vests stretch out a measuring tape.

Troy Beady lays one end on the forest floor at the higher elevation, then Jack Simonson stretches the other end straight ahead to a point where it meets the tree, about 5 feet above the descending ground. Then Beady tilts a hand-held meter until he can see the top of the mature alder they're surveying. He calls out the calculated figure, then Simonson adds on the length of the trunk below the tape and announces the tree's estimated height, in meters: "Thirty-five-point seven-five," he said. That's more than 117 feet tall.

Slowly, for the rest of this summer and maybe into next spring, the crew is circulating through Seattle, assessing city trees one plot at a time trying to answer a fundamental question: What is Seattle's urban forest worth?

The study is being undertaken with the U.S. Forest Service, the University of Washington, Cascade Land Conservancy, King County and Seattle. It will ultimately estimate the trees' economic worth to the city: how much pollution they absorb, how much summer cooling they provide and how much storm water they absorb in winter, showing how much they might save in energy and drainage costs.

The work advances beyond the estimates of the size of the city's tree "canopy," according to Ara Erickson of the Cascade Land Conservancy. It will be used to estimate the real-life economic benefits of having trees around. The answers might ultimately help the city decide how much more protections, if any, the city's trees need.

For the moment, tree-preservation advocates don't have economic arguments to make "except to say we have approximately this many trees," said Kathy Wolf, social scientist for the University of Washington and the U.S. Forest Service.

The new figures could provide tree advocates with more bottom-line suggestions, Wolf said. For example: "If we invest in more trees is that a favorable alternative to, say, investing in more light rail and investing in people car-pooling more?"

Erickson, the conservancy's green-cities director, said,"there's tons of benefits that are provided by urban forests" but it will help private landowners and development managers to have those quantified.