A recent Forest Service study indicates that trees are losing ground in many U.S cities, but Spokane appears to be an exception.
Researchers used aerial photographs to analyze tree canopies in 20 different urban communities. The overall trend was less tree cover – 17 cities had statistically significant net reductions in tree canopies over the past decade, the study found. In Spokane, however, the urban tree canopy appeared relatively stable.
Slightly less than 22 percent of the city’s landscape was covered by a tree canopy in 2007. That was down from 22.4 percent in 2002, but the drop wasn’t considered statistically significant, said David Nowak, project leader for the Forest Service’s research station in Syracuse, N.Y., which did the study.
Urban forestry is the research station’s focus. The Washington Department of Natural Resources had asked the researchers to provide baseline data for urban canopies in Spokane and Tacoma. Researchers expanded the study to look at urban trees in other cities, including New Orleans, where Hurricane Katrina took a heavy toll on urban tree populations, and Detroit, where an introduced pest is killing off the city’s ash trees.
Urban trees, Nowak said, are a result of both natural processes and human tinkering. Whether they’re ponderosa pines native to Spokane or ornamental nursery trees, urban trees provide tremendous social and environmental benefits, he said.
“Trees cool a city during the summertime, which has implications on energy use and human comfort,” he said. “Trees remove pollution from the atmosphere; they have a cleansing effect on air quality. They absorb carbon, which is important in terms of global warming. They provide wildlife habitat and they’re important aesthetically.”
Urban trees are vital for water quality, because they capture the stormwater runoff that carries pollutants to streams. Trees also soften the urban landscape, providing a calming effect for city residents.
“People’s bodies react to seeing vegetation,” Nowak said. “They change our body physiology in some fashion.”
Spokane already has one of the strongest urban forestry programs in the state, said Linden Lampman Mead, DNR’s urban and community forestry specialist. Through the Forest Service research, the community now has baseline data on its tree canopy, so it can measure future gains or losses, she said.
Nowak is encouraging communities to add to their urban tree data through field work, counting trees in parks and other public places, and documenting tree species. The landscape is always dynamic, he said, “and as we’ve seen in New Orleans, it can change rapidly over a short period of time.”
He also encourages communities to retain large, old trees in addition to planting new ones. A mature tree with a diameter of 30 inches or greater provides 60 to 70 times the leaf surface of a young tree with a 3-inch diameter. So, that older tree is providing 60 to 70 times the environmental benefits that the young tree provides, he said.
“I’d like to think that everyone out there is making an effort to preserve trees, but it’s sometimes difficult when you’re considering what the city is trying to do and what private landowners are trying to do,” said Angel Spell, urban forester for the city of Spokane.
To help Spokane residents appreciate the contributions of mature trees to the community’s quality of life, the city has a heritage tree program that recognizes trees of historical significance or trees with important or unusual attributes.
“They’re a public asset that appreciates over time,” Spell said of urban trees. “Even though planting trees is important, you won’t see the full benefit of the investment for decades to come.”