Why cities need trees: The science behind a 30 percent canopy

When Tacoma took on the goal of creating a 30 percent tree canopy in the city by 2030, officials relied on a study by the conservation organization American Forests to answer the question: How do you quantify a city’s need for trees?


Trees are vital in keeping a city’s air and water healthy for people.

The American Forests study, called “Calculating the Value of Nature,” analyzed the Tacoma-Everett-Redmond triangle using images from 1972 to 1996.

Dollar values were placed on the ability of trees to control stormwater flooding ($2.4 billion annually) and filter air pollution ($95 million annually.)

The loss, over that period, of 600,000 acres of highly canopied land meant a 35 percent increase in stormwater runoff, which affects salmon populations, pollution in Puget Sound, flooding on city streets, soil erosion and more.

“Each tree absorbs hundreds of gallons of water, filtering it through the root system so clean water goes back to the ground,” said Pierce Conservation District’s Melissa Buckingham.

And by filtering greenhouse gases such as carbon dioxide out of the atmosphere, trees contribute over time to cooling the air, as well as cooling it in real time with shade during summer.

According to California nonprofit Canopy.org, evaporation from one tree can produce the same cooling effect as 10 room-size air conditioners. That lowers the average summer temperature in highly canopied cities such as Palo Alto, California (which has 37.6 percent canopy), by up to 8 degrees compared with neighboring areas.

This effect will be increasingly important, officials say, during the hotter, drier summers forecast for Western Washington in climate change models.


As the city of Tacoma points out in its Urban Forestry Policy, numerous studies have shown that trees — the higher the number, the greater the benefit — raise property values by up to 15 percent.

Trees also increase profitability of business districts: Studies by University of Washington professor Kathleen Wolf show that tree-lined shopping streets increase shoppers willingness to pay for goods by 12 percent.

Properties rent more quickly, tenants are happier, and workers report higher productivity, Buckingham said.

That benefit is especially important in business districts such as McKinley, on Tacoma’s East Side, where a recent “depave” project replaced 7,000 square feet of sidewalk concrete with trees, shrubs and ground covers.

Because the trees are in the right-of-way, and thus the responsibility of property owners, the project was a partnership, lining up the city and Conservation District with business owners committed to caring for the young trees.

Now, even in winter, the shopping block in one of Tacoma’s low-income areas has gone from gray and bleak to more inviting, the pinks and reds of the sedums contrasting with the evergreen of pine and hemlock.

“It’s a good start to changing the feel of our neighborhood to warm and caring,” said resident and neighborhood council member José Chavez.


Canopy advocates such as Chavez and Buckingham also point out social advantages of trees: they slow traffic and shade sidewalks, making walking safer and more pleasant. They reduce noise and encourage folks to congregate.

“Adding trees gives a more welcome feeling,” Chavez said. “It shows that there are hardworking, caring people no matter what kind of neighborhood it is. … It means pride, too. That’s a good investment.”

Recent studies by the Pacific Northwest Research Station of the U.S. Forest Service go deeper into the social benefits of trees.

A 2011 Portland study showed greater tree canopy had positive effects on babies’ birth weights and thus lifelong health: Greater canopy cover within 50 meters of a mother’s house and the proximity to open space were associated with a reduced risk of having an underweight baby.

“It may be that trees improve birth outcomes through stress reduction,” said study author Geoffrey Donovan in the November 2011 issue of “Science Findings.”

His findings were backed up by a subsequent Spanish study showing that proximity to green space made positive birth weight differences in poor or low-educated socioeconomic groups.

Donovan’s study also found more tall street trees correlated with lower residential crime rates.

One explanation, he said, might be that mature trees create a pleasant setting for folks to congregate while not blocking views of houses, thus increasing the risk of exposure for criminals.

In addition, Donovan agrees with Chavez that trees indicate a cared-for neighborhood, which can be a disincentive for criminals.

Past studies have shown that views of trees speed healing in hospitals and reduce stress rates for residents.

In a 1984 study in the journal Science, R.S. Ulrich showed that surgery patients with views of trees spent fewer days in the hospital, used fewer narcotic drugs, had fewer complications and registered fewer complaints with nurses than patients with views of concrete walls.

Later studies also showed that patients responded better to brighter green trees and those shaped with a broad, spreading crown. One theory suggests this comes from our evolutionary ancestry on the African savanna, where trees are shaped like this.

Studies also have shown that views of nature can reduce stress in workers and university students.

As environmental scientist, author and academic Nalini Nadkarni points out, trees are also used to heal from loss, such as in the Living Memorials Project after the 9/11 attacks.

“For centuries, humans have looked to the symbolic resonance of nature ... to heal our sense of loss from death,” she said.

For Harvard biologist E.O. Wilson, the reason trees can heal goes far deeper. His “biophilia hypothesis” argues that humans have an innate emotional affiliation to other living organisms and that the health of the planet, trees included, is inextricably intertwined with our own.

“Trees aren’t just trees,” Chavez points out. “They’re many things.”


Trees don’t just contribute in just practical ways, says Nadkarni.

In her 2008 book “Between Earth and Sky,” Nadkarni, a former environmental studies professor at Olympia’s The Evergreen State College and founder of the International Canopy Network, also explores their spiritual benefits.

Citing her own personal and scientific experiences and those of other scientists and writers, she asserts that trees teach us about stillness, mindfulness, the interconnectedness of all life and what lies beyond the mundane.

“Trees at once enrich and instruct our lives in ways at once simple and complex,” Nadkarni writes. “Opening up to something as simple and pleasurable as climbing a tree — or sitting silently beneath it — can make humans feel at home with the world and with themselves.”