When Chicago’s Richard M. Daley, Jr., a self-proclaimed tree-hugger born on Arbor Day, became mayor back in 1989, he vowed to plant a half-million trees. Daley hated concrete and was still heart sore at losing so many American elms. New trees would help revive his Rust Belt hometown. “What’s really important?” he asked one interviewer. “A tree, a child, flowers. We need to soften the cities. Neighborhoods need nature…We can help people by planting trees, by putting in pocket parks that are habitat.” Moreover, his city’s air was the fourth most polluted in the nation, and the U.S. Environmental Protection Agency was pressing for improvements. Daley wondered, “Don’t trees clean the air?” But no one could provide an answer grounded in science. 

And so, Daley offered almost $1 million to U.S. Forest Service scientists led by Rowan Rowntree to set up studies in Chicago. Their June of 1994 scientific report, Chicago’s Urban Forestry Ecosystem, was a revelation. Chicago, home to about 2.8 million people, was also home to 4.1 million trees! Thanks to Greg McPherson, project leader, foresters could now say that while the city’s 416,000 street trees made up only a tenth of its urban forest, they also provided a quarter of the tree canopy or “24 percent of the total leaf-surface area.” Overall, the city’s 11 percent tree canopy was half the size it ought to be. And, almost half of existing street trees were Norway maple and honey locust. The city needed to plant a much wider palette of species. 

Greg McPherson. Photo provided by Jill Jonnes.

While everyone knew that trees cooled down buildings. McPherson’s chapter on the “Energy-Saving Potential of Trees in Chicago,” measured the actual energy and dollar savings involved. The shade from a large street tree growing to the west of a typical brick residence, he wrote, could reduce annual air-conditioning energy use by two to seven percent. Three 25-foot trees “located for maximum summer shade and protection against winter wind could save a typical Chicago homeowner about $50 to $90 per year (5 to 10 percent of the typical $971 heating and cooling bill).” Planting more trees in built-up city neighborhoods would cool urban heat islands, reducing energy use. “Trees,” McPherson wrote, “can help defer the construction of new electric generating facilities by reducing the peak demand for building air conditioning.” 

Fast forward to April of 2007. Scientist McPherson was now “focused on street trees,” for they delivered the most “services.” And, they were the main preoccupation of urban foresters. He says, “We worked on developing sampling methods using a thousand trees in any city that would give you a good enough estimate of the total number of street trees, their species, their size, what the management issues were, and then figuring out all the benefits from these street trees. That would serve as a basis for going to the city council, giving you a legitimacy and road map for educating the public.” 

In the years since Chicago, McPherson’s staff of fourteen scientists and technicians at the University of California at Davis had been laboriously gathering a set of standard street tree data for one American “reference” city in each of the nation’s 16 climate zones. In 2005, New York City’s Chief Forester Fiona Watt urged McPherson and his team: “Use Queens!” for their northeast “reference city.” And once she had lured them to Queens, she persuaded them to expand their sampling to the street trees in all five boroughs. 

Photo provided by Jill Jonnes.

By April of 2007, Fiona Watt was able to proudly present to City Hall technocrats McPherson’s full-scale New York “Municipal Forest Resource Analysis. ” Years of ground-breaking science was finally paying off. Armed with powerful new data, Watt could show that street trees were a crucial part of the cityscape—important green infrastructure that delivered quantifiable ecosystem services and merited major investment. And what exactly did New York’s street trees --- all these London planes, Norway maples, callery pears, honey locusts, and pin oaks (the top five street trees)-- do? The bottom line was that New York City’s street trees delivered annually $122 million in benefits, or about $209 a tree. 

In eco-system services, New York City’s street trees annually saved roughly $28 million, or $47.63 per tree. Air pollution: Each street tree removed an average of 1.73 pounds of air pollutants per year (a benefit of $9.02 per tree), for a total of more than $5 million. Street trees reduced storm water runoff by nearly 900 million gallons each year, saving the city $35.6 million it would have had to spend to improve its storm water systems. The average street tree intercepted 1,432 gallons, a service worth $61, a figure large enough to impress cost-conscious city managers. 

And then there were benefits associated with aesthetics, increased property values and economic activity, reduced human stress, and improved public health, which were estimated at $52.5 million, or $90 a tree. These findings drew on straight-up economic studies of real estate prices as well as social science research, which showed, for example, that hospital patients who could see a tree out the window of their room were discharged a day earlier than those without such a view. Other studies demonstrated that shopping destinations with trees had more customers than those that didn’t, and leafy public-housing projects experienced less violence than barren ones. 

At the time that Fiona Watt lured McPherson to New York to figure out all those street tree benefits, her forestry division received $8 million a year to plant and tend street trees, and spent another $6.3 million to pay personnel. McPherson’s study showed that for every $1 spent on a New York City street tree, it was generating $5.60 in benefits, or $122 million total. Net benefit? An impressive $100 million! Quite a handsome return. Mayor Michael Bloomberg was impressed, and quadrupled the city’s forestry budget, from $8 million to $31 million. He also launched MillionTreesNYC as part of his larger sustainability PLANYC to address climate change. Last year, New York City foresters planted their millionth tree two years early!

Big Bird joined Mayor Bloomberg in planting the first tree in the MillionTreesNYC initiative. Photo by Daniel Avila - NYC Parks & Rec.

Today, you can ask almost any question about what city trees “do” for us humans (aside from being lovely to look at) and get real data, something you can see for yourself on the U.S. Forest Service’s free public software platform i-Tree. For many urban issues, in part: “Trees Are the Answer.”

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Here at The Conservation Fund, we are passionate about projects that bring nature to cities and invest in urban trees. Find out more about the work we're doing around the country:

Chicago
An ambitious agenda to create a vibrant green infrastructure of protected and restored lands and waters in the four-state region surrounding Chicago has been the centerpiece of The Conservation Fund’s work in the Chicago Wilderness Region.

Raleigh-Durham
As the Raleigh-Durham area of North Carolina continues to grow, development threatens to cover valuable land in the Upper Neuse River basin, home to nine drinking water reservoirs serving more than half a million people in eight municipalities, including the city of Raleigh. The Conservation Fund is working with local governments and nonprofit organizations to create a strategy to protect priority land in an effort to improve water quality for the region.

Atlanta 
Conservation efforts, especially in urban areas, often are complicated; but we’ve taken on the challenge in Atlanta with both environmental and economic benefits for communities across the city. 

Detroit
The people of Detroit are proud and resourceful. They love their city and want to see it bounce back and be even better than before. We believe in Detroit’s future, and we’re helping its recovery be green.

Nashville
The Conservation Fund led a team to develop an open space plan for Davidson County based on our national expertise in green infrastructure planning. Our goal was to develop the most progressive open space protection strategy in the Southeast U.S. 

Los Angeles 
As one of the nation's largest metropolitan areas, the Los Angeles region should be a recognized leader in park and recreation opportunities but instead is known for gridlocked freeways, paved riverbeds, and concrete irrigation channels. We have collaborated on a plan to help change that. Also check out the blog post from our partners at TreePeople, who recognize that trees need people and people need trees!