Urban Forestry? The Value of Trees

The term urban forestry sounds at first like an oxymoron, but an article in the Wilson Quarterly by Jill Jonnes, What Is a Tree Worth?, describes it as a growing area of expertise. Jonnes not only provides us with some interesting insight into the ecology and economics of urban trees, but also makes us aware of some psychological benefits as well.



Early city planners had an intuitive belief in the beneficial effects of tree laden neighborhoods, but with waves of redevelopment and increasing populations, the utility of the space-needing trees had to confront an economic argument. According to the author, it was Chicago’s first Mayor Daley who officially asked the question “What is a tree worth?” What effect do they have on pollution, energy consumption, water economy, or on anything else? Chicago provided some critical funding at a time when a team of scientists were available to address this question. They proceeded to come up with the needed answers and could provide a quantified cost analysis. Their results also provided guidance for city planners on how to best take advantage of trees.

“Certainly the trees of Chicago had long sweetened the air and sheltered homes and streets from hot summers and freezing winters, but now here were actual data to show it. ‘In 1991, trees in Chicago removed an estimated 17 tons of carbon monoxide, 93 tons of sulfur dioxide, 98 tons of nitrogen dioxide, 210 tons of ozone, and 234 tons of particulate matter,’ Rowntree and his colleagues said in the conclusion to their report. In neighborhoods where trees were large and lush, they could improve air quality by as much as 15 percent during the hottest hours of midday. More trees and bigger trees meant cleaner air....Trees in the Chicago metro area sequestered about 155,000 tons of carbon a year.”

Jonnes reports detailed results of a study commissioned by New York City a few years ago.

“....the New York Parks Department asked them to value all of New York City’s 592,000 street trees. With the advances made over the preceding dozen years, McPherson could deliver a far more sophisticated report than he had for Chicago. Energy savings: New York City’s 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. The report also calculated that street trees reduced stormwater runoff by nearly 900 million gallons each year, saving the city $35.6 million it would have had to spend to improve its stormwater systems. The average street tree intercepted 1,432 gallons, a service worth $61, a figure large enough to impress cost-conscious city managers.”

These physical responses of trees within the urban ecology are not surprising, although the quantitative results might be. What are more interesting are the psychological or emotional benefits they were able to quantify.

“McPherson and his colleagues were also able to tally various 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 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 showed that shopping destinations with trees had more customers than those that didn’t, and leafy public-housing projects experienced less violence than barren ones.”

The net result was that New York began to see its trees as an asset that required not just maintenance, but science-based planning in order to accrue both economic and social benefits.

“All these data led to the finding that each year New York City’s street trees delivered $122 million in benefits, or about $209 a tree. As New York City’s parks and forestry officials well knew, they received $8 million a year to plant and tend street trees, and spent another $6.3 million to pay personnel. The netbenefit they were getting for all these trees was an impressive $100 million.”

“For the first time, urban forestry science had a dramatic effect on public policy: In 2008, Mayor Michael Bloomberg quadrupled the city’s forestry budget, from $8 million to $31 million (down last year to $27 million), when he launched Million Trees NYC, a partnership with entertainer Bette Midler’s nonprofit New York Restoration Project. McPherson was thrilled to see science elevate urban forestry above the level of ‘a kumbaya idea.’ The million trees (350,000 are in the ground so far) planted by 2018 will transform the Big Apple, and those lush, tree-lined streets and shaded parks may well become Bloomberg’s most visible legacy.”

Urban forestry apparently has become an established science with many cities and towns beginning to take a closer look at their arboreal practices. This can only be good news.

“While science and technology are transforming and expanding the way we think about trees, Rowntree, now a scientist emeritus with the U.S. Forest Service, estimates, ‘We are only 50 percent of the way to knowing what trees really do for us.’ What we have learned only proves the old proverb truer than ever: ‘The best time to plant a tree was 20 years ago, the next best time is today’.”