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Study Evaluates Costs of Reducing Carbon with Street Trees

London plane trees like the ones lining 10th Street in Manhattan, get high marks for carbon abatement, in part because of their long life span. Photo courtesy of New York City Department of Parks & Recreation. ST. PAUL, MN, October 23, 2013 - When it comes to carbon abatement, does a green roof do more good than planting a tree near a building? In which boroughs are street trees doing the most good? Which tree species is giving New York City more carbon abatement for its buck?

A new study by the University of Arkansas and the U.S. Forest Service gives MillionTreesNYC and New Yorkers answers to these and other questions related to the effectiveness of tree planting in carbon abatement. The study gives the London plane tree, trees planted where they shade buildings, and the boroughs of Staten Island and Queens the highest marks for carbon abatement.

The study, “The marginal cost of carbon abatement from planting street trees in New York City,” estimated the discounted cost of net carbon reductions associated with the MillionTreesNYC effort to plant and care for street trees over 50-year and 100-year horizons. Kent Kovacs, an assistant professor with the University of Arkansas, is the principal investigator with co-author Robert Haight, a research forester with the Forest Service’s Northern Research Station in St. Paul, Minn. The study was published recently in the journal Ecological Economics and is available on-line at: http://www.nrs.fs.fed.us/pubs/44348

“Because they lower our energy costs, store carbon, and tend to make human beings feel better, we think of urban trees as being priceless,” Haight said. “But in New York City and every city that is spending money to plant and tend trees and remove them when they die, ‘priceless’ comes at a cost.”

The average discounted cost per ton of carbon abated from planting trees near buildings for a 100-year planning horizon ranges from $3,133 per ton of abated carbon for the London plane tree to $8,888 per ton for the Callery pear, Kovacs and Haight found. That compares to an estimated annual cost per ton of carbon abatement in rural forestry programs of between $117 and $1,407 per ton of abated carbon.

“Cities are more expensive for both trees and people,” Kovacs said. “It costs much more to plant and maintain an urban tree than it does a tree in a rural forest. While their location makes them more expensive, it also makes them more effective – street trees are reducing more carbon than rural trees because they provide shade and block wind thereby reducing energy consumption in nearby buildings in addition to sequestering carbon.”

In terms of carbon abatement, Kovacs and Haight found that the long-lived London plane tree is the city’s best bargain, with the cost of carbon abatement varying from $1,553 to $7,396 tons of carbon abated annually depending on the tree’s location.

The study describes planting locations with the lowest average cost of carbon abatement as being 60 feet west of nearby buildings that are more than 60 years-old, one or two-stories tall, entirely residential, and without nearby tree canopy. While New York City’s five boroughs all have locations that fit that description, a spatial examination of the City showed that Staten Island and Queens have more than other boroughs, making them more effective locations for tree plantings.

The study did not address the entire range of benefits associated with urban trees, according to the study’s authors. “In addition to carbon abatement, planting trees in urban areas provides many other benefits, which our study did not attempt to quantify. A comprehensive assessment of all the benefits of urban trees, rather than a focus on carbon alone, is appropriate for deciding how much investment to make in an urban forest,” Haight said.

Urban trees store carbon through the growth process and reduce fossil fuel use by lowering cooling and heating energy consumption of buildings. Trees do this through a process called transpiration as well as by creating shade and the blocking the wind.

“More than 80 percent of Americans live in urban areas that, nationwide, contain over 100 million acres of trees and forests,” said Michael T. Rains, Director of the Northern Research Station and Forest Products Laboratory. “Forest Service research is helping improve people’s lives by making trees, forests and forest ecosystems healthier in cities throughout our country.”

 “With the recent news that New York City’s air is the cleanest that it has been in the last 50 years, there can be little doubt of the benefit of Mayor Bloomberg’s initiative of planting a million trees,” said NYC Parks Commissioner Veronica M. White. “We have just completed our biggest planting day ever, adding 20,000 trees to Rockaway Community Park on Saturday. We are nearing 800,000 trees planted and the success of the program has allowed us to move up the original target date by two years. When we complete the planting of a million trees in 2015, our air will be even cleaner and our homes and offices even cooler.”

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The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the nation’s forests and grasslands to meet the needs of present and future generations. The agency has either a direct or indirect role in stewardship of about 80 percent of our nation’s forests, amounting to 850 million acres including 100 million acres of urban forests gracing the nation’s cities, where 80 percent of Americans live. The mission of the Forest Service’s Northern Research Station is to improve people’s lives and help sustain the natural resources in the Northeast and Midwest through leading-edge science and effective information delivery.