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College of Food, Agricultural, and Environmental Sciences


Ohio State Research Shows Dollar Value of Urban Trees, Benefits NE Ohio City

December 15, 2011

WOOSTER, Ohio -- Do you know what the trees in your community are worth? Not how much they would cost at the local nursery or garden store -- but their true economic value in terms of the crucial environmental services they provide to you and your neighbors.

The city of Wooster in northeast Ohio knows exactly the value of its street trees, thanks to research conducted by a graduate student in the Department of Entomology at Ohio State University's Ohio Agricultural Research and Development Center (OARDC).

Alejandro Chiriboga inventoried and recorded various attributes, such as tree-trunk circumference and foliage condition, of Wooster's 3,229 municipally owned street trees in the summer of 2010. He then used the U.S. Forest Services' i-Tree Streets software to calculate the value of the environmental services, including carbon storage and air-pollution removal, made possible by those trees.

The result: $270,153, or roughly $83 per tree, in annual services.

This figure includes $85,310 in aesthetic and related benefits, $83,343 in energy conservation, $77,457 in stormwater remediation, $13,361 in air-pollution removal, and $10,682 worth of carbon (646 tons) removed from the atmosphere.

Wooster's street trees have also stored 3,980 tons of carbon, valued at $65,808, in aboveground tissues, such as branches and stems. The value of carbon storage is not included in the $270,153 figure because it's not considered an annual function of the trees.

"Essential environmental benefits of trees and their associated monetary values tend to be overlooked and frequently underestimated," said Chiriboga, a Ph.D. candidate. "Performing up-to-date structural and functional tree analyses that provide decisive environmental and economic information can certainly support local and regional governments, which are periodically seeking to sustain funding or generate new initiatives that benefit the environment and the well-being of their communities."

Compared to summer 2009 -- when Chiriboga conducted his first inventory and analysis -- the total value of Wooster's street trees increased in 2010 by $6,296, from $263,857. During that period, the net number of street trees in the city increased by 118.

Additional studies found that while 154 new trees in 2010 accounted for 1 ton of carbon removed from the atmosphere and 1.3 tons of carbon stored, the 65 trees that were cut down between 2009 and 2010 represented a loss of 16 tons of carbon removed from the air and 119 tons of carbon stored.

Such a drastic difference in carbon intake between new and older trees, Chiriboga said, underscores the importance of having well-established, mature trees in urban forests to maximize their contributions. Ohio State studies have found that urban trees provide their maximum environmental benefits (removal of dust particles and carbon monoxide from the air among them) when they reach an age of 20 or more years.

Chiriboga's study found that most of Wooster's street trees are relatively young and in good health -- which is good news for their future environmental and economic contributions to the city.

"The high proportion of healthy small- to medium-size street trees suggests that Wooster's urban forest has the potential to substantially increase environmental benefits over time if well managed," Chiriboga said. "On the flip side, only a few species (Callery pear, crabapple, and red and Norway maples) account for more than half of the city's street trees. This increases the vulnerability of Wooster's trees to devastation by exotic pests that attack one specific species."

Project adds trees to city

Another aspect of Chiriboga's research involves the evaluation of management strategies that can enhance urban trees' survival rate and health, consequently boosting their future environmental services.

One such strategy is treating trees with the insecticide imidacloprid before they are transplanted to urban environments, where studies have shown trees face a variety of stresses and obstacles to surviving and adapting -- so much so that only 60 percent of them survive the first five years.

"Beyond its insect-killing properties, imidacloprid has been shown to favorably impact the growth and stress tolerance of trees, increasing their total leaf area, number of leaves, and root growth," Chiriboga explained. "This is important because the faster trees can become established after being transplanted, the higher their long-term rate of carbon sequestration will be."

To test whether or not treating trees with imidacloprid in the nursery would boost their successful establishment in urban areas and their air-cleaning properties, Chiriboga partnered with the city of Wooster to use it as a giant 14.4-square mile outdoor laboratory, which 26,000 people call home.

The experiment involved transplanting, in the fall of 2010, 119 Homestead elm and Heritage river birch trees that were previously subjected to various fertility, insecticide and irrigation treatments at OARDC. Some of the trees were treated with the maximum-labeled rate of imidacloprid, while others did not receive any of the insecticide.

The trees -- which miraculously survived the Sept. 16, 2010, tornado that wreaked havoc on the OARDC campus, with only one of the original 120 succumbing to the storm -- were planted all over the city in sidewalk pits within the public right-of-way. Staff with the Wooster Parks and Recreation Division helped haul and plant the trees.

"The city of Wooster greatly appreciates the partnership we have with the OARDC," said Daryl Decker, parks division manager for the city. "Working together in this program the city benefits from a gift of 119 trees valued at around $29,000. These are trees that most likely would not have been planted last year due to our current budget constraints. We are fortunate to have internationally ranked research being carried on here in Wooster and privileged to be part of that ongoing work."

After the trees were transplanted, Chiriboga treated them with a second dose of imidacloprid. After that, the trees were to be left untreated with the exception of irrigation as needed and according to typical management practices for newly transplanted urban street trees. During 2011 and 2012, Chiriboga will study the trees to quantify the effect of imidacloprid application and nursery environment on growth rate, stress tolerance and pest pressure -- and how these factors impact carbon-removal rates.

"This collaborative research will benefit the urban forest of Wooster and other municipalities by developing methods that enhance the survival and growth of shade trees on city streets, which can be an inherently stressful environment for trees that are adapted to forests," said OARDC forest entomologist Dan Herms, who is Chiriboga's advisor.

"It also shows that a city can be a valuable laboratory for studying the economic impact of the environmental services provided by street trees. Ultimately, the benefits of this study will extend to urban forests throughout the world when the results are published in international journals."

Funding for this project was provided by Bayer Environmental Science. Bayer is the maker of several brand-name products containing imidacloprid -- including Merit, which was used in Chiriboga's experiments.

The largest university agricultural bioscience research center in the U.S., OARDC is the research arm of Ohio State's College of Food, Agricultural, and Environmental Sciences.


Mauricio Espinoza
Alejandro Chiriboga, Daryl Decker