OSU research shows the value of urban trees

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.

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.

Tree value

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 above ground 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.”

Increase

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.”

Strategies

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.

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 transplant, 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 were planted all over the city in sidewalk pits within the public right-of-way.

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.

Analysis

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.

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