iTree Helps Cities Value Trees

  • Pittsburgh's London plane trees pull up 1,400 gallons of water per year. Photo: Kara Holsopple

January 24, 2014

This story is part of “Think Outside the Pipes”, a local reporting initiative funded by the Park Foundation and sponsored by Penn State Public Media and its "Water Blues Green Solutions" documentary.

One type of green infrastructure that can help remedy the problems of aging sewer systems is older than any pipe or sewage tunnel. Simply put, it's a tree.  Sure trees have aesthetic value, but they can also soak up lots of water.  But to show decision-makers and citizens the value of trees on stormwater, stream flow and pollutant loads, the oaks and maples need some modern help.  Urban tree advocates in Pittsburgh and other cities around the country have been using a software application to calculate the value of trees.

Matt Erb is one of them.  On a rainy day on Wightman Street in Pittsburgh, Erb tries to keep dry under a line of London plane trees.

"The bark is kind of a camouflage color and it’s continually exfoliating, so there’s colors of green and brown and yellow. Really a pretty tree," Erb says.

Tree Pittsburgh is a nonprofit that restores and protects trees within the city.  Erb explains that these tall, broad trees hugging the curb, planted about 70 years ago, are more than a pretty face in this neighborhood. They’re workhorses.

"London plane trees in the city are pulling up 1,400 gallons of water a year, which is pretty considerable," he says. 

And they’re not alone.  Erb says throughout the city, 30,000 trees remove 42 million gallons of stormwater per year.  And that’s just street trees.  That’s water that’s not flooding streets and ending up in municipal storm sewers, causing sewer overflow problems. The trees also clean the water.

"The tree's vascular system kind of works like a straw and it just pulls water from the roots all the way up to the leaves," Erb says.

These London plane trees, a cross between an American sycamore and an Oriental plane tree, have wide leaves, too—creating a canopy that slows rainwater from making it to the ground. 

Tree Pittsburgh used a free software called iTree, developed by the U.S. Forest Service, to help determine the value that trees have for the city.  The number of trees in each neighborhood, and other factors like the size and condition of trees are entered into iTree, and out comes a dollar number. 

Working with community partners over several studies, Tree Pittsburgh determined that for every dollar spent on street tree maintenance in Pittsburgh, the city receives almost three dollars back in benefits—that’s an average of $2.4 million dollars in benefits per year from just street trees.   But the  value iTree calculated for the city took into account more than just decreasing stormwater run off.  Improving air quality, energy conservation, and removing carbon from the atmosphere are also part of the equation.

The iTree data directly informed a master plan for the future of trees in the city.

"So the greater we can maintain the trees, the better we can improve their health, the better stormwater benefits we’re going to get," Erb says. 

Another tool cities have is a version of iTree called Hydro.  It measures the impact trees have on stream flow, rather than just storm run off. Dave Nowak, the creator of iTree, says the movement of water—or hydrology—is the hardest factor to put a dollar value on. 

"If the stream goes up one inch or down one inch because of the forest, what value does that have in terms of all the things that it does—in terms or potential flooding or ecosystem services or fisheries habitat.  There’s not a lot of information out there on what that value is in terms of change in flow," Nowak says.

That is, until a stream reaches flood stage, and damages can be taken into account. That’s what got Vincent Cotrone, an urban forester with Penn State, involved with iTree Hydro. He used the prototype Toby Creek Watershed west of city of Wilkes-Barre.

It’s a watershed prone to flooding and stream degradation. The tree canopy covers half of the watershed, but there’s lots of new development.

The study showed that the trees in the watershed changed the streamflow by 11 percent—helping to prevent flooding.

"Somebody might look at that and go, that’s not a lot of reduction.  But you’re, in a sense, getting it for free.  You’re getting it from the land cover.  And there’s a ton of other benefits that come from those trees," Cotrone says.

Cotrone says that 40 years after the Clean Water Act, people don’t understand the connection between trees and clean water. And he says municipal managers and officials—even engineers—need to learn about tools like iTree and Hydro, and use them to create smarter plans for their towns and cities.