By Ben Schiller

What's the value of a tree?

David Nowak thinks he knows. For the last 20 years, he's been counting and analyzing trees in several cities (Oakland, Chicago, Baltimore, Philadelphia, Boston). And now he's leading an effort to standardize tree valuation.

The U.S. Forest Service's i-Tree platform calculates a city's "leaf surface area," and gives trees an economic value, from the environmental services they provide. How much carbon stored? How much ozone, nitrogen, particulates, removed? What's the health impact? The effect on building heating/cooling costs? Hydrology?

"Most of these are in tons or parts per billion, and have a dollar value associated with them," says Nowak, who heads USFS's Northeastern research unit. "If you buy a tree, you are affecting the environment--the carbon, air, water, UV. We show the costs and benefits of changing the structure of our cities through vegetation."

i-Tree simulates a city's trees using data from sampled areas (size, species, health). It brings in local weather and pollution data, and maps the trees across the city. It then computes the values based on official sources (like the U.S. government's 2010 "Greenhouse Gas Reporting Valuation," which prices carbon at $78 a metric ton).

New York, for example, has 876,000 trees, covering 23.1% of the city. They provide $11.2 million in annual energy savings, have a carbon sequestration value of $386,000, and a pollution removal value of 836,000. (Their asset value--what they would cost to replace--is much higher).

Nowak says the tool is important for two reasons. "One, it puts trees on a par with other items [on the budget], in terms of economics. But more importantly, it lets us understand what's out there, and understand its importance. We see trees all the time, we just haven't quantified them."

i-Tree is free to use, and there is now a mobile version (allowing you to input data remotely). Nowak's team has also built versions for Canada and Australia, and is talking with other countries.

Nowak says the service has helped people make the case for trees more successfully: "There are lots of success stories of people going to their mayor and having information to back up why trees are important. Instead of just saying 'we like them.'"

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