If you happened to be strolling through Chapman Square in Portland, Ore., this past April, you might have come across a curious sight: big, colorful “price tags” hanging from the park’s giant elm trees. Every tag said something different—one read, “This tree has given $20,000 worth of environmental & aesthetic benefits over its lifetime”—but all trumpeted the benefits of trees.
Those tags were part of Portland’s first ever Arbor Month. The goal was to get Portlanders to look differently at trees, to see all the ways in which trees are good for the environment and people’s health, from decreasing stormwater runoff to reducing atmospheric carbon dioxide to improving air quality. The city declared that for every dollar spent on a tree, an estimated $3.80 worth of benefits are returned.
So how did Portland come up with that figure, or the $20,000 figure for that matter? It used a modeling program called i-Tree, a suite of open-source software that allows cities, states and other users to “strengthen their urban forest management and advocacy efforts by quantifying the environmental services that trees provide.”
Introduced in 2006 by the U.S. Forest Service, i-Tree is in its fifth iteration. It has inspired cities from Baltimore to New York City to Milwaukee to Portland to set ambitious tree-planting goals. The free program has been downloaded more than 10,000 times so far. With so many states and localities pruning money from parks and tree-planting programs to balance budgets, i-Tree helps public officials put a monetary value on the benefits of growing them.
Take Pittsburgh. Last summer, the city approved a master plan for maintaining and expanding its tree canopy over the next 20 years. The decision came after a nonprofit group called Tree Pittsburgh used i-Tree to determine that the trees planted along sidewalks and medians throughout the city provided $2.4 million worth of environmental and aesthetic value every year. Since the city spends only $850,000 a year on street planting, that’s quite a return on investment: Pittsburgh gets about $3 in benefits for every dollar it invests in trees.
i-Tree works by calculating the “leaf surface area” of a city and assigning the canopy an economic value. The value comes from the environmental services trees provide, such as how much ozone, particulates and nitrogen are removed from the air; how much carbon is stored; the effect on building heating and cooling costs; and trees’ effect on hydrology, among other factors.
One especially neat feature is a module that links to Google Maps. It helps city foresters, homeowners and other users see the effects a tree would have if planted in a specific place. Researchers want the next version, which will likely be released in 2014, to enable modeling of trees and their benefits to ecosystems 30 to 50 years into the future.
For now, i-Tree is just a basic calculator that helps proponents make an economic case for why trees should be in the budget. A growing body of knowledge on the benefits of trees, however, could make i-Tree’s job even easier. Research has already shown that trees increase property values. And now, a new study has found that living near trees dramatically improves health.
Conducted over 18 years, research from the U.S. Forest Service has found a correlation between tree loss and human mortality. According to their findings, the loss of trees was associated with about seven additional deaths per year from respiratory causes and almost 17 additional deaths per year from cardiovascular causes per 100,000 adults. That, say researchers, comes out to more than 21,000 deaths in total. It seems trees have a value that goes far beyond dollars and cents.