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SCIENTISTS SAY TREES MAKE US BREATHE EASIER. HERE'S HOW.

Imagine your hometown is one of the nearly 100 U.S. cities in violation of EPA's new standard for ozone pollution. You're in a community meeting, listening to an engineer propose a novel solution. "My pollution filters require no electricity," boasts the engineer, "and if located properly, they will not only screen out impurities, they will also cool nearby homes, saving energy and reducing pollutants associated with energy production."

When the planning committee asks to see the blueprints, the engineer reveals that these pollution filters cannot be built. But they can be planted, he announces, by just about anyone.

In fact, green vegetation is one of the most easily manipulated - and influential - elements of an urban infrastructure. Trees have the power to change the temperature and humidity of the air, intercept rainfall and capture pollutants, and mediate chemical exchanges between the soil and the atmosphere. And, trees are a welcome addition to neighborhood aesthetics, a boost for property values, and a harbor for resident and migrating wildlife.

That's all well and good, says the committee, but caring for a green infrastructure must be evaluated against the many other projects competing for city funds. What they want to know is, how many tons of pollutants will trees remove? And how will this translate into dollars and cents?

Until recently, our tree-filter advocate might have been at a loss to answer their questions. Even though research had been conducted on the pollutant-screening power of trees, a tool that could turn the data into a report saying, "Here's how trees benefit our cities" had not yet been invented.

That changed in 1995, when AMERICAN FORESTS released CITYgreen, a software program that analyzes the urban ecosystems with an ordinary desktop computer. Developer Chris Daniel explains: "CITYgreen takes local data from aerial photographs, point sampling techniques, and ground checks, and uses this data to create Geographic Information System (GIS) maps of the city's existing forest.

"Built-in modules analyze what kind of services the forest is now providing - how much carbon is being stored, how much stormwater is being intercepted, how much heating or cooling energy is being saved because of trees, how many pollutants are being removed from the air, and even how many wildlife species are being housed. For planning purposes, a community can use CITYgreen to visualize "what-if" scenarios, predicting what would be lost if they cut down trees, and what would be gained if they planted more."

The pollutant-removal models are courtesy of David Nowak, a research ecologist with the U.S. Forest Service's Northeastern Forest Experiment Station in Syracuse, New York. Nowak says that his work differs from that of other Forest Service investigators in that, "Most scientists study what pollution does to trees, while I study what trees do to pollution."

in the process of transpiring and photosynthesizing, says Nowak, trees can make some pollutants disappear. Gases pass into the tree through the breathing pores of a leaf, where they are dissolved in watery films. A tree's outer surfaces can also intercept airborne particles and hold them temporarily. Finally, trees can shade buildings, lowering energy use and reducing pollutants emitted from power plants. The cooling effect of trees may also check pollutants whose emission rates rise with temperature.

The other way that trees clean our atmosphere is by storing carbon. They breathe in carbon dioxide (the most prominent greenhouse gas) and turn it into wood, leaves, twigs, etc. Acting as carbon 'sinks,' they help to absorb some of the excess carbon we put into the atmosphere through the burning of fossil fuels.

Nowak's urban forest effects model, called UFORE-D, calculates how many grams of ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide are deposited in tree canopies each hour, as well as how much particulate matter smaller than 10 microns is deposited each day. The carbon storage and sequestration model is called UFORE-C, and it estimates tons of carbon stored each year.

The amount of pollutant removed depends on where in the country trees are located. A city's pollution concentration, climate, and growing season can affect trees' screening abilities, making removal rates in Minneapolis very different from those in Los Angeles. Version 2 of CITYgreen used pollution modeling data from three cities; Nowak's group now has calculated this data for a total of 50. Incorporating this new information is expected to help sharpen future CITYgreen estimates.

What makes those estimates even more useful is a model that turns "tons per year" into "dollars saved." As Gary Moll, vice president of AMERICAN FORESTS' urban forestry center, says, "The secret of CITYgreen's success is its ability to convert scientific knowledge into a useable public format. For decisionmakers who must weigh the relative value of various projects, the most meaningful format is dollars."

To arrive at a dollar value, Nowak multiplied the tons removed by the figure that economists use to estimate what pollutants cost society in external impacts such as rising health care costs: $6,750/t (metric ton) for nitrogen dioxide and for ozone, $1,650/t for sulfur dioxide, $950/t for carbon monoxide, and $4,500/t for particulate matter smaller than 10 microns. The values, taken from the 1994 New-York State Energy Plan, are used collectively by CITYgreen to determine a dollar per acre value. Carbon storage was valued somewhat differently, based on the going price for carbon dioxide emission credits, which can be traded on the commodities market (companies can clean up their operations and then sell their credits to companies that are still polluting).

At an estimated going price of $22/ton, the city of Austin - whose trees sequester 5,728 tons of carbon annually - receives an estimated $126,000 in services from its urban forest. Milwaukee, where trees sequester 1,677 tons of carbon annually, is receiving $37,000 in services. In addition, by providing natural cooling that cuts back on air conditioner use, trees help cities avoid releasing carbon, which occurs at a rate higher than that of carbon storage.

Another way to put this data in perspective is to compare pollution removal rates to pollutant release rates. In a 1994 study in Chicago, Nowak estimated that the amount of carbon sequestered by trees in a year equaled the amount of carbon released during one week of transportation in the study area. "This is a relatively new field, and we're always looking for more accurate ways to value ecosystem services," says Nowak, who is researching other opportunities for urban trees by studying whether they could be counted as a "biotechnology" in EPA's State Implementation Plans for reducing pollutant levels. What's important, he believes, is that we use the best science available to arrive at a true reflection of forest values. As Robert Costanza; author of a recent Nature article that estimates the global contribution of ecosystem services at U.S. $33 trillion/year, has written: "We must begin to give the natural capital stock that produces these services equal weight in the decision-making process, otherwise current and continued future human welfare may drastically suffer."

"That's a lesson many cities are already learning," says Moll. "Cities like Atlanta, which lost 60 percent of its natural forest in the last 20 years, are realizing that built systems can't always replace natural ones. Every time you short-circuit a natural service like vegetative air filtration, you have to pay for more than an engineered solution that may not work as well." With the head-to-head comparison that CITYgreen supplies, trees begin to look like essential partners for urban ecosystems that aim to be sustainable.

"When CITYgreen is used extensively," says developer Daniel, "future cities will be leafier and leafier, and we'll all breathe a little easier. Behind all the bells and whistles, a better environment is the real goal of our program."

Janine Benyus overs science topics from her home in Stevensville, Montana.