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How many trees are there on New York City streets?

That question will be answered this summer by some thousand volunteers in a project called "Trees Count." Aside from counting every tree, the volunteers will collect basic data on each tree's location and species; they will also use handheld computers donated by Hewlett Packard to measure the diameter of the trunk and the crown.

All this data will be entered into a common database, then run through an economic model to determine not just how much it would cost to replace each tree, but also to assess how much each individual tree is worth.

"No one has really placed a value yet on the urban forest," said Parks Commissioner Adrian Benepe. "This will help us do that."

In a city that just earned yet another set of failing marks for air quality from the American Lung Association, parks managers are eager to quantify to the penny how New York City street trees improve the air and diminish energy consumption.

The first tree census in the city, conducted in 1995, came up with a total just under 500,000 trees.

Officials estimate that the number this time around will be about the same. This is true even in the face of a short-term reduction in the local tree population. Noting the recent discovery of an Asian longhorn beetle eggs, Parks Commissioner Benepe promised an "aggressive" response. "Any trees found to have insects will be chopped down, chipped, and the chips will be sent to the incinerator," he said.

Such promises reflect the management philosophy that, in an era of diminishing budget resources, it's better to amputate and be safe than waste precious time and resources developing a more nuanced approach. One reason for the tree census, notes Jennifer Greenfeld, director of the New York Tree Trust, is to give future resource managers a clearer picture of what corners of the city are doing well and what corners aren't.

"[A census] helps us figure out how to balance our resources, how to adjust our palette in terms of how we respond to different problems ," says Greenfeld. "This is the one thing that helps give us a snapshot so we can plan what to do."

Although the 1995 tree census took note of tree condition, this year's count will take more notes on the types of things that lead to the biggest maintenance headaches -- things like powerline and sidewalk interference and the tree guards which protect trees over the short term but can restrict growth over the long term. Losing larger, more established trees to such problems is especially painful from a management perspective, Greenfeld says, because larger trees provide exponentially more shade, oxygen and pollution-filtering power than their sapling replacements.

"A tree is probably the only investment that a city makes that increases over time," Greenfeld says. "Unlike a sewer line, which gets less useful the older it gets, you want to keep [a tree] in the ground as long as possible."

To help prove that point, New York Tree Trust will be relying on an economic model developed by the Center for Urban Forest Research, a laboratory at the University of California at Davis that is funded by the U.S. Forest Service. Dubbed STRATUM, the model estimates growth rates by species and location and then multiplies the resulting size estimates with biological data and economic estimates on the social cost of airborne pollution and energy production. The end result, says center director Greg McPherson, is a bottom line summary of each tree's ability to reduce carbon, pollution, and energy bills over time.

"It provides the other side of the equation which is that trees are an investment," says McPherson. "If you increase your investment, you can provide yourself more of a return over time."

McPherson's lab has analyzed census data from cities such as Berkeley, Calif., Minneapolis, and Bismarck, N.D. Estimates on the "return on investment" for each tree planting vary by city. In Berkeley, the return on every dollar spent was $1.37. In Bismarck, the return was $3.09 for every dollar spent.

New York has tried to assess its trees before, in a 2001 study that concluded that urban trees statewide had a compensatory value of $84 billion and a carbon storage value of $500 million. David Nowak, lead author of the 2001 study, says pricing models are popping up in court cases to assess the value of damaged trees or stolen timber and could lead to even more creative uses. Last year, for example, the Office of the New York Attorney General mandated $47,000 worth of tree planting in a January, 2004, settlement with four school bus companies accused of excessive idling, one of the first attempts by a government agency to include trees in the bargaining process on air pollution.

Nowak expects this trend to continue, noting that trees are a classic example of a benefit enjoyed by society as a whole coming at a cost only to the individual or agency that planted the tree. This is the exact opposite of air pollution, which comes at no cost to the factories or automobiles, etc. that produce it, but produce damage felt by society as a whole. Economists call the first example, the tree, a "positive externality," and the second, the pollution, a "negative externality." Both are "external" to the ordinary marketplace's ability to set a price. Assessing the damage or benefits often falls on state agencies.

"A lot of things we can't quantify yet, but some of the things we can," says Nowak.

The Trees Count project was launched with an Arbor Day tree planting ceremony outside the Little Sister of the Assumption Family Health Service in Spanish Harlem, a neighborhood currently experiencing the highest childhood asthma rates in the country. According to a 2003 study involving researchers from Columbia University and Harlem Hospital, a quarter of all children in east Harlem suffer from asthma, more than four times the national rate. Looking past the current tree census, both Benepe and Greenfeld point to Trees for Public Health, managed by the Hunts Point environmental group Greening for Breathing.

Trees for Public Health seeks to reduce local air pollution levels in a corner of the city plagued by automobile exhaust and has, to date, relied on a patchwork funding from city, state, and federal agencies. One way to guarantee a steady stream of funding, Benepe says, would be to confirm a direct causal link between money spent on trees and money saved at local asthma treatment centers thanks to declining admissions.

"We've gotten to the point where people understand anecdotally why urban trees are valuable," says Benepe. "Our next step is to quantify that value. For that we need the help of the scientific community."

The 2005 Trees Count project starts on June 1, 2005. Volunteers who would like to help out can register by calling 311 or visiting the city's "Trees Count" web page. The Parks Dept. is asking volunteers to donate at least 40 hours of their time, plus a three hour training session. The training sessions begin in the second weekend of May. Event sponsor Bank of America, meanwhile, is offering $1,000 to the adult New York City resident who comes closest to guessing the final street tree tally. The free entry form, which requires physical address and birthdate, is located here.

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