David Nowak would rock on Jeopardy.

"I'll take Baltimore for $100, Alex."

Alright, David, here's your answer: They outnumber people more than four to one.

"What are Baltimore's trees?"

That's correct. In fact, you could call Baltimore a forest with a lot of people in it--an urban forest. And thanks to Nowak, it's one of the best-studied urban forests in the country. Nowak, a senior researcher with the U.S. Forest Service, is leading a unique long-term study of the trees growing on 200 patches of land randomly scattered across the city, in suburban backyards, in industrial parks, on highway shoulders, in vacant lots where rowhouses once stood--any place a tree might grow.

"Baltimore for $200, Alex."

American beech, black locust, elm, white ash.

"What are the most common trees in Baltimore?"

Right, Dr. Nowak. As a part of part of the Baltimore Ecosystem Study, a research program funded by the National Science Foundation (see "Where the Wild Things Are," July '08 Urbanite), forestry researchers first surveyed their 200 plots in 2004, sorting the trees out by species, measuring their height and mapping their density. They'll come back for another survey in 2014. By tracking the trees over time, forestry researchers are learning what makes urban forests unique. How does the stress of city life affect the trees' growth? Which species thrive and which can't survive? Do the trees make subtle biological adjustments to living in an urban setting, and if so, how does that affect the plants and animals that share the cityscape with them?

"Baltimore for $300, Alex."

Here's your answer: More than 2,000 tons a year.

"How much pollution and smog do trees take out of the air just in Baltimore and the 'burbs?"

That's right, and if utilities and industries had to pay for the smog removal that trees do for free, an extra $30 million a year would be tacked onto local customers' utility bills. In fact the 9 million or so trees in the Baltimore region produce more than $55 million in ecosystem services, Nowak said. Urban forests cool the air, lowering energy costs. They reduce the flow of polluted runoff into local streams and the Chesapeake Bay. And they take carbon dioxide out of the atmosphere, helping to slow global warming.

"Baltimore for $400."

The answer is: 15 years.

"What's the average lifespan of a city tree?"

City trees have it rough. Urban forests can't survive on their own. The city will remain green and thriving into the next century only if people continually plant trees. City foresters planted 1,800 trees last year.

"Baltimore for $500."

The last answer in this category: Zero.

"How many street trees will Baltimore city plant this year?"

Baltimore City's budget cuts have fallen hard on the city's forestry department. A two-cent bottle tax that the Baltimore City Council okayed just minutes before its final budget vote last month was not enough to save the city's forestry and horticulture divisions, which eliminated most of their jobs. The few remaining forestry workers will only remove dead trees and do emergency pruning. The city "will focus on encouraging private property tree planting," according to the mayor's budget statement.

That leaves the city with only one tree-planting program--TreeBaltimore, which is largely funded by corporate donations and focuses its efforts on private land. It also slows progress toward a goal set by former mayor Sheila Dixon to increase the city's tree cover from 27 percent to 40 percent, as part of creating a "Cleaner Greener Baltimore." www.cleanergreenerbaltimore.com

Nowak and his fellow researchers have already looked at what would happen if Baltimore stopped planting new trees on public and private land. At first existing trees' growth would make up for losses, but within five years the city's leafy canopy would start to shrink.

That means that for now, the survival of Baltimore Forest is in our hands.