Rich Hallett, a research ecologist with the US Forest Service, has an unusual job this fall. Using an eight-foot-tall slingshot, he's shooting beanbags at the leaves atop New York City's trees.

When Hallet is on his game, the beanbag loops over a branch and pulls down a clump of leaves, which he collects to test their chloropyhll levels. He wants to measure the tree’s photosynthetic capacity —how much energy it is producing — to gauge its health.

Hallett and fellow ecologist Nancy Sonti study "street trees" — the trees that grow out of the sidewalks. Trees in an urban landscape are fascinating, Hallet says, because they have to work so much harder than the trees in forests and mountains. “They have to put up with soils that are man-made, that they didn’t evolve to be growing in," Hallet explains. “And they have harsh conditions — chemistry, exhaust from cars and just a lot of stress.”

On an early fall day, Hallet stands at the base of a red maple that was flooded by Hurricane Sandy. Oak, maple, black cherry and gingko are some of the common species in the New York City.

To an untrained eye, the tree looks like it's in good shape. But to find out how stressed this tree is, Hallet and Sonti look at more precise indicators like leaf discoloration, twig growth and photosynthetic capacity.

Hallett’s goal is to create an urban forest among the concrete and skyscrapers of New York City. He and Sonti are collecting detailed data to get a clearer picture of how city living affects these trees. “We’re working to understand how trees interact with air quality, with greenhouse gas emissions, storm water capture in tree pits and just general hydrology,” Sonti says.

New York City officials are also interested in that data. In the past, the city had two different categories for trees: dead and alive. Now, they have a keener awareness of the benefits of an urban forest. Trees soak up harmful greenhouse gas emissions and help offset the “urban heat island” effect, in which cities are much warmer than the areas around them.

On hot summer days, streets, buildings and sidewalks absorb heat; at night, all those structures release that heat. That leads to warmer temperatures, higher air pollution levels and increased energy use. The cooling effect of trees helps reduce this effect. It’s one reason for the city-wide campaign to plant one million trees over the next decade.

New York City still has 100,000 more trees to plant to reach its goal. Many of the already-planted trees are still very small. “As they mature, I think it really will change the face of our urban forest,” Sonti says.

She and Hallet are just beginning to understand how complicated an urban forest ecosystem really is. What they learn today could help change the concrete jungle into something a little greener.

This story is based on a report that aired on PRI's Science Friday with Ira Flatow