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Urban Forestry's New York City Roots

by Sam Williams, Aug 06, 2004
 
 

It was 84 degrees in Manhattan with humidity just under 80 percent, but the hikers wouldn't know it standing in the middle of Inwood Hill Park.

With trees rising as high as 140 feet in the air, blocking out sunlight while leaving just enough head room to catch the occasional wind draft, they could feel the summer warmth, but didn't have to wear it like a blanket. Manhattan might as well have been a thousand miles away. But of course, it wasn't. Inwood Hill Park is in Manhattan.

"I'm pleased with it," said Tim Wenskus, a biologist with the New York City Department of Parks Natural Resources Group and the man in charge of making sure this forest in the northern tip of the island remains tranquil.

"You should be," said Mark McDonnell, associate professor and divisional director of the Australian Research Center for Urban Ecology. "You've done a great job."

Both Wenskus and McDonnell are urban foresters, a small but growing group of scientists who believe habitats such as this one are just as deserving of study and protection as anything you'd find in the Amazon rainforest.

The four men on the hike were having something of a reunion. In the 1980s, Mark McDonnell helped dig and document the many plots the group is now revisiting, in the city that provided a North American launching pad for the field of urban forestry 20 years ago.

"Traditionally, ecologists never looked at cities," said Richard Pouyat, who now works at the Forest Service. "We always looked at areas where people weren't living. Mark was really a pioneer of looking at cities as their own ecosystems. Twenty years later, people have come around to his point of view."

Trees do more than provide an aesthetic backdrop in city environments. Urban environments, once denuded of native vegetation, become dangerous heat conductors, a phenomenon called the "heat island" effect. Coupled with the heat displaced by air conditioners, urban environments like Manhattan average daily temperatures 10 degrees hotter than surrounding regions.

"If you want to see the effects of global warming, just look at any big city," McDonnell said. "At a local scale, it's just pure environmental change."

The first sign of change, notes McDonnell, is the rapid loss of native species evolved to survive in a more temperate climate. In Tokyo, the disappearance of birds and insects has prompted some businesses to install outdoor speakers to supply the missing sounds. Here in New York City, native hardwoods such as oak, hazel, sassafras and the tulip tree, a fast growing eastern giant that can age up to 300 years, are giving way to invasive immigrants such as ailanthus, Norway maple, and a host of exotic vines and worms.

Then again, the central paradox (and saving grace) of urban ecology is the fact that urban forests often provide the most faithful glimpse of what primeval forests once looked like before industrialization.

The reason is simple: City dwellers, starting in New York City, were the first to witness the consequences of rampant overdevelopment. To maintain a semblance of contact with nature, 19th century civic leaders pressed for parks development, effectively creating a museum of native ecosystems. In terms of species distribution, the trees you find in Central Park and Inwood Hill offer a more authentic glimpse at the pre-Columbian ecology of North America than most upstate forests, where logging and agriculture continued unabated until well into the 20th century.

"It's a worldwide phenomenon," says McDonnell. "All cities have older trees than the surrounding countryside because that's where the protection started."

This paradox is the chief explanation for the "Lord of the Rings"-like canopy here at Inwood Hill. Though its 200 acre habitat is microscopic compared to state preserves like Adirondack or Catskill Park, it exudes a majesty that puts other northeastern forests to shame, with towering trees that in upstate forests would have been cut down and hauled off to the mill a long time ago.

Such majesty puts pressure on forest managers like Wenskus, however. Every time a giant oak or tulip tree dies, there is a natural rush to fill the gap. Invasive species like Norway maple and porcelain berry enjoy an advantage in this race because they can smother out smaller plants' sunlight, killing off the plants that help make soil chemistry favorable to natives. Biologists and managers with the parks department's Natural Resources Group use a variety of tactics to keep invasive species at bay. Still, Wenskus says, they can only do so much. Forestry is not gardening, and even if the group had the resources to treat it as such, native species must ultimately fend for themselves.

As proof of that stoic philosophy, Wenskus pointed to a fist-thick porcelain berry vine running all the way up a middle-aged oak and shook his head sadly. Only two outcomes are possible at this stage: Either the vine will die on its own or the tree will succumb because of diminished sunlight.

"Porcelain berry grows so fast," he said. "It's a bad customer."

If invasive species share a common characteristic, it's their ability to grow fast and look good doing it. Porcelain berry, for example, is a relative of the grape and can quickly transform a backyard into a leafy arbor. Norway maple throws out a dense, low-to-the-ground leaf cover, perfect for sun baked sidewalks, less perfect for native woodlands. Lately, however, the tree has come under attack by yet another immigrant, the Asian longhorn beetle, diminishing migratory pressure on local woodlands.

Other young, upwardly-mobile plant species include European White Poplar -- "Robert Moses's favorite," grumbles McDonnell -- and ailanthus, a scrappy weedlike tree sometimes known as the "tree of heaven."

For a patient forester, however, the signs are encouraging. Twenty years ago, wrecked cars littered the undergrowth and periodic fires killed off promising adult trees in Hunter Island, part of Pelham Bay Park in the Bronx, which was the next stop on the group's tour. Now the forest is protected by firebreaks and barred to vehicle traffic. Native plants are getting a chance to push the canopy upward, allowing room for a thick underbrush. It isn't Inwood Hill Park but in a few more years you might be able to find yourself visiting this corner of the city and wondering if it has been untouched since the 17th century.

"In all honesty, the best stuff is the stuff you can't tell apart from nature," said Wenskus, summing up his work and the general goal of the Natural Resources Group. "If it looks like nature made it, I figure I did my job right."

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