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Urban Forestry's New York City Roots

by Sam Williams
Aug 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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