The Bronx's old growth lab.

Deep in the heart of the Bronx, you can find something very rare in the United States - an old-growth forest. In fact, it's the largest remnant of the original forest that once existed in the New York metro area. Some 40 acres at the New York Botanical Garden have slipped between the cracks of progress to remain a green retreat from the urban world.

Wood-chip paths lead visitors through forest that features an odd combination of scattered towering red oaks and tulip-trees, skeletal hemlock groves stripped by woolly adelgid blight, and fire hydrants. Granted, these 40 acres aren't a museum tableau of the forest that Henry Hudson found in the region in 1609, and they've been trampled and managed so much they probably no longer meet the spirit of old-growth, but they're a valuable research site for ecologists studying the disruptive effects urban civilization has on natural forests.

For the past decade, scientists from the Institute of Ecosystem Studies in Millbrook, New York, have had a particular interest in the forest's soil, finding that it holds more lead, copper, and nickel than rural dirt. It also repels rainwater, a trait called hydrophobicity, and generates nutrients more rapidly because earthworms there digest the leaf litter much faster than fungi does on the forest floor in rural areas (see As the Worm Turns, page 34). But these studies have simply laid the groundwork for new research into why plant and tree species prosper or fail in this environment.

"All we know now is that there are differences in the soil and in the forest structure between the urban and rural forests. Certain species are increasing, and others are not. We don't know why," says Janet Morrison, a forest ecologist who joined the NYBG staff in October 1994. Do some species produce more seeds than others? Do squirrels and rabbits prefer certain seeds? Do some seeds germinate better than others in this soil? In short, researchers must study each step of the life cycle to learn where and why each species falters or gains.

The losers are oaks, which for uncertain reasons haven't regenerated, and hemlocks, which have declined for much of the century. "We have beautiful old archival photos of hemlocks that were 250 or 300 years old," she says. But years ago, as a precaution against fires, the NYBG staff removed fallen trees from the forest floor. Since young hemlocks often sprout from dead trunks, the workers inadvertently aborted the next generation of trees, and the hemlock cover has shrunk from half the forest to just small patches. Since 1990 the wooly adelgid has killed 40 percent of the remaining trees.

But to the NYBG this blight is simply nature at work. Morrison has begun studying the new species starting to grow in the dying hemlock groves. "There's always something coming up," she says. "We don't have bare ground."

Other trees are prospering, such as tuliptrees, wild black cherries, American beeches, black birches, sweetgums, and two alien trees with unpopular reputations among urban foresters - Japanese cork and Norway maple. Morrison isn't so much interested in exotic species - common at this horticultural Mecca that collects plants and trees from around the globe - as in invasives that spread with Napoleonic intensity. Japanese knotweed and garlic mustard plants have both migrated from their sunny habitats into the forest shade. The NYBG has weeded oriental bitter-sweet vines and may soon attack some other invaders.

"We don't want the forest to become dominated by five or six species," Morrison says. And yet she's fascinated by questions of why the invaders do so well. Are individual plants highly adaptable to their surroundings or does tire broad population have diverse members suited to different settings? She says that ecologists really haven't answered a lot of basic questions about these exotic species, which urban forest managers must grapple with everyday.

Morrison herself has a special interest in plant diseases. She has surveyed the flowering dogwoods that somehow survived anthracnose blight and found a surprising number - more than 200. "All show signs of this fungus," she says. "Some are doing great and others not so great."

She will compare these survivors with those in a rural forest to study whether the urban trees fared worse in this attack because they live amid so many other stresses. As a theory it makes sense, but Morrison would much rather have data sets that prove it right or wrong.

"This is a great opportunity for an ecologist," she says of the forest considered a showcase for how humans alter nature. "This is a great lab for studying human impacts on the natural world."