As the worm turns.

A dirty, rotten story about earthworms and fungi and what they are doing to the soils in our cities and suburbs.

In the mid-1980s, Richard Pouyat, then a soil ecologist for the New York City Parks Department, spent his days roaming the forests in two of the Bronx's big parks, the Staten Island Greenbelt, and forest parks elsewhere in the city. Often he turned up the same mystery: The tree stands seemed more like retirement homes than natural forests. They had towering old trees but had somehow lost the generation of younger trees that should be patiently waiting in the understory to replace the falling elders. Unlike rural forests, which rise in layers of shrubs, understory trees, and high canopies, many urban stands shot straight up from knee-high weeds to leafy ceilings.

Pouyat was surprised - and puzzled. He was surprised because urban forestry experts are aware of research showing that downtown trees live much shorter lives than those in suburban and rural areas; these New York City park trees, however, seemed older than those in the nearby countryside. He learned that these parks had been set aside for park use long ago - thus the big trees.

But another factor - the lack of any understory trees or brush - convinced him that something else was impairing the natural growth process here. And he knew it had to be more subtle than overuse. Many New Yorkers retain a medieval-like fear of the remote corners of their parks as hideaways for thieves and sinners.

Pouyat, who is now a forest ecologist with the USDA Forest Service, decided to start his search for answers in the dirt. "In my mind, the soil is a barometer in many ways for how an ecosystem functions," he says. It provides nutrients for the plants, collects pollutants that may harm the system now or later, and helps determine what grows and what doesn't (a hemlock stand, for instance, sheds needles that makes the soil repulsive to neighboring deciduous trees).

In 1988, Pouyat, by then a doctoral student at Rutgers University, and Mark McDonnell and Steward Pickett of the Institute of Ecosystem Studies in Millbrook, New York, began an ongoing study of nine plots in mature red-oak forests in sites from the Bronx through suburban Westchester County in New York and northwest to rural Connecticut. At first, the researchers found what they expected. The urban soil had upwards of five times as much lead as rural soil, Pouyat says, and two or three times as much copper and nickel. Over the years these heavy metals from automobile exhaust pipes and factory chimneys had settled as urban dust.

But they were surprised to find an even greater difference in the soils: The urban parks had been overtaken by earthworms.

For centuries, people dismissed earthworms as wiggly pests, until Charles Darwin's book about them inspired new respect for their role in nature. Today many farmers and suburban homeowners view earthworms as a blessing.

"In general, they enhance the fertility of the soil," says Patrick Bohlen, a researcher at the Institute for Ecosystem Studies, who has written his own book about the busy creatures. They decompose dead organic matter, help aerate the soil, dig out pores that absorb water, and create more room for roots to grow. And they seem so common - from fishbait to food for robins on the lawn - that many people would be surprised to learn that the earthworms found outside the South are usually alien invaders from Europe and Japan. The glaciers of the last Ice Age drove our native worm species south, if they survived at all; the immigrant species arrived later on in the root balls of exotic plants and trees imported from abroad.

Many ecologists worry about the exotic species that are invading our native ecosystems. Earthworms are no exception, although researchers have only lately discovered them in the forests after studying them on farms for years. Their effect there seems to be quite different.

"What's great for agricultural land may not be great for native forests," says Margaret Carreiro, an assistant professor of biology at Fordham University, who joined the urban-to-rural forest studies in 1989. Farms and lawns are disturbed ecosystems that may be helped by the worms' presence. But they may be "unsettling" to northern forests, which have evolved since the last Ice Age without them.

"Everyone worries about zebra mussels," she says, referring to the exotic pest found in northern lakes and rivers. "What about earthworms?"

The country forests not yet hit by the worm invasion have always had a profusion of fungi. Their carpets of leaves sprout mushrooms rather than worms. At her field station - the Louis Calder Center in Armonk, New York - Carreiro seems to be on the front lines of the urban worms' march north into suburban forests. All they may need to make the move are gardeners planting vegetation from nurseries or developers carving new suburban lawns from the woods.

Carriero and I don white overalls perfumed with DEET for protection against ticks, and walk into the woods to examine two different versions of a forest floor - one created by fungi, the other by worms.

Squatting down, Carreiro rolls back a mat of damp old leaves as thick as a Sunday newspaper. In a native forest, the decaying foliage often lays three or four inches deep because some leaves decompose very slowly - red oak leaves, for example, take five years. After my eyes adjust to this tiny world, I see that the mat is full of fungi. Fine white filaments spread like lace across a leaf that has leached to the color of parchment. Furry white mold spills from a layer of slick black leaves. Some bits of extra-fine dental floss - actually fungal filaments braided together - are knotted among the tiny roots in the soil duff. All told, Carreiro says, a square meter of leaves one-and-a-half inches thick holds 500 to 5,000 miles of fungal filaments, a vast highway that few people ever notice except for the fungal mushrooms that sprout in late summer and fall.

The forest floor, in essence, acts as the ecosystem's stomach, transforming fallen leaves and plants, branches and tree trunks, and animals into nourishment for the next generation of life. And it proceeds at a finely calibrated rate that recycles 80 percent of the forest's nitrogen. In a way, the forest slowly cannibalizes itself at a pace that wastes little food or energy.

But the earthworms change the nutrient cycle, shifting the stately pace of fungi digestion into overdrive. By consuming new leaf litter in a year, rather than upwards of five years, they flood the soil with ammonium. Bacteria convert this excess plant food into nitrate, a much trickier compound to absorb for many roots. Though ammonium is the "general diet" for all plants, Carreiro says, nitrate is a "special diet" for some plants. The Europeans, who spotted their forest earthworms long before we did, now identify some plants as "nitrophiles," but Americans don't yet know which of our trees and plants love it or hate it.

Over time, however, worm-infested forests could be filled with our own "nitrophiles," and may be doing so already. Pouyat has begun studying the Norway maple's appetite for nitrates. If the exotic species turns out to be well-adapted for our changed soils, it may be that the New York City parks department and others are waging a futile campaign to cut them down (see "Hot Cars & Hardwoods, page 28).

In the rain, nitrate leaches from the soil much faster than ammonium. In theory, the forest could slowly starve itself by dissipating this vital nutrient, because nitrogen, unlike minerals, does not occur naturally in the soil. Carreiro speculates that the leaching may be offset by fresh nitrogen from the nitrogen oxide released in automobile exhaust. Still, this leaking nutrient could alter the local ecosystem.

"When the nitrates get into lakes, rivers, and estuaries, the algae love it," she says. The algae decompose and rob oxygen from water, killing fish and other invertebrates.

Carreiro has no luck digging up worms today, but promises better luck if I return later in the year and stand around for 10 minutes. "You'll have four or five of them crawling on your sneakers," she says.

Even though we don't see them, the worms' impact is obvious. They consume the leaves and excrete what look like tiny black millet grains. The worms can chew through an autumn season of leaves in less than a year, tuning what should be a thick brown carpet of old foliage into a threadbare sheet of leaves or plain black dirt, and displacing the fungi, bacteria, and tiny invertebrates. The dirt also looks different under the surface: The worms have destroyed the typical layer of root duff and mixed all the soil together. On this hillside, some leaves have eddied behind fallen logs, where Carreiro finds her fungi, but large patches of ground look as bare as the dirt around a potted plant. The change looks startling; what it ultimately means for the forest is much harder to gauge.

To Carreiro, a fungi aficionada, the forest at the Louis Calder Center simply doesn't look right, and she's not just referring to aesthetics. "Where are all of the trilliums, the moccasin flowers, the trout lilies, the Canada mayflowers?" she asks. Maybe the crush of suburban deer have eaten them, or maybe the ground no longer nourishes them.

What other effect could the worms' presence have on the forest? In the country forest, the thick mat of leaves hides large tree seeds, like acorns, from predators such as chipmunks. The litter is fertile ground for mature forest trees: oaks, hickories, beeches, butternuts, walnuts, and others that drop nuts in various shapes and sizes. The small seeds that flutter down as wings or buttons from maples, ashes, and elms often get buried in these damp leaves and rot. But if worms devour the leaf litter, Carreiro suggests, the predators might feast on the exposed bounty of acorns or nuts, making it possible for the smaller seeds to flourish. Over time, she says, the forests could convert from large-seeded trees to small ones. Goodbye oaks, hello maples.

For now, the ultimate consequence of the earthworms remains a topic for further scientific studies. With the groundwork now done, Carreiro, Pouyat, and others must turn to the worms' effect on particular tree species. Having uncovered this tale of two soils, they must untangle the more complicated stories of full forest ecosystems. But some urban forest managers already await their results.

"We can't figure out where our leaf litter goes," says Marianne Cramer, the planner for the Central Park Conservancy. In the Park's north woods, the site of a great deal of restoration work since 1989, the ground is soon bare no matter how many leaves are shed each fall. Do earthworms eat them? Do they blow away without the mortar of fungal filaments? She would like to know.

"It worries us," Cramer says, adding that worms may not be the most threatening creatures in Central Park, but they could be changing the forests as indelibly as anything else."

WILL NIXON - is a freelance writer in New York City.