Magazine article American Forests

As the Worm Turns

Magazine article American Forests

As the Worm Turns

Article excerpt

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. …

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