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To travelers zooming along the highways of the northeastern United States, the deciduous woodlands that decorate the landscape might look like a sign of improving environmental health. Surely, this is the way things were before Europeans set foot on these shores. Or is it?

It is true that the Northeastern woodlands have refilled much of the space once taken up by farms, that the trees are growing bigger and that animals like black bears, beavers, wild turkeys and even moose have returned. But does this mean that the forests have recovered from two centuries of destruction, or that they ever will?In many ways, the character of the forest has been transformed beyond recognition. It has been cut up into so many fragments that much of its native biological richness has been lost. Where once there were elms and prothonotary warblers now there are Japanese honeysuckles and cowbirds.

The Northeastern deciduous forests are remnants of a band of temperate-zone woodlands that encircled the globe 10 million years ago. Since then, they have proved remarkably resilient in the face of almost constant disturbance.

The last ice age pushed the trees south, and when they finally began to recover their territory, within the past 10,000 years, they encountered a new disturbance: Indians.

To make the landscape more passable and attract game to the tender re-growth, Indians regularly set fire to the woods. The burning favored thick-barked oaks over thin-barked species like maples, birches and beeches, creating large oak-studded open stretches.

Contrary to myth, then, much of the forest that greeted the first European settlers was not a dark wilderness but rather something like the settlers' own tame English parks. One 17th-century arrival wrote that these areas made the country "very beautiful, and commodius."

Still, the settlers wasted no time in clearing the rest of the land to make room for farms. At various points in the 18th and 19th centuries, farmers managed to cut all but about 2 percent of the North-east's trees.

Finally, though, the forest got a break. About a century ago farmers began abandoning the Northeast and moving to the Midwest, where the soils were better.

This simple economic act, more than any deliberate restoration effort, was the reason that the woods made a comeback. When the farmers left, the trees began to reclaim their fields.

In general, the Northeastern forests have more recovery potential than many other ecosystems, simply because they get more rain. According to Reed F. Noss, a conservation biologist at Oregon State University, who was an author of a recent federal study of endangered ecosystems, the forests of the Northeast tend to regenerate on their own if they are simply left alone.

Ecologists say the woods still have some distance to go, though, and they may never get back to their pre-settlement state.

"What we now have," says Mark McDonnell, a botanist and ecologist at the New York Botanical Garden's Institute of Ecosystem Studies at Millbrook, N.Y., "is a forest that's 80 to 90 years old" - middle-age, but not yet mature from an ecological standpoint. (Contrary to popular belief, many of the older and bigger trees are in cities and older suburbs, where they escaped destruction by farm-ers.)

The percentage of Northeastern land now in forests ranges from 89 percent in Maine and 87 percent in New Hampshire to 31 percent in Delaware, 42 percent in New Jersey and 43 percent in Maryland, according to the latest estimates by the U.S. Forest Service. Sixty-two percent of New York State is in trees, and 59 percent of both Connecticut and Pennsylvania.

But these forests are different from their predecessors. Chestnuts and elms have been all but wiped out by diseases imported from abroad. The region's magnificent and stalwart oaks, deprived of the Indians' fires that once kept competing trees in check, are having trouble regenerating, and they are threatened by a fungal infection called oak wilt.

Certain types of maples have proliferated, but the sugar maples and red maples have been attacked by a disease of unknown cause and by maple wilt, a fungus.

All told, says McDonnell, the future of at least 10 forest trees, including beech, flowering dogwood, sycamore, white ash, balsam fir and hemlock is clouded by disease or pests.

What is replacing the old trees? The forests are being taken over by exotic plant species like the well-known ailanthus (or tree of heaven), the Japanese honeysuckle, the bittersweet, the Norway maple and the Amur cork tree.

Although exotics were introduced as ornamental trees (and the city of New York continues to recommend some of them for urban planting), many have escaped to the wild.

For example, Norway maples, with their thick, low canopies, may look pretty in a front yard, but their naturalized relatives are crowding out native species. They cast a deep shade that ruins sun-loving trees' chances for survival. And because their leaves do not decompose well, the plants near them on the forest floor do not always thrive.

Roads, power lines, encroaching ex-urbanites, high-tech industries and vacation homes have also taken their toll. According to McDonnell, 71 percent of the forest in the rural New Jersey highlands, regarded as a reasonably wild area, consists of patches smaller than 50 acres.

This fragmentation isolates plant and animal populations, so that each species is more vulnerable to local extinction. Fragmentation also creates lots of forest "edges" - places where non-native species invade the forest and displace native ones.

At the forest's edge trees are more susceptible to being blown down and to being eaten by game animals like deer. The more forest edges there are, the smaller the range of deep-forest creatures.

For instance, songbirds like the red-eyed vireo, the wood thrush and the prothonotary warbler are now being pushed aside by raucous bluejays and parasitic cowbirds. And the "edge effect" can ripple surprisingly deeply into the forest.

To many people, one kind of forest is as good as another. But for ecologists something irreplacable is slipping away. "We have enough quantity, but what we're losing is the quality habitats" and the native plants and animals that go with them, says McDonnell. "We're losing that natural heritage."