A tree unique to southwest Virginia was thought to be probably extinct for more than 60 years and was listed as such by the Smithsonian Institution until a young biology teacher hiked into the woods and found one three years ago.

Now that same time, the Virginia round-leaf birch, or Betula Uber, is endangered again.

The reason: people apparently have been digging them up for souvenirs or research.

When it was discovered in 1975, the round-leaf birch was healthy and multiplying in a small area containing 15 trees and 21 seedings in rural Smyth County near the Tennessee-North Carolina border.

Since then, three trees have died and 10 seedlings are gone. The seedlings "just disappeared," according to Peter Mazzeo, a botanist with the National Arbo-retum who was called in to confirm the identity of the trees. In their place are 10 holes in the ground.

At that time Mezzeo tried to get other scientists interested in searching for the "probably extinct" tree in that area.

But "most people laughed and thought the whole thing crazy," Mazzeo said this week.

So it was a complete shock at 4 p.m. Aug. 28, 1975, when Mazzeo received a call from his friend Leonard Uttal in Blacksburg, Va.

"What would you expect to be the greatest news in the history of Virginia botany?" Uttal asked.

"I don't know. I give up," Mazzeo said.

"Betula Uber has been rediscovered," Uttal shouted with glee. Its last record sighting was in 1914.

Douglas Ogle, the biologist who found the round-leaf birch, then came on the line.

Mazzeo was so ecstatic, he said, he pummeled Ogle with questions to be certain it was indeed the long-sought birch.

Where is it? How big is it? How old is it? How many are there? Mazzeo asked. But most important: Is it reproducing? Are these seeds and fruits on the branches? Mazzeo said he was almost frantic.

But Ogle couldn't say. He wasn't the expert Mazzeo was.

Within a month Mazzeo arranged a trip to Cressy Creek to see for himself. Sure enough, they were the rare birch. And, incredibly, they were reproducing.

Since then, Mezzeo has brought back hundreds of cuttings from the seedlings along Cressy Creek and has successfully rooted about 30 seedlings at the National Arboretum in the District.

His priority now is stopping them from dying out in the wild.

All but two of the trees and all of the seedlings were on property owned by Ray Haulsee and Garland Ross, both farmers. Both men were anxious to cooperate in preserving the trees, and, at their own expense, erected eight-foot high fences around the birches. Whoever has taken the seedlings has had to climb those fences to get to them.

The two trees on U.S. forest land are located within the Jefferson National Forest on Flat Ridge Road just south of Sugar Grove, Va.

The Forest Service is now preparing the site around one of the trees for visitors and tourists, according to Kirby Brock, area ranger in the Mount Rogers National Recreation Area.

The round-leaf birch resembles its relative the black birch. Both have a wintergreen aroma and black bark and stand about 40-feet-tall when fully grown.

But the round-leaf birch has a more rounded leaf - as its name implies - fewer veins in its leaves and a poorer chance of survival in its natural habitat.

Although the black birch is abundant in Virginia and elsewhere, the round-leaf birch's only habitat is within the high southwest Virginia mountain valley (altitude about 2,800 feet) on the edge of the Blue Ridge with the Appalachians on the southwest.

Just why the trees are dying is unknown to Mazzeo. At least one of them was washed away in floods this spring, but the others died for no apparent reason. Boring of two of the trees indicates they are between 40 and 50 years old.

Their only chance for survival may be through the efforts of Mazzeo and others like him who have formed a committee to study the trees along Cressy Creek and to attempt to propagate the seedlings in the Arboretum. They have agreed not to do anything to those in the wild until they have a better idea of their particular ecology, Mazzeo said.

The only other trees currently listed by the U.S. Department of Interior and the Smithsonian Institution as "probably extinct" is the Franklinia, a member of the tea family. Although no longer in its wild habitat of southern Georgia, the Franklinia is cultivated nationwide, including one on the U.S. Senate grounds.