WASHINGTON - Recent images from space satellites reveal hundreds of little-known primeval forests and stands of ancient trees scattered all across the United States. Scientists say these trees -- some dating before the rise of the Roman Empire -- provide an unequaled record of droughts and floods that can help them understand historic disasters and predict environmental changes.

In addition to California's famed redwoods and giant sequoias, researchers have discovered that millions of very old trees remain in their pristine state in dozens of states from New England to the Carolinas and across Texas to Arizona and Nevada.

"We can still find unmolested virgin forests," said David Stahle, a forest scientist at the University of Arkansas in Fayetteville. "There are still trees that are thousands of years old, the last relics of the great forest primeval that has been heavily disturbed or completely destroyed by man."

According to Stahle, the largest old-growth forest left in the United States consists of ancient blue oaks covering more than 4,000 square miles of the California foothills. But even in the thickly populated eastern United States, Stahle thinks, more than 2,000 square miles of old-growth woodlands survive to this day.

"People used to think there were no ancient trees in the eastern United States. That is not the case," Stahle said. "The abundance of ancient forest sites strongly contradicts the common misconception that most ancient forests were destroyed by logging and agricultural development."

Old-tree hunter Robert Leverett, executive director of the Friends of the Mohawk Trail in Deerfield, Mass., has discovered a 626-year-old black gum tree in New Hampshire. There are 400-year-old red oaks on a Massachusetts mountain in view of the Boston skyline. Only 50 miles north of Manhattan, 500-year-old pitch pines cling to a mountainside in the Hudson River Valley.

Farther south, bald cypress trees, 1,500 to 2,000 years old, dwell along North Carolina's Black River. Stands of 900-year-old junipers survive in the Ozark Mountains of Arkansas. About 500 square miles of post oaks up to 400 years old remain in eastern Oklahoma, some only 15 miles from downtown Tulsa.

Leverett and Stahle were reluctant to identify the precise locations of old trees, for fear of endangering them. Often, park rangers will help.

"The Blue Ridge Parkway in Virginia and North Carolina still has many areas of old chestnut oak along the windy ridgelines," Stahle said. "Millions of visitors drive by them every year without realizing the antiquity of these handsome, gnarly old oaks."

Most very old trees are found on rugged terrain, arid land or steep slopes unsuitable for development or agriculture. They are slow-growing, stunted and twisted, and hence were never harvested for lumber.

Stahle compared their worn appearance to the "silver hair and wrinkled skin" of elderly humans.

"The ruggedness of the mountainous lands and individual landowner and society preferences allowed for uncut forested spots to slip through the cracks in almost original condition," Leverett said. "As a result, pockets of old-growth forests in New England survive today in approximately the condition we might have witnessed before Europeans came to the shores of New England in the 1600s."

To find unmolested trees, Stahle and his associates study images taken by NASA's Landsat satellite, looking for remote areas where they are likely to survive. After identifying a promising region, they select random samples and bore narrow cores about as thick as a pencil into the centers of the trunks. The cores are taken back to Stahle's Tree Ring Laboratory at the University of Arkansas, where they are polished and the tree rings are counted.

As has been known since the time of Leonardo da Vinci, a tree grows a new ring each year, providing a remarkably precise measure of its age.

Tree-ring records show that the oldest-known continuously living organisms on Earth are 5,000-year-old bristle cone pines in the western United States. The rings in dead trees also can be counted, such as 8,000-year-old logs lying on the forest floor. In Europe, 12,000-year-long climate chronologies have been constructed from fossil logs buried in Irish peat bogs and German river sediments.

Tree rings tend to be wider in years of plentiful moisture and narrower in dry years. The patterns closely match human records of rainfall, and can be used to fill in the blanks before people began to keep weather data.

Though they have little or no economic value, these old trees are "archives of environmental history," said Ben Patrusky, executive director of the Council for the Advancement of Science Writing in New York.

The rings in California's blue oaks, dating from 1604, show that San Francisco Bay became saltier during dry years, when less fresh water flowed into the streams feeding the bay. The most extreme salinity recorded in the last four centuries was during the California drought of 1976-77.

Tree rings show the impact of climate on human history. For example, the driest single year in records covering 800 years along the eastern coast of North America was 1587. That was the year when both Sir Walter Raleigh's "Lost Colony," on what is now Roanoke Island, N.C., disappeared and Spanish colonists abandoned their Santa Elena settlement on Parris Island off South Carolina.

In the American Southwest, the decade-long Dust Bowl drought of the 1930s was the worst in the last 500 years of tree-ring records.

An even longer set of rings, dating to 137 B.C., before Julius Caesar was born, revealed a 30- to 40-year megadrought 400 years ago that helped end Native American colonies from New Mexico to Guatemala, according to a paper published by Mexican epidemiologist Rodolfo Acuna-Soto.

Years of more than normal rain created overly optimistic expectations early in the 20th century. This led to excessive development in the West, such as assigning more water from the Colorado River for cities and farms than could be sustained in dry times.

Scientists hope that understanding the cycle of wet and dry weather, preserved in the tree rings of ancient forests, can help prevent such mistakes.

"In terms of science, the old forests provide living laboratories for researchers to study," Leverett said.

For more information, go to the University of Arkansas Tree Ring Laboratory Web page, at www.uark.edu/dendro. To find ancient trees in your area, check with national and state park rangers.