The amount of land covered in forest in Loudoun County actually has increased in the past 2 1/2 decades, but many of the remaining forests are fragmented, and those that were cut down have been replaced by "trash trees," researchers at George Washington University have found.

The researchers used satellite images obtained last summer to map forested land in Loudoun. The images were subjected to an analysis using a computer program known as a geographic information system that can track the fragmentation of forest cover.

The results show that between 1973 and 1999, Loudoun experienced a net gain in forest cover--from 30,281 forested acres to almost 38,000 acres--primarily because patches of fast-growing juniper trees have taken root in recently developed areas and in parts of central and southeastern Loudoun that once were used for agriculture.

The study confirmed that the greatest loss of forest has occurred in the eastern end of the county, where large tracts of forests have been removed, leaving behind fragmented woodlands.

The researchers said that juniper patches and fragmented forests are not as desirable as more mature forests because they lack the diversity of habitat that attracts and sustains a variety of birds and other wildlife.

"As you drive around the county, you can see a lot of junipers coming in to fill in the fields," said Dorn McGrath, director of the Institute for Urban Environmental Research at GW. "People say, 'Oh, yeah, we're getting trees back.' The fact is we're getting trash trees back. . . . It's not the same kind of habitat as what was there originally. It's not as diverse. It takes a long time to get back to a forest of maples and other hardwoods."

The forest study is one component of the Loudoun Environmental Indicators Project, undertaken by a phalanx of scientists in 1998. Eventually, the project--which is being financed by private groups and citizens, the Loudoun County Board of Supervisors and the Virginia Environmental Endowment--will document the state of the environment in Loudoun and try to predict its future.

The analysis of the satellite images includes data on forests in the county's 11 watersheds, with the greatest loss of forest recorded in the Potomac and Catoctin watersheds and the greatest gain recorded in the Broad Run and Bull Run areas.

The greatest fragmentation was mapped in the Sugarland Run watershed.

The images tell the tale of tracts of forested land that were cut into fragments--islands of trees--to make way for the rapid development that made Loudoun County the third-fastest-growing county in the country last year.

The forests in Loudoun already were fragmented in 1973--the year against which this year's data were compared--but in the past 26 years, the big fragments became even more fragmented, said Douglas O. Fuller, a geography professor at George Washington who is in charge of the tree cover analysis for the Loudoun environmental project.

"You get smaller and smaller islands of forests, almost like tiny archipelagoes," Fuller said. "They're small, isolated."

More mature forests in Loudoun typically have 30- to 40-year-old hardwood trees--hickory and oak--of various heights. The newly grown forests typically have trees that are all about the same height.

Juniper trees proliferate because--much like weeds--they establish root systems and grow quickly. But the sprawling and scraggly patches of junipers, which typically live only about 30 years, will not prevail in the long run if slower-growing oaks and other hardwoods--whose seeds are dropped by passing birds--also succeed in taking root, scientists said.

"Ultimately, the junipers are outcompeted by the hardwoods, so it's basically a sort of race, and the junipers ultimately are overtopped," Fuller said. But in the meantime, foxes, raccoons, songbirds and other animals that thrive in the older forests typically cannot manage in the juniper patches, Fuller said. And animals that require a large forest are challenged to survive in forest fragments that let in more noise, wind and pollution.

"Songbirds like to live in contiguous forests," Fuller said. "They don't like to fly over subdivisions. They don't like to fly over shopping centers. They just don't survive under those conditions."

Scientists will present the first of five annual reports on the project Dec. 2 at the university's campus near Leesburg.

CAPTION: Scientists have mapped forested land in Loudoun, such as this tract of trees along Potomack View Road in Sterling.