Montgomery County's agricultural reserve is home to 577 farms and 350 horticultural enterprises, 12,000 horses and 2,201 head of cattle. Its rolling hills, rustic roads and pockets of open space make up one-third of the county's land area.

So when Barnesville illustrator Tina Thieme Brown embarked on a year-long project to create a map of the county's 93,000-acre expanse -- where development is restricted to one housing unit per 25 acres -- she knew it wouldn't be easy.

"It was a little overwhelming at first," she said.

With input from naturalists, farmers and county officials, Brown created a map that depicts the roads that link the more developed, suburban "downcounty" to the less built-out, rural "upcounty." The map shows the reserve's reservoirs and rivers, its farms and nurseries, its historical sites, and its flora and fauna.

"What I'm hoping is that we have created enough of a pictorial history . . ." she said. "The hope is that people will get out here and dig a little deeper."

That's the goal of county officials as they observe the 25th anniversary of the agricultural reserve, the most prominent of land-preservation efforts in the metropolitan region. Through such events as farm tours, lectures, and hikes, officials said, the county will step up efforts to educate downcounty residents about the merits of the agricultural reserve. County officials estimate that the reserve contributes about $250 million annually to the county's economy.

"I think that many of the farms that we have today wouldn't be here if the ag reserve had not been created," said Jeremy Criss, the county's agricultural services chief.

Since 1980, the county has offered farmers incentives to not sell their land to developers. One incentive program allows landowners to sell builders transferable development rights that can be used to construct housing units in other, more urban areas of the county rather than within the reserve. Officials have also restricted development in communities buffering the reserve, creating such areas as rural cluster zones, where housing is restricted to one unit per five acres.

Yet even as the county touts its land-preservation efforts, there is evidence that the pressure to develop the land continues to be a threat and that farming has become a more difficult enterprise. Though the market value of agricultural products sold grew from $28 million to $42 million from 1997 to 2002, the amount of farmland dipped from 77,266 acres to 75,077 acres in that time, according to Census of Agriculture figures released last year by the U.S. Department of Agriculture.

Criss said the county recognizes that it is going to lose some farm acreage and hopes to have 70,000 acres protected by 2010.

"Think about it: residential development in Montgomery County. We have extreme pressures on land," Criss said. "We're so close to the nation's capital. All of those factors contribute to the loss of acres . . . . We're going to see development continue."

"There's a lot of different things going on," said Terry Cummings, director of the Poplar Spring Animal Sanctuary, a nonprofit animal refuge inside the reserve in Poolesville. "There's so many people moving to this area and huge pressure to build. There's not much space in Montgomery County and there's a huge area here and people say, 'Why not use it?' "

As she conducted a tour of the sanctuary on a recent day, Cummings compared Montgomery's upcounty skyline with that of Loudoun County's across the Potomac River.

"There's a lot of history here," she told the small group of downcounty residents who also visited several farms and orchards. Then she pointed to the view across the river. "Unfortunately, now we have to look at those high-rises."

Perry Kapsch, second vice president of the Historic Medley District, a nonprofit group that focuses on preserving open space in western Montgomery County, said part of the problem is that many county residents do not know of the reserve's existence or of its goals.

"It's not land being held back from development. It's land that was developed as farmland. . . ." she said. "It's a reserve, not a holding ground for land for future suburbia."

One pressure that the County Council plans to take up in the fall, when it returns from its summer recess, is the presence of megachurches in the reserve. Taking advantage of current policy that allows them to build anywhere in the county, the churches are buying land in the reserve because they cannot find enough affordable property in southern Montgomery.

It's not just churches that are taking up space in the reserve. Some large homes have been built on big lots, making it difficult to have contiguous farmland.

Also, talk of a "techway" bridge linking Montgomery to Northern Virginia through the reserve has never completely gone away. The decision by the state to build the Intercounty Connector from Gaithersburg to Laurel through the eastern part of the county has intensified some northern Montgomery residents' fears of an outer Beltway, even though county officials have rejected such proposals in the past and say they have no plans to change their stance.

Then there's the farming industry itself. Though one of the goals of the reserve is to conserve land for food and fiber production, traditional farming has become less viable. As a result, horticultural businesses and farms that produce hay for the growing equine industry are taking up more space in the reserve, officials said.

Criss said the county is trying to embrace nontraditional forms of farming because they contribute to the economy, but zoning laws require such businesses to apply for special permission to operate in the reserve. "It's a process that is very expensive, very [time-consuming] and highly uncertain," he said.

As a result, county officials have worked to ease restrictions on such enterprises as equestrian facilities and horticultural businesses.

Despite the challenges, officials and ag reserve residents say the future is bright for the land-preservation program.

And Brown, a 52-year-old artist who also did the illustrations for a book on the nearby Sugarloaf Mountain, said she hopes the 18- by 14-inch map she designed can help the effort. "We have a sacred landscape," she said. "This map project was great, we feel, because it was a great way to get the story out."

About 2,000 copies have been printed and will be sold for $12 each. The project was financed through grants: $2,500 from the Montgomery County Historic Preservation Commission and $5,000 from the county's Department of Economic Development. Historic Medley, another sponsor, put in $1,500.

A five-person committee decided what roads, farms and images of animals and vegetation to include in the drawing. Brown said she wanted the map to be as authentic as possible. That meant, for example, that her original decision to prominently display a rooster on the border was vetoed by other committee members. The reason? There isn't a substantial poultry industry in the county, she said on a recent morning, as she ruffled through dozens of watercolors that she had scanned into her computer and arranged onto the map.

The other images on the map -- a horse nuzzling the ground, a farmer on a green tractor, two sheep taking a stroll -- all came from real people and animals she spotted as she rode her bike around the reserve, her home for about six years.

"Everyone who lives here, and a lot who don't, understand what we have here," she said. "It's a legacy."

Vincent Cordova stacks crates of peaches he picked for Homestead Farms, one of 577 working farms in Montgomery County's agricultural reserve. A herd of rescued pigs dozes in a barn at Poplar Spring Animal Sanctuary, a refuge operating inside Montgomery's agricultural reserve. Monocacy Aqueduct, a site on the agricultural reserve map. The reserve is credited with contributing $250 million annually to the county's economy. The agricultural reserve has about 12,000 horses, including this one at Poplar Spring.Artist Tina Thieme Brown explains her work on the map of the county's agricultural reserve.Rescued cows lounge at Poplar Spring Animal Sanctuary. The ag reserve has more than 2,000 head of cattle and 12,000 horses on its 93,000 acres. Ben Allnut works at Homestead Farms, where the peaches at right were produced. Images of a cardinal and a tractor with its operator adorn Tina Thieme Brown's agricultural reserve map, which took her a year to complete. About 2,000 copies of the map have been printed.Lynn Butler tends to rescued mule Miss Glory. A rescued cow at Poplar Springs Animal Sanctuary.