Fairfax, a city of 6.2 densely packed square miles in the middle of a densely packed county, is embarking on an ambitious program of buying land to preserve its disappearing open space.

The purchase of 50 acres scattered around the city is being funded by a property tax increase Fairfax City voters approved two years ago. A 3-cent jump in the city's tax rate, now 96 cents per $100 of assessed value, already has raised $1.5 million, city officials said.

"Concern about rapid growth led to this," said City Council member R. Scott Silverthorne, who came up with the idea of a land-purchase program in 2000. At the time, Northern Virginia's booming economy was saturating the city of 21,000 with new residential and commercial development, and residents were starting to feel the results in traffic jams and crowded schools.

"I thought, is there a way to slow the growth down?" Silverthorne said. "The only way was to buy land and preserve it."

Voters by a 2-1 margin backed a tax increase of up to 5 cents. City officials said they hope to use the acquired land to build parks, trails and possibly athletic fields.

The city's effort to save its remaining open space comes at a time when other area localities are seeking creative ways to preserve undeveloped land and manage development. Montgomery County has preserved huge swaths of land for open space, and voters in Fairfax County approved a $20 million bond issue in November that will allow the park authority to spend $15 million on open-space acquisition and $5 million to turn previously purchased land into parks.

"You need that breathing room, particularly in dense communities," said Josephine F. "Jolly" de Give, director of planning services for the Warrenton-based Piedmont Environmental Council. "These are pocket parks. They give you that breathing room."

Concern about the city's rapid growth dominated the city's mayoral race last spring, when City Council member Robert F. Lederer defeated longtime Mayor John Mason after portraying the incumbent as a leader who embraced excessive development. Lederer said he believes in the program, though initially he was skeptical that buying such a relatively small amount of land would make a difference.

"I thought it was more of a political stance than a practical tool for managing growth," Lederer said yesterday. "But it has turned out to be a very nice tool for a small city to manage its growth."

The city has begun to acquire four parcels and a 50-foot trail easement between Sager Avenue and Main Street. The four parcels are: 3.25 acres in the Orchard Knolls/Fair Oaks area, three acres west of Providence Park, 4.6 acres on Lee Highway off Rebel Run and a 39.2-acre parcel at 10090-10120 Lee Highway. The city has decided to take the last parcel by eminent domain after failing to reach agreement on a sale with the owner. The final cost of the land will not be known until the acquisition process is complete.

The tax increase took effect in 2001 and is scheduled to expire by 2006. But the City Council could increase it to the 5-cent maximum, Silverthorne said, depending on what other properties the city might want to acquire.

The typical Fairfax City homeowner pays about $120 more a year to fund the program. Despite that increase, the city's tax rate remains the lowest in Northern Virginia. The total revenue generated for the program is expected to be about $5 million.

With the exception of Leesburg, Fairfax City has the lowest ratio of parkland to total area of any regional jurisdiction. About 4 percent of its area is parkland. The city of Alexandria and Fairfax and Arlington counties devote about 9 percent, 8 percent and 7 percent respectively of their land to parkland.