The Prince William Board of County Supervisors last week tightened the requirements of a tax-break program designed to preserve open space from development. The supervisors took the step because they feared the program could be subject to abuse.

But the supervisors backed away from a staff plan to eliminate a similar tax break for forest land, after three hours of angry testimony from landowners and environmentalists who said the change could lead to runaway development.

Under the use-value taxation program, commonly referred to as "land use," open space and agricultural and forest land is taxed according to its use, rather than its market value. The use value can be only one-tenth or less of the market value. The programs cost Prince William $6.5 million in additional tax revenue in 1989.

Proponents of the program say the tax breaks help stave off development when property values, and hence real estate taxes, begin to rise. Landowners who might feel pressured to sell their land to developers to pay their taxes can hang onto their land longer.

"We would have to sell if they did away with {the tax break}. No way can you meet the theoretic market price values," said Anne D. Snyder, who has 180 acres in the forestry and agricultural programs.

Developers also take advantage of the tax break to minimize taxes on land they are not ready to develop. In the current real estate slump, such programs greatly reduce developers' expenses and allow them to wait until the market improves.

For example, 80 percent of the tax savings gained under the forestry category went to landowners living outside the county, including two major developers, Anden Group and the development arm of Exxon Corp.

People who get the tax break can develop their land at any time, although they must pay five years of back taxes when they do.

Prince William tax assessors feared that several recent changes in state law made the open-space program subject to abuse.

In 1988, the General Assembly greatly broadened the "open space" category, which previously had been reserved for private recreation spaces, such as golf courses. The new rules allowed virtually anyone with five undeveloped acres to get the tax break.

Although only 12 Prince William landowners have applied to put their land in that category, the changes made 5,300 more acres eligible, and county staff members feared the lost revenue could exceed $500,000. The supervisors concurred, and voted to limit the open-space program to parcels of 20 acres or more.

"If you have 19 acres of {agriculturally zoned land}, you can't subdivide it anyway, so we aren't getting anything {for the tax break}," said Manager of Assessments John Cunningham.

The board rejected the companion plan to eliminate the forestry tax break because supervisors were concerned that some property owners might be forced off their land.

Instead, the supervisors intend to ask the General Assembly for authority to create small forestry districts in which landowners get the same tax break but are required to make a commitment to the program for at least four years.

"We're not prepared to change {the program} unless we have an alternative," said Supervisor William J. Becker (R-Brentsville), whose district includes many beneficiaries of the tax break.

Supervisors said they also were concerned that they would lose the five years of back taxes. Landowners who were forced out of the land use program and then develop their property do not have to pay the back taxes.