Prince William supervisors put teeth behind their long-standing pleas for tree preservation last week, enacting an ordinance that requires developers to save or plant enough trees to cover a minimum amount of their property.

The law requires developers to plant or preserve trees to ensure that trees will shade 20 percent of most residentially zoned land and 10 percent of most commercially zoned land.

"It was long overdue," said Supervisor John D. Jenkins (D-Neabsco). "Trees improve the quality of the air . . . and stabilize the topsoil."

The Board of County Supervisors timed its action to take advantage of legislation passed last year by the General Assembly. Cities and counties that pass tree ordinances before July can require developers to meet minimum tree standards within 10 years, but jurisdictions that adopt ordinances after that date must give developers 20 years to meet the minimum standards.

"The 1989 regulations provide a lot of incentive for preserving the existing cover {because they set a shorter time table}," said Supervisor Kathleen K. Seefeldt (D-Occoquan), who has been fighting for a tree preservation ordinance for seven years.

The county had to wait for action by the General Assembly because the courts had thrown out a similar Fairfax County ordinance, saying the local government did not have the authority to require tree preservation.

"I'm glad to see they finally did adopt a tree ordinance. They have always said they needed the state {to authorize such an action}. Now it's good to see them following up," said Del. John A. "Jack" Rollison III (R-Woodbridge), who first sponsored the enabling legislation in 1986.

At a marathon hearing last week, the ordinance came under heavy fire from the building industry and some nursery owners. They argued the 10-year requirement would prompt developers to plant what they called "trash trees" -- fast growing species that would quickly provide the minimum canopy and then die.

"What they passed just wasn't right. It was done in a hurry," said Thomas F. Neil, owner of White Oaks Nursery. "The county said 'Quality doesn't count, we're going for quantity.' "

Neil, who testified against the ordinance, said he wants the county to establish a rating system, where developers could plant fewer trees of higher quality.

While Keith E. Hawkins of the Virginia Department of Forestry agreed, "Some developers are going to put in the cheapest thing they can," he said he believed the 10-year requirement would prompt developers to save more existing trees.

And Seefeldt said, "These are the same tired, ineffective arguments I've heard before."

Both Loudoun County and the City of Manassas are considering passing their own ordinances. To insure compliance, the Department of Development Administration will study each site plan and estimate how much cover the planned trees are expected to provide in 10 years, said Assistant County Attorney Angela Lemon, who drafted the ordinance.