The largest tract of privately owned green space in Philadelphia is now protected from development into perpetuity.

Officials at the Schuylkill Center for Environmental Education reached an agreement for a conservation easement on 325 acres of woods and fields in Upper Roxborough, virtually all the land the center owns.

In doing so, the center gave up the potential for tens of millions of dollars if it had sold the property to a developer. And it traded the land for $750,000 in state grant money, minus administrative fees.

"This is really satisfying," said John Howard, an architect and board member who years ago promised an ailing founder of the center that he would not leave the board until he saw the project through.

"There aren't many things in life that one can do that really extend into the far future with assurance," Howard said. "But this is just about the best way I can think of. Land is so precious. And once it's gone, it's gone."

Officials of the center signed an agreement Wednesday with the Natural Lands Trust, a nonprofit land-conservation organization, which will monitor the easement and, if necessary, enforce it.

"Philadelphia is blessed with a lot of open space . . . but when you think about the consequences of losing something like this, and what the impacts would be to the community and the ecology of this area, it's difficult to overstate the importance of this," said Molly Morrison, president of the trust.

Neighbors chimed in as well.

"It's an extraordinary thing they've done," said David Cellini, president of the Residents of Shawmont Valley Association in Upper Roxborough.

The center traces its origins to the mid-1960s, when descendants of Henry Howard Houston, a railroad magnate who died in 1895, donated land.

One of them was Henry Meigs, who secured the promise of preserving the land from Howard.

Meigs' son, Massachusetts sculptor Binney Meigs, is now president of the board. He grew up on the property - bordered by Spring Lane, Hagys Mill Road, Port Royal Avenue, and what is now the Schuylkill Bike Trail - roaming its woods, he said in a phone interview Thursday, and "it holds huge power for me."

The property encompasses two "first-order streams," Smith's Run and Meigs' Run, which are largely unpolluted from their headwaters to where they empty into the Schuylkill, a rarity in any city.

It has nesting populations of rare species like the blue-winged warbler and trees believed to be 250 years old.

One section of about 20 acres has had a deer-exclusion fence for nearly a decade, and the area inside has been cleared of invasive species and replanted with native ones.

Now, "it doesn't look like the rest of the forest," Howard said. "One hopes and dreams that this actually looks like the forest as William Penn found it."

But for most of its life, the nonprofit center, which has an annual budget of $1.2 million, has been under "substantial pressure" to lease or sell off portions, Meigs said.

At one point, a previous executive director contemplated leasing part of the land for a communications tower. "That was a rude awakening," said Bob Turino, president of the Upper Roxborough Civic Association, who also praised the easement.

Meigs said his family always wanted to see the land preserved, but the center was founded when the land's development value was "relatively modest."

Today, he said, "it's extremely valuable."

He said the family had "enormous concerns," but now "we can all rest assured that this green space will be there in perpetuity."

No one really knows how much it would have been worth, but $40 million is a figure that gained some traction in recent years.

Those involved credit Anne Bower, an associate professor of conservation biology at Philadelphia University, with tireless efforts to make the deal happen.

She came to the center first as a researcher, then joined the board and is now its vice president.

For her, it was "two years of some of the most intense work I've ever done in my career." It included overseeing surveys of plants, animals, and water resources.

Two of the main buildings at the center are a wildlife rehabilitation clinic and an education center, with labs, classrooms, and an interactive children's exhibit, a bookstore, and the Green Woods Charter School, which serves students in kindergarten through eighth grade.

Another descendant, Eleanor Smith Morris, had sued in Philadelphia Orphans' Court to evict the school and halt other land development. She said some projects the center was contemplating, including a solar farm, violated the intent to preserve the land as an oasis of nature.

That was later resolved, and although the school is at the center, officials have announced it will leave in 2012 because it will have outgrown its space.

The easement process was stalled during the litigation, and officials had to race to meet a Jan. 1 deadline, when the state grant would expire.

The easement confers three levels of protection on the property, recognizing what's there now. The bulk of the property - nearly 260 acres - will have the highest protection status, in which virtually the only permitted activities are efforts to preserve it.

Thirty-four acres have midlevel protection that would allow such activities as agriculture.

A final 31 acres, which includes barns and other buildings, has the least protections.

This was a disappointment for former board member Christina Kobland, a wildlife advocate who lives in Lafayette Hill. She felt that all the property should have received the highest level of protection.

Cindy Dunn, deputy secretary of the state Department of Conservation and Natural Resources, which awarded the grant, said the easement "really touches all the right points for our conservation program."

The grant money comes from the Keystone Fund, which is generated from a portion of the real estate transfer tax.

"A lot of land opportunities come to us, so we look for extra values," Dunn said. "It's rare that we get to do a large land-conservation project so close to the city.