The North Shore Land Alliance maintains a low profile by design.

The only indication of the group's presence in donated space in a historic barn off Post Road in Old Westbury is a tiny sign by the door that reads "NSLA."

In its nine years of operation, the group has learned that to preserve open space, especially land owned by wealthy Gold Coast residents, it's better to tread lightly.

"It's kind of the nature of our work," said president Lisa Ott. "Land is a very personal thing."

So the six-woman Alliance staff works primarily behind the scenes -- often with governments or other partners -- to keep pristine woodlands and farms from sprouting McMansions that would despoil the groundwater.

To date, it has protected more than 150 acres at 13 sites in villages in Oyster Bay Town and one in Cold Spring Harbor through land easements in which the owners donate their development rights to the organization in return for tax breaks.

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The Alliance has donated money and in other ways helped Nassau County and the towns of Oyster Bay, North Hempstead and Huntington preserve another 700 acres.

The Alliance last year saved the historic 1844 Trousdell House in Oyster Bay from demolition by purchasing it and is now seeking a sympathetic buyer.

And it has worked with local governments to make their laws more effective in preserving open space and run programs to educate the public about the importance of doing so.

"We don't actually own that much land, but we had our hands in the protection of a lot of pieces of land," Ott said.

The organization is about to assume ownership of six nature preserves -- 126 acres -- from the Nature Conservancy, a national organization that helped form the Alliance, after managing the preserves for three years.

"Because Nassau, unlike Suffolk, has no permanent government land preservation programs, the private initiatives have been key to getting any preservation done in Nassau," said Richard Amper, executive director of the Long Island Pine Barrens Society. "The Alliance has attracted private money and government money as a result of [its] advocacy."

Huntington Supervisor Frank Petrone said, "We are fortunate to have such a capable and committed organization willing to support local land protection efforts."

The Alliance operates on an annual budget of about $800,000 but raises additional funds for special projects through memberships -- there are almost 2,000 members -- and events, including an annual wine auction. Annual membership costs $40.

Because the Alliance's board members are a who's-who of the Gold Coast and the group's focus is largely on that area, Ott said the organization is sensitive about appearing elitist. "That's what we try to do, as much as we can, with education," she said to explain the value of open space to all of the region's residents and how it helps protect groundwater in particular.

The Alliance has offered programs at Roosevelt Middle School and Roosevelt Charter Academy for several years and organized events at Nassau County's Roosevelt Preserve.

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The group last year held its first Small Farm Summit to promote saving land through the development of local small-scale farming. The event drew 800 people. This year's summit is April 14 at Hofstra University in Hempstead.

The Alliance calculates there are about 12,000 acres of open space left along the North Shore from the Queens border to Smithtown. "That's our target," Ott said.