IF a local land trust had not stepped in 38 years ago and started preserving the banks along the Sudbury River from development, parts of the large stretch of marshland and open space might now resemble the urban sprawl that has crept from Boston westward.But through the efforts of the Sudbury Valley Trustees, 6,000 acres in the Sudbury River Valley have been saved, some of it now part of a national wildlife refuge. The land trust, which started with three individuals, now has nearly 2,500 members and has evolved from an all-volunteer organization to a group with eight employees. "We expect to continue to grow rapidly in size of membership as well as reach of communities we serve," says Lisa Curtis, executive director of the Sudbury Valley Trustees. Land trusts - nonprofit organizations dedicated to preserving open space - are one of the fastest-growing conservation movements in the United States. During the past decade, the number of land trusts doubled from 429 in 1980 to nearly 900 today. They have helped protect more than 2.7 million US acres, according to the Land Trust Alliance, the national organization for such trusts. Jean Hocker, president of the Land Trust Alliance, says the growth stems from a heightened environmental awareness among individuals and the belief that people can make a difference through grassroots efforts. "I think people are seeing things in their community that they don't like, and they don't see anybody else fixing it. They're getting angry about it, and they're forming organizations to do it themselves," she says. Land trusts operate in every state on the local, regional, or state level. Most are run by volunteers, but a large number have full- or part-time staffs. From magnificent scenic areas like the California coast at Big Sur to an urban garden in Chicago, land trusts have helped protect many tracts of valuable property. Farmland, forests, historic areas, wetlands, wildlife habitat, and islands have also been preserved. The conservation methods land trusts use vary. Many trusts receive property through donations or buy it and then resell it to a park or government agency. Another approach is acquiring land through conservation easements - permanent, legal restrictions on development. Ms. Curtis says this method has become increasingly popular among landowners here. Often, when a landowner turns property over to family members, the family is forced to sell it to developers because it can't afford to pay the expensive estate taxes. Conservation easements give development rights to the land trust while allowing the family to continue to live on the land and qualify for tax benefits. Terry Blunt, manager of the Massachusetts Department of Environmental Management's Connecticut Valley Action Program - a program which helps conserve land along the Connecticut River - says land trusts are cost-effective and benefit all involved. The first land trust was formed in 1891 when the Massachusetts legislature incorporated the Trustees of Reservations. But the most most phenomenal growth of land trusts occurred during the 1980s. Curtis says the Sudbury Valley Trustees also experienced rapid growth in membership at this time. And although the faltering Massachusetts economy ended the development craze of the middle to late '80s, she predicts development will start up again. During this lull, the Sudbury Valley land trust has taken an active role in talking to landowners who are doing estate planning and informing them how they can preserve their land. Ms. Hocker predicts that the land trust movement will continue to grow. "What's going to happen is more people will get involved in protecting more open space in communities across the country and will build a strong constituency for open space protection."