The county plans to add five Vashon properties, including its newly acquired site on Maury Island, to a preservation program that would provide the parks and natural areas added environmental protection. An open house to discuss the additions will be held tomorrow from 6 to 7:30 p.m. at Dockton Park.

Under the preservation program — an amendment to the county charter overwhelmingly approved by voters in 2009 — the sale of designated properties, or any change in their use, would require a supermajority vote by the King County Council as opposed to a simple majority.

Sharon Clausen, a project manager for King County Parks, said the program is like a safeguard, protecting land purchased for conservation from sale or other significant changes. She added that the program doesn’t have any bearing on rules for community use at the parks.

“The county wants to keep (this land) in perpetuity, and one way to do that is to make it difficult to make changes,” she said. “Over time, elected officials change, management changes. This is kind of like a guarantee.”

Nine properties on Vashon are already protected under the ammendment, including Island Center Forest, the Shinglemill Creek Preserve, Dockton Forest and the Maury Island Marine Park. Proposed additions include the Point Heyer Natural Area, Paradise Valley Natural Area, recent additions to Island Center Forest and Raab’s Lagoon, and the former Glacier property on Maury Island, now owned by the county.

Tom Dean, director of the Vashon Maury Island Land Trust, agreed that the program provides important natural areas and preserves on Vashon added protection. Sale of critical lands, he noted, has happened in other areas when agencies needed the funds.

“It’s just an extra layer of protection against some horrible downturn in the economy,” he said.