OAKLAND — Clearing the way for a landmark Sonoma County conservation deal, the state Coastal Conservancy board Thursday approved a $10 million contribution toward the purchase of Preservation Ranch, a 19,652-acre property that sprawls across the county's northwest corner.

The state agency's board voted 6-0 after hearing its staff and county officials, including Sonoma County Supervisor Efren Carrillo, describe the advantages of preserving the property that had been eyed for a controversial forest-to-vineyards project covering nearly 1,800 acres.

Securing the conservancy's $10million contribution was "the most significant financial hurdle" to completing a $24.5 million purchase of the property, Carrillo said after the meeting. "The sun is bright on the horizon for finalizing the deal by the end of May."

The next major step is confirming a $4 million contribution from the Sonoma County Agricultural Preservation and Open Space District at its May 7 meeting.

Carrillo and Bill Keene, the Open Space District general manager, said they foresee no obstacles to completing purchase of the land from CalPERS, the giant state workers pension fund.

The negotiated purchase includes $10.5 million in private funds. The purchase would "put to rest the most contentious land use debate in Sonoma County," a Coastal Conservancy staff report said. It also would be the largest conservation purchase by acreage in Sonoma County history and the largest along the North Coast in years.

Once home to the Kashia Pomo, the rugged, hilly ranch was heavily logged in the 1950s and '60s. It includes 20 miles of streams inhabited by steelhead trout, while mountain lions, bobcats and deer roam the land.

CalPERS, the $257.5 billion state workers pension fund, has controlled Preservation Ranch for nearly a decade. For most of that time, the ownership was through a Napa-based vineyard development firm that bought the property in 2004 for $28.5 million. Since late 2011, the pension fund has had direct control of the land and the Preservation Ranch project.

CalPERS' plan to convert up to 1,769 acres of forest and grassland to vineyards, removing more than 300,000 trees, and to possibly subdivide the ranch into 60 estates has not been withdrawn, Carrillo told the state board. CalPERS still may be interested in proceeding if the conservation purchase does not occur, he said.

The deal would grant the Coastal Conservancy a share of net revenue from timber harvests and carbon offset sales from the property, after annual management and restoration costs are met. The money would be used for conservation projects on the California coast.

Doug Bosco, a Santa Rosa attorney who serves as chairman of the conservancy board, questioned the property's revenue prospects. "Where's the profit in this?" he asked.

Chris Kelly, California program director for The Conservation Fund, which would own and maintain the property, said it could generate carbon credit revenues of "several hundred thousand dollars a year." The property's timber value is minimal, he said, because it has been clear-cut twice and now is a young forest with a timber value of $1.7 million to $2.8 million.

As the forest matures, its carbon credit and timber values will both increase, Kelly said.

Bosco said he was willing to bet on the prospects of carbon credits. "It seems like there's a lot of value to us as an investment," he said. (Bosco is an investor in Sonoma Media Investments, which owns The Press Democrat.)

The Conservation Fund, a Virginia-based group, already owns two large parcels in Mendocino County adjacent to the northern end of Preservation Ranch. It will contribute $6million and secure an additional $3.5 million for the deal. Sonoma Land Trust, a private nonprofit group, would contribute $1 million through a grant from the Gordon and Betty Moore Foundation.

The conservancy's funds come from a 2002 voter-approved bond for water, park and coastal conservation projects. Acquiring the ranch, which lies between Skaggs Springs Road and the county line, would establish a 58,000-acre uninterrupted swath of protected land in the two counties.

Board members encouraged the county and The Conservation Fund to establish public access to Preservation Ranch, which is currently accessible by Kelly Road, a private road used only by property owners.

"There's a lot of public money going into this," board member Mary Shallenberger said.

Kelly said his organization intends to come up with a public access plan over the next two years.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.