Sonoma County has the third-highest tally of protected open space in the nine-county Bay Area.

But more land in the county also remains threatened by development than in any other part of the bay region, according to a new report from the Greenbelt Alliance, an open space preservation group.

About 115,000 acres — an area more than four times the size of Santa Rosa — could be transformed by development within the next three decades, the group said.

That's more than a third of the 322,000 acres across the Bay Area likely to be threatened by development over the next 30 years, the report said.

The environmental group, which has produced the report every six years since the late 1980s, said it provides a snapshot of the mix of building pressure and open space protection throughout the 4.5-million-acre Bay Area.

Open space advocates and government officials said it serves as a useful report card for growth and land-use issues.

"It reminds everybody what's at risk and what we need to do," said Ralph Benson, executive director of the Sonoma Land Trust, a private nonprofit organizations that has protected about 25,000 acres in Sonoma County.

A local building industry leader, however, said it appears to overstate development pressure that has hardly existed since the recession.

That dropoff, combined with the county's low population growth in the past decade, government zoning and subdivision restrictions and public and private land protection efforts, have stemmed any risk of overdevelopment, said Keith Woods, chief executive officer of the North Coast Builders Exchange.

"The threat doesn't seem real to me," Woods said.

Because of Sonoma County's size — at just over 1 million acres, it is the largest county in the Bay Area — open space leaders and building representatives said they were not surprised it ranked at the top of the group's list for acreage that could face future development pressure.

The county's tally is higher than the combined acreage identified as at risk in Santa Clara, Alameda and San Mateo counties, three of the region's largest by population, but with a far smaller store of open land.

Rural lands at low risk of development still make up the widest swath of Sonoma County — about 649,200 acres, or almost two-thirds of the total acreage.

The area protected in parks, open space and farmland, meanwhile, has grown by 28 percent since 2006, to 171,200 acres, or 16.9 percent of the county. Across the Bay Area, 1.1 million acres are protected and an additional 2.2 million are at low risk for development.

Work by Sonoma County's tax-supported Agricultural Preservation and Open Space District, which has set aside more than 85,000 acres since 1990, has been a main factor fueling the increase of protected land locally.

Other factors include the voter-approved urban growth boundaries now ringing all nine cities in the county. Most date from the mid- to late 1990s and government planners say their effect has been to contain most development to urban areas.

Subdivision limits and rules to protect open land between cities, called "community separators," have also been a factor in the unincorporated area, said Jennifer Barrett, the county's deputy planning director.

"Most development now follows the urban corridor," she said.

But a quarter of the county still has little protection from urban sprawl or rural subdivision, according to the Greenbelt Alliance. The highest-risk parcels, representing 13,800 acres, could be transformed into houses and other uses in the next 10 years, the report said.

Lands the group said were at risk encircle the county's three largest cities — Santa Rosa, Petaluma and Rohnert Park — and ring Sonoma and Sebastopol. They were selected based on zoning rules, planning documents and interviews with sources, said Amanda Bornstein, a Santa Rosa-based senior field representative with the group.

Other parcels in Sonoma Valley, Healdsburg and Cloverdale show up because of development proposals that have either been approved or are in the pipeline, Bornstein said.

Preservation Ranch, the large timber-to-vineyard conversion project proposed in rural northwestern Sonoma County, makes the list because its latest proposal includes the potential for a number of homesites, she said.

She said stronger coordination in other areas between cities and counties has helped those jurisdictions limit the threat of sprawl.

"Sonoma County has done a great job, but there's still more work to do," Bornstein said.

Much of the development potential put forward by the group is in an indefinite holding pattern or may never materialize, said Woods, the builders' exchange executive.

"The marketplace is dictating that it will be a serious long time before big development will take place and most of that development is going to be within urban-growth boundaries," he said.

But conservation officials said the report could help them with decisions in the interim about where to steer the now-diminished amount of public open space dollars.

"We all know that everything is cyclical," said Bill Keene, general manager of the county Open Space District. "The next boom is around the corner. Will it be like past booms? Maybe not. But you want to be prepared. Things can change very quickly."

You can reach Staff Writer Brett Wilkison at 521-5295 or brett.wilkison@pressdemocrat.com.