Over the past five years, Sonoma County has nearly cut in half the amount of open space that could be paved over by future development, according to a new report from a national environmental group.

A combination of voter-approved growth boundaries, greenbelts separating cities and taxpayer-funded land conservation means Sonoma County fares much better than other Bay Area counties in protecting its open space, the San Francisco-based Greenbelt Alliance found in its report, released Tuesday.

Overall, the span of open land considered to be at risk of future development in the county has dropped from 115,000 acres in 2012 to 58,400 acres — an area more than twice the size of Santa Rosa.

The number of acres most at risk — meaning they could be paved over in the next decade — dropped 70 percent in that period, to 4,100 acres, the group found.

“Clearly, Sonoma County has been a leader on protecting open space,” said Teri Shore, the Greenbelt Alliance’s North Bay regional director.

The report called out the 900-acre Sonoma Developmental Center campus in Sonoma Valley as one of the properties at risk of being developed. The state is set to close the facility at the end of 2018 and a coalition of interest groups have united in an effort to protect the center’s open space.

The group credited the county with the “largest recent strides in land protection” in the Bay Area, where 293,100 acres of natural space and farmland is at risk of being developed over the next 30 years, according to the report.

That amount is nearly 10 times the size of San Francisco, though it marks a 9.2 percent decline of at-risk acreage since the Greenbelt Alliance’s 2012 analysis.

Among the region’s nine counties, Contra Costa had both the largest total acreage at risk and the largest amount Ultrabet of land at high risk for development over the next decade.

Sonoma County had the second highest tally for threatened open space, a factor mainly of the county’s vast size, at more than 1 million acres, its rural landscape and growing economy.

The report lauded the county’s recent actions to protect its greenbelts, including the overwhelming passage of Measure K, which extends for another 20 years protections on tens of thousands of acres of largely undeveloped lands separating cities in the county. The Greenbelt Alliance spearheaded the campaign for the measure.

“The strength with which Measure K passed … really indicates the value that our communities place on open landscapes,” said Dave Koehler, the executive director of Sonoma Land Trust, which has protected more than 48,000 acres in the county.

The report also called out the 2013 purchase of the 20,000-acre property formerly known as Preservation Ranch. It was the largest conservation deal in county history, protecting a sprawling landscape of forest and wildland in the northwestern corner of the county near Annapolis.

Along the more developed Highway 101 corridor, the report marked several areas at highest risk for development, including lands around Rohnert Park, Windsor, Healdsburg and Cloverdale.

Shore pointed to Rohnert Park’s University District project, under construction near Sonoma State University on the city’s eastern edge, as an example of the development that could that could chip away at open space around city limits. Shore acknowledged that the property falls within the city’s urban growth land and the development was planned for years.

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