Sonoma County supervisors hailed a landmark conservation deal Tuesday as they unanimously approved $4 million in open space dollars to help with the $24.5 million purchase of Preservation Ranch in the county’s northwest corner.

A little known state agency that has poured $68 million into Sonoma County conservation projects is running low on cash and planning to scale back its mission of protecting and enhancing vast forests and coastal lands.

A national conservation group has reached an agreement to buy nearly 20,000 acres of timberland in northwestern Sonoma County, a move that derails the long-disputed, forest-to-vineyards conversion project pushed by CalPERS, the giant state workers pension fund.