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Irvine Co. Chairman Donald Bren signed over 20,000 acres of rugged, dramatic landscape to OC Parks on Tuesday amid windblown grasses and hulking oaks.

“That was painless,” he said after signing a ceremonial deed created for the occasion, while Orange County supervisors, Irvine city officials, park rangers, naturalists and open-space advocates looked on.

Then, before turning away from both the microphone and his role as landowner for some of the county’s most untrammeled wild spaces, Bren, 78, shook the hand of OC Parks director Mark Denny.

“It’s all yours,” he said.

“Yours,” in this case, means all of Orange County. The four major canyons that make up the gift include Black Star, expected to become the 2,000-acre “Black Star Canyon Wilderness Park” within three to four years.

It is the largest gift of land in county history.

Orange County supervisors accepted the 20,000 acres in June, though Bren’s proposal had been announced the year before – and anticipated for 20 years. It increased OC Parks’ landholdings by 50 percent in a single stroke, and caps Irvine Co. land donations over the past century that amount to more than half of the historic Irvine Ranch that stretched across the county’s midsection.

Public access to what was once the domain of cattle and cowboys will gradually increase in coming years.

Hikers, bicyclists and horseback riders already have free access once a month to the Limestone Canyon section, spanning more than 5,000 acres adjacent to Whiting Ranch and home to “The Sinks” – a “breathtaking geological formation” that is “one of the wonders of Orange County, and should be seen by all,” Orange County supervisor Bill Campbell told the group.

And there are other programs and outings on the property led by docents.

Bren’s gift also includes Fremont Canyon, full of poppies in spring, Weir Canyon, full of oak woodlands and mule deer, and Loma Ridge, from which the ocean and downtown Los Angeles were visible Tuesday, with skies blown clear by wind.

Mountain lions frequent the property, raptors hunt rodents in the scrub, owls hoot at night.

“It’s almost like having an entire national park, as a centerpiece, located right here in the middle of Orange County,” Bren told the group. “What’s more, it’s the largest urban open space in the United States. In fact, more than 30 million people live less than 30 minutes from this pristine natural treasure.”

Much of the land is protected under Orange County’s Natural Communities Conservation Plan, an umbrella of land management meant to preserve suitable habitat for a variety of native species.

So OC Parks must balance public access with habitat protection – perhaps keeping some sections closed even as more of the land is opened to the public in the years to come.

For the next three years, the Irvine Ranch Conservancy, created by the Irvine Co. to manage wild lands, will continue conducting research, education and restoration on the property.

Environmental groups, including some that questioned the land transfer and the county’s ability to manage and fund it, studied the proposal carefully before lending their support.