A view of Marble Mountains, within Mojave Trails National Monument.

President Obama declared three national monuments in Southern California on Friday, creating the world’s second-largest desert preserve and nearly doubling the amount of land he has unilaterally protected while in office.

The new Mojave Trails, Sand to Snow and Castle Mountains national monuments cover more than 1.8 million acres of land and include sand dunes, Native American petroglyphs and one of the continent’s youngest volcanoes. The designations under the 1906 Antiquities Act connect an array of existing protected areas, including Joshua Tree National Park, Mojave National Preserve and 15 wilderness areas, creating a nearly 10 million-acre arid land reserve that is surpassed only by Namibia’s Namib-Naukluft National Park.

Brian O’Donnell, executive director of the Conservation Lands Foundation, called it “one of the most significant land conservation gains in the past two decades.” He said the designation “will help the desert, its wildlife and communities adapt to a changing climate and maintain its scenic beauty.”

House Natural Resources Committee Chairman Rob Bishop (R-Utah), by contrast, called it “presidential bullying.” While Sen. Dianne Feinstein (D-Calif.) had conducted hearings on a similar legislative proposal and an array of local groups and officials had endorsed the idea, the designations effectively bypass a competing Republican bill and are not subject to the extensive federal review that other land use plans must undergo.

“It’s an authoritarian act that ignores people under the guise of preservation,” Bishop said.

The president flew over part of the national monuments Friday on his way from Los Angeles to Palm Springs, Calif., though he opted to make the designations through three written proclamations.

In the presidential proclamation for Mojave Trails, Obama said the area “is a stunning mosaic of rugged mountain ranges, ancient lava flows, and spectacular sand dunes. It is a landscape defined by scarcity and shaped by travel.”

The 1.6 million-acre stretch, which includes more than 350,000 acres of already designated wilderness, features the longest intact stretch of Route 66 as well as an ancient Native American trading route and former World War II training site for the troops of Gen. George S. Patton Jr., who were preparing for battle in North Africa.

Sand to Snow, which has 100,000 acres of already designated wilderness and another 54,000 acres that just won new protections, boasts more than 240 species of birds and 12 threatened and endangered wildlife species, along with more that 1,700 Native American petroglyphs. It also includes 30 miles of the Pacific Crest Trail, the setting for Cheryl Strayed’s celebrated memoir, “Wild.”

The region is also home to the world’s smallest owl species, the elf owl, whose numbers have declined sharply in North America as its habitat has shrunk.

“Sand to Snow’s peaks and valleys have long provided physical and spiritual sustenance to native people,” said Agriculture Secretary Tom Vilsack, whose agency will jointly manage the monument along with the Interior Department’s Bureau of Land Management. “Today, they are also an inspiration and recreational beacon to millions.”

Castle Mountains, the smallest designation at 20,090 acres, links two mountain ranges and serves as a migratory range for bighorn sheep, mountain lions and bobcats.

Feinstein, who ushered through a landmark desert conservation law in 1994, tried to protect all three areas in legislation but asked the president to step in last summer after a compromise proved elusive. In a statement Friday, the senator said she and her staff “have spent hundreds of hours working with the diverse range of stakeholders” to forge a consensus, but in the end Obama needed to use his executive authority “to carry out the protection needed.”

Obama has now conserved more than 265 million acres through the Antiquities Act, which allows him to put federal lands and waters off limits to development without congressional approval. In an interview Thursday, the Sierra Club’s Michael Brune said environmentalists are still hoping the president will impose restrictions on federal lands ranging from the 1.8 million acres surrounding the Grand Canyon to the Arctic National Wildlife Refuge’s coastal plain.

“Throughout his second term, the president has been very clear and very consistent that he’s going to use the authorities vested in him to both fight climate change and protect our natural and cultural resources,” Brune said. “We anticipate that there will be many more announcements to come.”