The Fish and Wildlife Service determined Monday that whitebark pine, a tree found atop mountains across the American West, faces an “imminent” risk of extinction because of factors including climate change.

The decision is significant because it marks the first time the federal government has identified climate change as one of the driving factors for why a broad-ranging tree species could disappear. The Canadian government has already declared whitebark pine to be endangered throughout its entire range; a recent study found that 80 percent of whitebark pine forests in the Greater Yellowstone ecosystem are dead or dying.

The Natural Resources Defense Council asked the Fish and Wildlife Service to place the tree on the endangered species list. In its determination, the agency said that it found a listing was “warranted but precluded,” meaning the pine deserved federal protection but the government could not afford it.

The whitebark pine will remain a candidate under the Endangered Species Act and will come under review annually.

An invasive disease, white pine blister rust, along with insects such as mountain pine beetle, has infiltrated the historically colder altitudes where whitebark pines thrive. Fish and Wildlife Service biologist Amy Nicholas said these factors, along with fire patterns and global warming more broadly, are undermining the tree’s viability.

The Fish and Wildlife Service estimates that the whitebark pine could disappear within two to three generations — from 120 to 180 years from now.

Nicholas said that is why the agency would like to list the species as endangered. However, she added, “we’ve got definitely a limited amount of budget and a limited amount of staff to address all these species. There are other species that are worse off than whitebark pine.”

The tree is a critical part of the West’s high-elevation habitats: It helps to slow the annual melt of snowpack and provides food for animals such as grizzly bears and Clark’s nutcracker, a bird that can cache thousands of pine seeds in different places and remember later where it put them.

“If you lose those forests, there are so many impacts, not just in wildlife, not just in grizzly bears, but in the whole hydrology of the ecosystem,” said Jesse Logan, who was project leader for the Forest Service’s West-wide bark beetle project before retiring in 2006.

Logan added that he and colleagues began modeling how climate change would threaten Western tree species such as the whitebark pine in the early 1990s, but “no one had any sense that it would be as dramatic and catastrophic as it has been.”

It is unclear when the tree would make it onto the endangered species list, since some plant and animals languish on the candidate list for years. This month, the House Appropriations Interior Subcommittee voted to eliminate any funds for listing species under the Endangered Species Act as part of the 2012 budget.

Andrew Wetzler, director of the Natural Resources Defense Council’s land and wildlife program, said that while “it’s undeniable” the Fish and Wildlife Service has “an enormous backlog of species,” he and others will have to scrutinize background documents on the whitebark pine decision to see whether it should have ranked higher on the agency’s priority list.

“As today’s announcement demonstrates, this is exactly the wrong time for Congress to be debating cutting off all funds for protecting species under the Endangered Species Act,” Wetzler said. “There are few enough funds to protect endangered species as it is. We need to strengthen the safety net for fish, wildlife and plants, not weaken it.”

Logan said he remained optimistic that the agency’s decision would spur action to save the tree. “I’m hopeful this decision will focus effort and resources on this critical ecosystem,” he said. “There’s also resiliency in this ecosystem, so it’s not a hopeless issue.”