A Clark's Nutcracker perches in a whitebark pine/Frank D. Lospalluto/Flickr. (CC BY-NC-ND 2.0)

Not quite a decade after agreeing whitebark pines were a species in need of Endangered Species Act protection, the U.S. Fish and Wildlife Service now is proposing to list the iconic trees as threatened under the act.

The pines, which are viewed as a "foundation species," grow high in the upper reaches of parks such as Yellowstone, Grand Teton, Glacier, Yosemite, Crater Lake, and Kings Canyon. Roughly 70 percent of the species' range lies within the United States, according to Fish and Wildlife staff, with the remaining 30 percent in Canada, where the trees can be found in all of that country's mountain national parks - Waterton Lakes, Banff, Kootenay, Yoho, Mount Revelstoke, Glacier, and Jasper.

Whitebark pines create the conditions necessary for other plants and animals to get established in harsh alpine ecosystems. These high-elevation trees produce a calorie-rich nut that grizzly bears in the greater Yellowstone ecosystem like to feast on in the fall. It's a nut that also feeds red squirrels and the Clark's nutcracker.

The sheer stature of the tree also helps maintain watersheds. In winter its bulk serves as natural snow fences, and in spring that same bulk helps shield the resulting snowbanks from the sun, thus allowing for a relatively slow and even snow melt.

But the trees, a member of the "stone" pine family, for years have been under pressure from a warming climate that has brought boring beetles and disease to them. Unlike lodgepole pines, which co-evolved with mountain pine beetles and developed defenses against them, whitebark pines did not and so are defenseless against the bugs.

And whitebark pines are slow to reproduce; they can take 75 years of growth before they sprout their first pine cone. Plus, they rely largely on the Clark's nutcracker, which feasts on whitebark seeds, to, basically, plant new whitebark pine stands by caching seeds that later go on to germinate.

Back in July 2011 the Fish and Wildlife Service agreed after a "review of all available scientific and commercial information" that the species merited listing as threatened or endangered. However, the agency added at the time, that listing was "precluded by higher priority actions to amend the Lists of Endangered and Threatened Wildlife and Plants."

Having now proposed to list whitebark pines as threatened, the agency is taking public comment on the matter until February 1.

"We have determined that the primary stressor driving the status of the whitebark pine is white pine blister rust, a fungal disease caused by the nonnative pathogen Cronartium ribicola," the agency said in a Federal Register announcement about the proposed listing. "Whitebark pine is also impacted by the mountain pine beetle (Dendroctonus ponderosae), altered fire regimes, and the effects of climate change."

Forest ecologists long have viewed the pine beetle as a natural agent for thinning forests. But climate change has enabled this beetle to make longer and longer forays into the upper elevations of the ecosystem, inroads that are seriously jeopardizing whitebark pines, whose disappearance could have dire ramifications for the overall health of the Greater Yellowstone Ecosystem in general and the grizzly bear specifically.

The warming climate has allowed pine beetles to thrive. Forest entomologists say it typically takes a ten-day cold spell of 40-below-zero weather to kill these beetles.

Pine beetles kill their host trees by boring through a tree's outer bark and then setting up shop in the spongy phloem layer, where they lay their eggs and eat the phloem; in the process, the beetles cut off the flow of water and nutrients between the tree's roots and its needles.

Blister rust is an invasive disease thought to have arrived in North America around 1900 from France with imported eastern white pine seedlings. The fungus kills by girdling branches on infected trees.

Concerns over the fate of whitebark pines are not new. Back in February 1991 the Fish and Wildlife Service was petitioned by the Great Bear Foundation of Missoula, Montana, to list the whitebark pine under the act. The foundation cited the same underlying reasons that the agency now is saying pose threats to the species: impacts from mountain pine beetles, white pine blister rust, and fire suppression.

There also are concerns that the loss of whitebark pines will lead to the loss of Clark's nutcrackers.

At Crater Lake National Park in Oregon, in 2010 park staff were fighting both beetles and blister rust that had infected some stands of whitebark pine in that park. Among their tools were packets of synthetic hormones to drive pine beetles away from healthy trees.

In 2011, the Greater Yellowstone Coordinating Committee released a strategy for how to conserve whitebark pine in the face of both blister rust and pine beetles. The plan, five years in the making, called for a variety of measures to be implemented, from inventorying whitebark stands to assess their condition from these two threats to identifying trees resistant to blister rust and which could be used as a seed source for growing resistant stands of whitebark.

At the time, data obtained by the committee showed that more than 50 percent of whitebark pine stands in the ecosystem "have already suffered high to complete mortality of overstory trees (from pine beetles) and 95 percent of forest stands containing whitebark pine have measurable mountain pine beetle activity."

Parks Canada researchers have been working since at least 2016 to raise a veritable disease-resilient forest of whitebark pines. The agency has protected seed-producing whitebark pine trees that show resistance to the introduced blister rust. The agency then enhances natural regeneration by planting stock grown from these trees’ seeds. Some seedlings are also inoculated with spores of a native fungus called ‘Siberian slippery jack,’ which has a symbiotic relationship with pines and helps the young trees acquire nutrients. When applied to the roots of seedlings prior to planting, the survival and health of those seedlings is greatly improved.

Prescribed fire in Canada also is used to restore and improve whitebark pine habitat in sub-alpine regions. These fires replicate natural conditions under which whitebark pine previously evolved and thrived. The process removes competing vegetation and creates nutrient-rich habitat suitable for planting putatively blister rust-resistant whitebark pine seedlings.

Glacier National Park in Montana has been helping Parks Canada by growing whitebark pine seedlings for transplant.