Montana’s Rocky Mountain Front has long been characterized by ancient, gnarled limber pines with distinctively broad crowns and wavering limbs, carved over centuries by the harsh conditions unique to their rugged landscape. Described as “windows into the history of the land,” Indigenous people cultivated the pines through fire while buffalo rubbed tree trunks clear of lower branches. Today, limber pines still provide essential shelter for Montana’s iconic species, including grizzlies, elk and mule deer.
But new research suggests the ancient pines appear to be dying as they are pushed to their ecological limits, primarily due to a nonnative fungus known as white pine blister rust.
Known by the scientific name Cronartium ribicola, white pine blister rust has caused extensive crown dieback and mortality since the invasive pathogen was first introduced to limber pine populations east of the Continental Divide in the 1930s.
Limber pines, Pinus flexilis, are five-needle pine trees that thrive in harsh environments from lower to upper treeline elevations. Blister rust, an invasive pathogen from Eurasia, however, is putting the future of some Montana limber pine populations “under extraordinary threat,” according to Andrew Larson, professor of forest ecology at the University of Montana and the director of the Wilderness Institute.
“The pathogen’s primary hosts are five-needle pines, and because white pine blister rust is from Eurasia, our five-needle pines are native [and] don’t have evolved resistance to this pathogen,” said Larson. “They’ve been decimated in many populations.”
Blister rust requires wet, damp spring conditions that facilitate the successful inoculation and completion of the fungus’s life cycle, and Larson notes that limber pines in wetter regions have been “hammered.”
More recently, the combined impacts of rust, mountain pine beetle (Dendroctonus ponderosae), drought and other climate-driven stressors have contributed to shifting forest structure, particularly along the grassland-montane ecotone where limber pine is a key species. While mortality is not solely attributed to blister rust, the pathogen remains a persistent factor shaping long-term stand dynamics in the region.
Blister pine rust causes a lesion or canker under the bark. Once blister rust has progressed far enough into the tree, it will cut off and kill the tissue above the canker. Infected trees have flagging of branches that have turned red and died, or the crowns of the trees have died, while the bottom portions may still have live branches on the tree. Bark becomes thin and translucent, and in some portions of the tree, the fungus produces bright orange fruiting bodies.
White pine blister rust has caused extensive crown dieback and mortality since the invasive pathogen was first introduced to limber pine populations east of the Continental Divide in the 1930s.
Since 2017, the U.S. Forest Service has partnered with The Nature Conservancy and the Montana Department of Natural Resources under the Montana Forest Action Plan. This multiyear collaboration is working to establish a baseline understanding of Montana limber pine populations.
“Our ongoing monitoring seeks to better understand the interactions between rust and these additional factors,” said Christy Cleaver, a Forest Service plant pathologist, “especially how cumulative damage influences mortality rates and regeneration potential. While the pathogen remains widespread, its influence on long-term trends likely varies based on elevation, site conditions and genetic susceptibility within tree populations.”
The study established 74 long-term plots across public and private lands along a 50-mile stretch west of Choteau, Montana, spanning diverse topography and ownerships. The program assesses site conditions, stand structure, regeneration and damage agents affecting tree health, including the incidence and severity of blister rust.
Initial findings indicate that 43 percent of limber pines in the study area are dead, while another 21 percent were in decline or dying. Just 37 percent of limber pines in the population area were classified as healthy. Of dead trees recorded between 2017 and 2018, 40 percent of deaths were attributed to blister rust. By comparison, bark beetles were responsible for less than 1 percent of deaths, while nearly 40 percent were attributed to unknown causes. Blister rust, meanwhile, was observed in all of the plots surveyed.
Dave Hanna, TNC’s Crown of the Continent director, says he’s most interested in limber pines occupying the grassland mountain ecotone, where the Great Plains and mountains come together.
“The growth form and density of those older trees were shaped by Native people managing fire, managing buffalo, and they also give us an insight into the past history,” Hanna said. Some of these trees may be around 500 years old, and in some cases, he notes, it’s these old trees that often house the locations of bird cavity nests, marking the importance of maintaining old legacy trees and creating new ones for the future.
“Humans are intervening in these landscapes no matter what,” Hanna said. “It’s important for us to understand the landscapes in the ways that we can, and think about what’s important for us to try to maintain. Their form reflects the kind of tough life that [the trees] have to live.”
As people continue to influence the ecosystems in and near where they inhabit, he adds, humans must also play a role in ecosystem-wide restoration.
