Pinus albicaulis

The whitebark pine has been classified as endangered by the IUCN.[1] Severe population decline in whitebark pine communities is attributed to various causes, most significantly infection with white pine blister rust, recent outbreaks of mountain pine beetles (2000–2014), disturbances in wildland fire ecology (including fire suppression), forest succession, and climate change. A study in the mid-2000s showed that whitebark pine had declined by 41 percent in the western Cascades due to two primary threats: blister rust and pine beetles.[7] Whitebark deaths in North Cascades National Park doubled from 2006 to 2011.[7]

Many stands of Pinus albicaulis across the species' entire natural range are infected with white pine blister rust (Cronartium ribicola), a fungal disease introduced from Europe. In the northern Rocky Mountains of the United States, whitebark pine mortality in some areas exceeds 90 percent, where the disease infests nearly 143,000 acres (580 km²). Cronartium ribicola occurs in whitebark pine to the northern limits of the species in the coastal ranges of British Columbia and the Canadian Rocky Mountains. The blister rust has also devastated the commercially valuable western white pine in these areas and made serious inroads in limber pine (Pinus flexilis) populations as well. Nearly 80 percent of whitebark pines in Mount Rainier National Park are infected with blister rust.[7]

There is currently no effective method for controlling the spread and effects of blister rust. However, a small number of trees (fewer than 5%) in most populations harbor genetic resistance to blister rust.[4] Restoration efforts undertaken by the U.S. Forest Service, Bureau of Land Management, and National Park Service in the northern Rocky Mountains involve harvesting cones from potentially and known resistant whitebark pines, growing seedlings, and outplanting seedlings in suitable sites. In California, where the blister rust is far less severe, whitebark pine is still fairly common in the High Sierras.[8]

Unusually large outbreaks of mountain pine beetle (Dendroctonus ponderosae), a species of bark beetle native to western North America, have also contributed significantly to the widespread destruction of whitebark pine stands.[9] The beetles both lay their eggs and introduce pathogenic fungi into their host trees, which include many other species of pine, and the combination of larval feeding and fungal colonization is typically sufficient to kill old or unhealthy trees. However, the beetles have recently expanded their attacks to younger, healthier trees as well as older trees, and climate change has been implicated as the primary culprit. Since 2000, the climate at high elevations has warmed enough for the beetles to reproduce within whitebark pine, often completing their life cycle within one year and enabling their populations to grow exponentially. Entire forest vistas, like that at Avalanche Ridge near Yellowstone National Park’s east gate, have become expanses of dead gray whitebarks.[10] Scientists have attributed the recent warming trend to anthropogenic global warming.[7][11]

In 2007, the U.S. Fish and Wildlife Service estimated that beetles had killed whitebark pines across 500,000 acres (200,000 ha) in the West, while in 2009, beetles were estimated to have killed trees on 800,000 acres (320,000 ha), the most since record-keeping began.[7] The pine beetle upsurge has killed nearly 750,000 whitebark pines in the Greater Yellowstone Ecosystem alone.

Fire suppression has led to slow population declines over the last century by altering the health and composition dynamics of stands without the fire ecology balancing their habitat and suppressing insect-disease threats.[12] In the absence of low-level wildfire cycles, whitebark pines in these stands are replaced by more shade-tolerant, fire-intolerant species such as subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii). In addition, senescent and blister rust-infected pine trees are not destroyed by natural periodic ground fires, further diminishing the whitebark pine forest's vitality and survival.[13]

In 2012 the Canadian federal government declared whitebark pine endangered in accordance with the Species at Risk Act. Accordingly, it became the first federally listed endangered tree in western Canada.[14] In 2022 the U.S. Fish & Wildlife Service also acted. It listed whitebark pine in the lowest category of vulnerability: "threatened." Four distinct threats were described, beginning with white pine blister rust as "the primary stressor." Mountain pine beetle, altered fire regimes, and "the effects of climate change" add to the challenges.[15] This listing marks the first occasion in which a tree regarded as ecologically important over a vast range in the United States is acknowledged as vulnerable to extinction.[16]

In response to the ongoing decline of the tree throughout its range, the Whitebark Pine Ecosystem Foundation was formed. Their mission is to raise awareness and promote conservation by sponsoring restoration projects, publishing a newsletter called "Nutcracker Notes", and hosting an annual science and management workshop for anyone interested in whitebark pine.[17] This U.S. group collaborates closely with the Whitebark Pine Ecosystem Foundation of Canada.[18]