Pinus albicaulis

Pinus albicaulis occurs in upper subalpine forests of many western North American mountain ranges. It is, however, severely threatened in the majority of its range by introduced White Pine Blister Rust (Cronartium ribicola), outbreaks of Mountain Pine Beetle (Dendroctonus ponderosae), succession resulting from decades of fire suppression, climate change resulting in decreases in suitable habitat, and various synergies between these factors. Although a few areas such as the southern Sierra Nevada in California and the interior Great Basin ranges, as well as scattered stands in the rest of the range, still appear to contain large numbers of relatively healthy trees, it is expected that the blister rust will eventually become abundant in the vast majority of the range, causing significant tree mortality. Tree mortality rates exceeding 50% have already been documented in numerous parts of the range. A small percentage (1-5%) of trees appear naturally resistant to the blister rust, and restoration strategies hope to propagate these genotypes for use in restoration, although even rust-resistant trees will remain threatened by other factors. In addition, it has relatively low genetic variation and exists as a fragmentary species, making it more vulnerable than its range might indicate. This is a keystone species of high-elevation western ecosystems whose decline is expected to have cascading effects on ecosystem function and biodiversity.

Whitebark Pine occurs in subalpine and timberline zones from west-central British Columbia east to west-central Alberta and south to central Idaho, southwestern Wyoming, and southern California (Murray, 2005; Ward et al., 2006). Its distribution is split into two broad sections, one following the Coast and Cascade ranges and the Sierra Nevada, and the other following the northern Rocky Mountains. Scattered populations occur between the two sections in Great Basin regions of eastern Washington and Oregon and northern Nevada (Fryer 2002, Burns and Honkala 1990). Although historical sources include Utah in the distribution, more recent workers have not found it to occur there (FNA 1993). The easternmost extent of the species is in the Wind River Range in Wyoming (USFWS 2016). COSEWIC (2010) evaluated current estimates of the extent of occurrence of Pinus albicaulis and found the best estimates to be 190,067 sq km in Canada and 147,000 sq km in the U.S. for a total of 337,067 sq. km. (COSEWIC 2010).

Information to delineate and rank individual occurrences throughout the range of Whitebark Pine is lacking. However, based on its range and presence in isolated stands, the number of populations rangewide exceeds 300. The number of extant occurrences in British Columbia is 47 and in Alberta is 730 (NatureServe Network Database as of November 2017). There are 3,517 observations in Montana (Montana Natural Heritage Program 2018). Occurrences are not tracked in Washington, Oregon, Idaho, California, Nevada, and Wyoming as of 2017. Goeking and Izlar (2018) estimated the total area with Whitebark Pine in the western United States covers more than 4 million ha, with the vast majority of this area in Montana, Idaho, and Wyoming. Notably, only 15% of the area with Whitebark Pine was classified as Whitebark Pine forest type; the Lodgepole Pine, Engelmann Spruce/Subalpine Fir, and Subalpine Fir forest types each contain more area than does the Whitebark Pine forest type (Goeking and Izlar 2018).

This species is seriously threatened by White Pine Blister Rust (Cronartium ribicola), increases in Mountain Pine Beetle (Dendroctonus ponderosae), fire suppression, climate change, and synergistic effects of these factors, there are minor impacts from road-building, development projects, and logging. White Pine Blister Rust is believed to be the greatest threat throughout the range (USFWS 2016, COSEWIC 2010, Ward et al. 2006, Murray 2005, Fryer 2002, Murray and Rasmussen 2000).

WHITE PINE BLISTER RUST: White Pine Blister Rust, a fungal disease caused by the pathogen Cronartium ribicola, was inadvertently introduced to Vancouver, British Columbia in 1910. In most parts of Whitebark Pine's range today, the majority of surveyed stands are declining in condition as a result of blister rust infection. For example, blister rust infection was found in 96% (164 of 170) of surveyed stands in Washington and Oregon (Ward et al. 2006), 83% in Bob Marshall Wilderness Complex in Montana (Keane et al., 1994), "the vast majority" of stands in Alberta, and "all of the regions sampled" during the most recent British Columbia survey (Alberta Sustainable Resource Development and Alberta Conservation Association 2007). Throughout the species' range there are few stands that show no infection (Alberta Sustainable Resource Development and Alberta Conservation Association 2007).
In general, moist, humid conditions are believed to promote the spread of White Pine Blister Rust and dry conditions to slow it, therefore, increased global warming trends should favor the spread of the disease (see below). Stands that combine high elevation, dry conditions, and a fire regime of non-lethal underburns at long intervals are believed to be some of the healthiest remaining, avoiding the worst impacts of both blister rust and fire suppression; such stands occur mostly in the southern parts of the range in the Rocky Mountains (less than 10% of range) (Keane 1999). Nevertheless, environmental conditions at the extremes of Whitebark Pine's distribution - including cool temperatures, shorter growing seasons, and greater aridity - that were initially thought to provide some refuge from blister rust infection (e.g. USFWS 1994) are now known to only slow its spread. The epidemic is still spreading into and increasing within environments previously considered inhospitable, and the vast majority of the natural range is now believed to harbor the pathogen (Alberta Sustainable Resource Development and Alberta Conservation Association 2007, Wars et al. 2006).
The percentage of individual trees infected appears to vary widely throughout the range, with some areas having very high infection (> 90% per Alberta Sustainable Resource Development and Alberta Conservation Association 2007) and a few areas believed to have low impacts as yet. The highest infection levels (50-100%) are believed to occur in the northwestern U. S. and southwestern Canada in the northern Rockies and Cascades (Whitebark Pine Ecosystem Foundation 2006, Tomback 2002); Tomback (2002) also notes high infection in the intermountain ranges. Ward et al. (2006) state that "there is a high degree of localized variation in the prevalence of blister rust infection...hot spots of higher damage can occur...even in areas of moderate infection." In Oregon and Washington, the average percentage of infected living trees per stand (for stands that had infected trees) ranged from 11% to 95% (Ward et al. 2006). At two locations east of the Continental Divide in the northern Rocky Mountains, Montana, 35% of sampled trees were infected (Resler and Tomback 2008). At four biogeographically variable sites in the Greater Yellowstone Ecosystem, 85% sampled trees were infected (Tinker and Bockino 2007). In Alberta, approximately 60% of sampled trees were infected in the northern region, 16% were infected in the central region, and 73% were infected in the southern region (Alberta Sustainable Resource Development and Alberta Conservation Association 2007). In British Columbia, in 483 stands distributed over the major mountain ranges, forest district levels of infection ranged from 18% to 53% (average 34%) (Zeglen 2002 cited in Alberta Sustainable Resource Development and Alberta Conservation Association 2007). In contrast, in California, Whitebark Pine is not believed to be in serious decline in the Sierra or Warner Moutains; T. Keeler-Wolf (pers. comm. 2008) states that "despite the White Pine Blister Rust problem and others [elsewhere in the range], there is still tons of [Whitebark Pine] in the Sierra and although some has died from disease it is still the most abundant subalpine conifer." A 2002 survey in Sequoia and Kings Canyon National Parks, in the southern Sierra Nevada, documented cankers on few to no trees (Duriscoe and Duriscoe 2002 cited in Whitebark Pine Ecosystem Foundation 2006). The interior Great Basin ranges also appear to be minimally impacted by blister rust at this time (Whitebark Pine Ecosystem Foundation 2006).
Within each stand, the percentage of dead trees (mortality) from blister rust alone, and from all causes combined, tends to be significantly less than the percentage of infected living trees. Nevertheless, some areas have already experienced substantial (>50%) mortality (Alberta Sustainable Resource Development and Alberta Conservation Association 2007). Furthermore, most mature trees infected with blister rust suffer loss of reproductive potential well before mortality occurs; thus many infected trees are no longer contributing to the maintenance of the population even though they remain alive (Alberta Sustainable Resource Development and Alberta Conservation Association 2007). In Oregon and Washington, the average mortality per stand from all causes ranged from 2% to 41% (Ward et al. 2006). In a study that measured 17 permanent plots in western Montana at two intervals separated by 20 years, there was an average mortality rate of 42% over the 20 year period (Keane and Arno 1993 cited in Alberta Sustainable Resource Development and Alberta Conservation Association 2007). At four biogeographically variable sites in the Greater Yellowstone Ecosystem, 52% of the Whitebark Pine sampled were dead from multiple causes (Tinker and Bockino 2007). In southern Alberta, the average mortality in a recent survey was 61%; at eight permanent plots in this region, the mortality rate increased from 26% to 61% between 1996 and 2003. However, mortality is lower in central and northern Alberta (Alberta Sustainable Resource Development and Alberta Conservation Association 2007). In British Columbia, in 483 stands distributed over the major mountain ranges, the range of mortality caused by blister rust was estimated to be between 4% and 22% (average 10%) and the range of mortality from all causes was estimated to be between 6% and 31% (average 19%) (Zeglen 2002 cited in Alberta Sustainable Resource Development and Alberta Conservation Association 2007).
Although projections of the future should technically not be considered in evaluating short term trend, it is worth mentioning that several studies have found these to be grim. In Mt. Rainier National Park, Washington, without any management intervention, 150-175 year simulations predict a 65-94% chance of Whitebark Pine extinction in the Park (Ettl and Cottone 2004 cited in Alberta Sustainable Resource Development and Alberta Conservation Association 2007, Cottone 2001 cited in Ward et al. 2006). In Crater Lake National Park, an overall decline of 0.4 percent per year for mature trees is predicted, which would lead to a 20 percent reduction in the Park within 50 years (Murray and Rasmussen 2000, 2003 cited in Ward et al. 2006). Murray (2005) predicts 95-99% mortality in Oregon populations due to blister rust.
However, there is some hope for the persistence and recovery of this species since naturally resistant trees have been found at many locations. Natural resistance to White Pine Blister Rust infection is believed to exist in approximately 1-5% of the total Whitebark Pine population (Keane 1999), although resistant trees are still susceptible to other causes of mortality such as mountain pine beetle attack. Some researchers familiar with the species do expect it to persist, although noting that the structure of stands and the landscape pattern of their distribution may be different than the historical condition.

MOUNTAIN PINE BEETLE (Dendroctonus ponderosae): Mountain Pine Beetle is native to western North America and appears to have periods of higher and lower population density over time. For example, between 1909 and 1940 and again from the 1970s to the 1980s, outbreaks of Mountain Pine Beetle killed Whitebark Pine throughout the U.S. Rocky Mountains (Whitebark Pine Ecosystem Foundation 2006). Drought and warmer temperatures in recent years have allowed unprecedented increases in beetle abundance and distribution. The first decade of the 20th century has seen further outbreaks within much of the U.S. range as well as attacks in British Columbia and Alberta of unprecedented scope and severity.
Studies in various parts of Whitebark Pine's range suggest the severity of impacts. 2006 aerial surveys indicated large-scale outbreaks of beetles in Whitebark Pine in northern Idaho, west-central and southwestern Montana, and the Greater Yellowstone Ecosystem (Gibson 2006 cited in Whitebark Pine Ecosystem Foundation 2006). In the Greater Yellowstone Ecosystem, the current Mountain Pine Beetle outbreak is unprecedented in scope and severity: more than 700,000 Whitebark Pines were killed by beetles in 2004 (Whitebark Pine Ecosystem Foundation 2006), and at four biogeographically variable sample sites, 70% of Whitebark Pine were attacked by the beetle (Tinker and Bockino 2007). Observations in 2005 suggested that Mountain Pine Beetle occurrence in Whitebark Pine is increasing in Oregon and Washington locations as well, such as Okanogan and Wenatchee National Forests and Crater Lake National Park (Ward et al. 2006). While Whitebark Pine losses due to Mountain Pine Beetle in British Columbia and Alberta were relatively minor prior to the 1980s, warming climates have led to an expansion of beetle outbreaks into higher elevation forest containing Whitebark Pine (Campbell and Antos 2000, Campbell and Carroll 2007 cited in BC CDC 2008). In the 1980s outbreak, the beetle is believed to have affected a large decrease (30-40%) in mature Whitebark Pine canopy cover in southern Alberta. In British Columbia, 2007 aerial surveys indicated widespread beetle infestations and tree death, with about 7% of BC forests containing Whitebark Pine infested (Campbell and Carroll 2007 cited in BC CDC 2008) and impacts expanding into Alberta. This epidemic is projected to continue over the next few years (BC CDC 2008). A small percentage of Whitebark Pine trees (3-5%) appear able to resist Mountain Pine Beetle attack (BC CDC 2008).
As of 2016, Mountain Pine Beetle predation is less than in recent years, however, since epidemics are cyclical and climatic conditions increasingly favorable to the beetle, it is anticipated that future outbreaks will be more frequent and severe (USFWS 2016).

FIRE SUPPRESSION AND SUCCESSIONAL REPLACEMENT: Prior to about 1930, the replacement of Whitebark Pine by later successional species such as spruce and fir was usually interrupted by naturally occurring fires. However, decades of fire suppression have allowed spruce and fir to become dominant in many forests that were historically dominated by Whitebark Pine. This threat appears to be particularly significant in the northern Rocky Mountains of the United States and the intermountain region (Whitebark Pine Ecosystem Foundation 2006), and in moister areas at lower elevations (Ward et al. 2006). Whitebark Pine survives low severity fires better than its competitors because it has thicker bark, thinner crowns, and deeper roots. It is also well-adapted to recolonizing burned areas, as its seed disperser, Clark's Nutcracker, appears to prefer open sites for seed caching (Keane 1999). Prescribed burn to control blister rust often results in reinfected trees returning in greater numbers that regenerate more slowly than non-infected trees (Tomback et al. 1995).

CLIMATE CHANGE: Major reductions in habitat suitable for this subalpine species are expected as the climate warms (BC CDC 2008). Modeling mostly predicts a decline in Whitebark Pine due to global increases in temperature and more frequent summer droughts (Mattson et al. 2001, McCaughey and Tomback 2001). Climate modeling for Yellowstone National Park predicts that independent of other agents of decline such as blister rust, Whitebark Pine is the most at-risk conifer in the Park due to drying conditions in high-elevation habitats (Bartlein et al. 1997). However, impact of climate change on Whitebark Pine is inconclusive: Keane et al. (1996) and others predict expansion of Whitebark Pine in Glacier National Park due to more frequent fire return intervals resulting from global warming. Increased global temperature makes the species more vulnerable to fungus such as blister rust (Murray 2005).

SYNERGIES: Numerous studies have found that Whitebark Pine trees stressed by blister rust are more susceptible to attack by mountain pine beetle. Mortality from the combination of blister rust and Mountain Pine Beetle has apparently exceeded 50% in areas including Glacier National Park, northwestern Montana, north-central Idaho, and northern Washington, and mortality is increasing rapidly in the Cascades and Sierra Nevada Range (Whitebark Pine Ecosystem Foundation 2006). Furthermore, the significant threat from both White Pine Blister Rust and Mountain Pine Beetle threatens the success of restoration strategies based on cultivating tree resistant to either threat alone (Whitebark Pine Ecosystem Foundation 2006). In addition, the tendency of both of these agents to kill mature, reproductive trees accelerates successional replacement processes resulting from fire suppression; and fire suppression itself is believed to have an inhibitory effect on Whitebark Pine's recovery from major beetle outbreaks (Keane 1999). Finally, climate change interacts significantly with the Mountain Pine Beetle threat, as warmer temperatures increase he proportion of Whitebark Pine's range vulnerable to beetle attack (BC CDC 2008). Warmer temperatures also appear to permit the beetle to complete its life cycle more quickly (i.e. in one year) and to make summer dispersal flights more dependably (Alberta Sustainable Resource Development and Alberta Conservation Association 2007). Logging has been noted as a threat in British Columbia; while it is not a significant threat on its own, since it occurs in healthy stands it reduces the number of intact stands as yet minimally affected by other threats, which may be important for future survival (E. Campbell, pers. comm., 2007 cited in BC CDC 2008).