Whitebark Pine

Whitebark pine is a picturesque tree of the subalpine forest and treeline of the Rocky Mountains, Coast and Cascade ranges, and the Sierra Nevada (Fig. 1). Slow growing and long-lived, it is typically more than 100 years old before it produces cones. Whitebark pine's growth form ranges from a krummholz mat to a moderately tall, upright tree, but it is often short and heavily branched, with multiple stems.

Fig. 1. a) Natural distribution of whitebark pine with amount of mortality from all causes since presettlement; b) white pine blister rust infection rates in whitebark pine. In northern Canada and the southern United States, blister rust is present but infection rates are unknown (revised from original map in Kendall 1995).

Whitebark pine typically grows with other high mountain conifers but can form nearly pure stands in relatively dry mountain ranges (Arno and Hoff 1989). Where associated trees are capable of forming closed stands, whitebark pine can be a long-lived dominant seral species if periodic disturbance, such as fire, removes its shade-tolerant competitors. On a broad range of dry, windy sites, however, whitebark pine is a climax tree because it is hardier and more durable than subalpine fir and other tree species (Arno and Hoff 1989). The sites where whitebark pine is seral tend to be moister and more productive than sites where the tree is climax (Arno 1986).


Until recently, little was known of whitebark pine status because it occurs in rugged terrain and has limited use as a commercial timber species. In the last decade, however, its role as a keystone species has been recognized. Whitebark pine seeds are a preferred food of the threatened grizzly bear and many other mammals and birds (Fig. 2).

Fig. 2. Whitebark pine cones and large, wingless seeds.
Courtesy K. C. Kendall, USGS

Because whitebark pine can grow in cold, dry, and windy conditions tolerated by no other tree, and because it pioneers disturbed sites, it plays an important role in tree establishment in high-elevation open sites. Whitebark pine helps stabilize snow, soil, and rocks on steep terrain and has potential for use in high-elevation land reclamation projects (Arno and Hoff 1989). Because of their spreading crowns and penchant for establishing on windswept ridges (Fig. 3), whitebark pines accumulate and retain snow, extending the snowmelt period into the growing season, when water is needed. With the growing appreciation of these values has come more interest and mounting concern over dramatic declines in whitebark pine stands (Arno 1986; Kendall and Arno 1990; Keane and Arno 1993; Lanner 1993).

Fig. 3. Whitebark pine occupying a windswept ridge in Glacier National Park, Montana.
Courtesy B. R. McClelland, National Park Service (retired)

Causes of Decline


Sixty years of fire suppression have advanced forest succession at the expense of seral whitebark pine communities. The transport and caching of whitebark pine seed by Clark's nutcrackers and the hardiness of seedlings on exposed microsites give a competitive edge to whitebark pine over less hardy wind-dispersed conifers, such as spruce and fir, in reforesting large burns. However, without fire, whitebark pines are shaded out by other trees, and there are few open sites for whitebark pine regeneration. Before fire exclusion, the average whitebark pine stand burned every 50 to 300 years. Even with the prescribed natural fires that have been allowed to burn in wilderness and national parks in the past 25 years, fewer than 1% of seral whitebark pine stands have burned during that period--an average fire return interval of more than 3,000 years (Arno 1986; Keane 1995a). The Selway-Bitterroot Wilderness in Montana has one of the most extensive prescribed fire programs in the United States, yet between 1979 and 1990, burning in the whitebark pine zone was less than half the area burned each year in presettlement times (Brown et al. 1994; Arno 1995).


An exotic fungus, white pine blister rust, has killed many whitebark pine trees in the moister parts of its range. White pine blister rust, which was introduced from Europe to western North America around 1910, has spread to most whitebark pine forests. Although white pine blister rust can damage all North American white pine species, whitebark pine is the most vulnerable (Fig. 4); fewer than 1 in 10,000 trees is resistant to blister rust. Because whitebark pine cones form in the top third of the tree and blister rust tends to kill trees from the top down, a tree's ability to produce seed is eliminated by the rust long before the tree dies (Fig. 5).

Fig. 4. Heavy mortality from blister rust in a whitebark pine stand in Glacier National Park, Montana. Courtesy S. Gniadek, National Park Service

Declines from Historical Levels


Natural whitebark pine abundance before the recent decline has been summarized by Arno and Hoff (1989). Near the northern end of its range in the British Columbia coastal mountains, whitebark pine is a minor component of treeline communities. In the Olympic Mountains and on the west slope of the Cascades, it grows primarily on exposed sites near treeline. East of the Cascade crest, it is abundant within both the subalpine forest and treeline zone. Whitebark pine is a major component of high-elevation forests in the Cascades of southern Oregon and northern California. Near the northern end of their distribution in the Rockies of Alberta and British Columbia, whitebark pines are generally small, scattered, and confined to dry, exposed sites at treeline. Whitebark pine becomes increasingly abundant southward, especially in Montana and central Idaho. It is a major component of high-elevation forests and the treeline zone in western Montana. In western Wyoming, it is abundant between elevations of 2,440 meters to 3,200 meters. Before the advent of blister rust and fire control, whitebark pine was an important component on about 10%-15% of the forested landscape in the Rocky Mountains of Montana, Idaho, and northwestern Wyoming (Arno 1986). On about 1.2 million hectares of this area, whitebark pine communities are seral.

Fig. 5. An ancient whitebark pine in the Mission Mountain Wilderness, Montana, dying from the top down from the introduced fungus, white pine blister rust. Courtesy K. C. Kendall, USGS

Although there is not comprehensive information on whitebark pine throughout its range, recent studies have begun to piece together the current status of this species. An assessment of the interior Columbia River basin found that the area of whitebark pine cover types has declined 45% since the turn of the century (Keane 1995b). Most of this loss occurred in the more productive, seral whitebark pine communities; 98% of them have been lost. Practically all the remaining whitebark pine stands are old. In southwestern Montana, a project to reconstruct landscape patterns found that 14% of the sampled stands were dominated by whitebark pine around 1900, but none of them were by the early 1990's (Arno et al. 1993). Moreover, the extent of stands with significant cone-bearing whitebark pine trees had declined by half.


Nearly half of the whitebark pine trees in Glacier National Park and the Bob Marshall Wilderness Complex in northwestern Montana are dead (Fig. 6). Of the remaining live trees, more than 80% are infected with rust and more than a third of their cone-bearing crowns are dead (Keane et al. 1994; Kendall et al. 1996a). Much of this mortality has been recent; few whitebark pines had suffered significant damage from rust in the early 1970's (Keane and Arno 1993). Blister rust is now present throughout the range of whitebark pine in the Canadian Rockies, with the highest rust infection rates and mortality within 125 kilometers of the United States border (Smith 1971; R. Hunt, Forestry Canada, unpublished data).

Fig. 6. A ghost forest of whitebark pine in Glacier National Park, Montana.
Courtesy B. R. McClelland, National Park Service (retired)

In southern Montana and Wyoming, whitebark pine health improves as the climate becomes drier (Fig. 7). In the Gallatin National Forest and Yellowstone and Grand Teton national parks, an average of 7% of the whitebark pines are dead and 5% of the live trees are infected with rust (Kendall et al. 1996a,b). The highest infection rates (up to 44%) are found in the Teton Range, where conditions are moister than in neighboring areas to the north. Whitebark pine is reported to be functionally extinct on the Mallard Larkins Pioneer area in the Idaho Panhandle National Forest (Zack 1995). Rust infection rates in the Sawtooth National Recreation Area in central Idaho are generally light, but low elevations may harbor some heavily infected sites (Smith 1995).

Fig. 7. A healthy whitebark pine in Yellowstone National Park, Wyoming.
Courtesy D. Reinhart, USGS

There is less information about the status of whitebark pine west of Idaho. As a rule, blister rust is present and whitebark pine infection levels and mortality are high in the Cascade and Coast ranges. For a time, the dry conditions in the Sierra Nevada were believed to protect most white pine stands there, but in 1976 and 1983, unusually favorable weather produced heavy waves of rust infection in California white pines (Kinloch and Dulitz 1990). Although sugar pine has been the most affected and studied of these, rust is also present at low levels in some whitebark pine stands. In Kings Canyon and Sequoia national parks, fewer than 1% of the whitebark pine sampled in 1995 was infected with rust (Duriscoe 1995).


In Washington state, northern Idaho, northwest Montana, and southern Alberta and British Columbia, 40%-100% of the whitebark pine is dead in most stands, and 50%-100% of the live trees are infected with rust (Fig. 1) and have lost most of their capacity to produce cones (Kendall and Arno 1990; Kendall 1994a,b; Kendall 1995). Mortality and rust infection levels decline in the drier areas to the south.


Future Trends


Successional replacement due to fire exclusion is a major cause of whitebark pine decline (Keane et al. 1994). Whitebark pine cannot maintain its functional role in mountain ecosystems unless areas suitable for its regeneration are available across the landscape. Modern fires are restricted in whitebark pine habitats because they normally burn only at the height of very active fire seasons and, under those conditions, managers choose to suppress new fires (Arno 1995). Options for providing sites for whitebark pine regeneration include allowing wildfires to burn near historical levels, having more management-ignited burns with slash cut to help carry the fire in moderate fire weather, and selectively removing whitebark pine's competitors.


It is clear that the blister rust epidemic in whitebark pine has not yet stabilized, even in regions with the longest history and highest infection levels of rust. The most likely prognosis for whitebark pine in sites already heavily infected with rust is that they will continue to die until most trees are gone. In the southern Rockies and Sierra Nevada where there is currently little or no infection of whitebark pine, waves of infection are expected to occur within a few decades (Kinloch and Dulitz 1990; Kendall et al. 1996a). Eventually, whitebark pine in these areas is likely to suffer heavy losses.


Whitebark pine possesses some ability to defend itself from white pine blister rust (Arno and Hoff 1989), and there is evidence that natural selection has already started to enhance that ability. Forty percent more seedlings from stands with high blister rust mortality survived artificial inoculation with rust than seedlings from low mortality stands (Hoff 1994). In the future, whitebark pine trees will be all but absent in most areas, and small, isolated populations will be lost until rust-resistant types evolve. Without intervention, this is expected to require hundreds--if not thousands--of years, because whitebark pine matures slowly and most of the population soon will be lost (Fig. 8). Management strategies such as breeding whitebark pine for rust resistance and establishing natural selection stands will speed this evolution (Hoff et al. 1994).

Fig. 8. Winter comes to a whitebark pine stand in Yellowstone National Park, Wyoming. Courtesy K. Kendall, USGS
Katherine C. Kendall
U.S. Geological Survey
Biological Resources Division
Glacier Field Station Science Center
West Glacier, Montana 59936-0128