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Limber Pine


Limber pine is a five-needled pine widely distributed in the mountains and foothills of the Rocky Mountains in the western United States and southern Canada (Fig. 1). It is adapted to dry and windy conditions and can grow in some of the driest sites capable of producing trees (Pfister et al. 1977). Limber pine ranges from upper treeline and midelevation sites to lower treeline where the mountain forests give way to shrub steppes or prairie grasslands (Fig. 2). In most old stands, limber pines are widely spaced dominant trees with a short, bushy form; on moister sites, though, limber pines are moderately tall trees. The large wingless seeds of limber pines are a favored food of many animals. Limber pines are not usually commercially harvested because of their low productivity and poor form, thus, information about the species status is scarce. Recent observations of limber pine mortality have sparked increased interest in trends of limber pine communities (Kendall et al. 1996).

Fig. 1. Distribution of limber pine showing mortality zones and blister rust infection rates (adapted from Little 1971).

Limber pine, like the related whitebark pine, has been damaged extensively in some areas by white pine blister rust (Kendall 1995; Kendall et al. 1996). Blister rust is an exotic fungus for which limber pine has evolved few defenses, so the tree is extremely susceptible to this deadly disease. Limber pine is less affected by fire suppression than whitebark pine; in some areas, limber pine has expanded its range by invading grasslands where it was previously excluded by fire.

Fig. 2. Typical limber pine savannah near the town of Gardiner in southwestern Montana.
Courtesy K. C. Kendall, USGS

Status

 

Limber pine has suffered extensive mortality and blister rust infection in northwest Montana and southern Alberta (Fig. 1). On average, more than a third of the limber pines are dead and 90% of the remaining live trees are infected with rust (Kendall et al. 1996; Fig. 3). The status of limber pine in the northern Canadian Rockies is not known (Smith 1971). To the south, limber pine rust infection is reduced. In southwestern Montana, northwestern Wyoming, and adjoining areas of Idaho, limber pine mortality and incidence of rust are low to moderate, with a few hot spots of heavy infection. Blister rust incidence on limber pine in the Bighorn Mountains of north-central Wyoming has increased dramatically in the past few years (Lundquist 1993). No rust has been found in Craters of the Moon National Monument in southern Idaho (Smith 1995; Kendall et al. 1996). Blister rust has not been reported in limber pine south of Wyoming (Hawksworth 1990; Duriscoe 1995), and little is known of its status in Utah, Nevada, and California.

Fig. 3. Ghost limber pine forest on the Blackfoot Indian Reservation, with the mountains of Glacier National Park in the background.
Courtesy R. Keane, USDA, Intermountain Research Station

Outlook

 

Because limber pine grows in very dry areas, biologists hoped that blister rust, with its higher moisture requirements, would not be able to make significant inroads in limber pine stands. Unfortunately, it is now apparent that for most sites it may be just a matter of time before the necessary climatic conditions combine to produce a large wave of infection, even in drier climates in the southern parts of the limber pine range (Kinloch and Dulitz 1990). Once infected, most trees will die.

   

White pine blister rust was recently discovered in southwestern white pines in southern New Mexico (Hawksworth 1990). The nearest known rust occurrence is 1,000 kilometers to the north on limber pines in southern Wyoming. It is not clear whether this outbreak is a result of infected cultivars brought to the region or a result of long-distance transport of rust spores, but it is likely that few limber pine stands are ultimately safe from rust.

   

Other North American white pines have a small degree of rust resistance that can be strengthened by natural selection or tree-breeding programs. It is likely that the same is true for limber pine, but this potential remains unexplored. Because it is also likely that some individual trees are naturally resistant to blister rust, limber pine probably is not threatened with extinction. Some isolated populations, however, will be lost, and limber pine will be functionally extinct in areas suffering from heavy mortality for the hundreds of years that will be required for rust-resistant types to emerge. Natural selection could be speeded by a breeding program and establishment of stands where more limber pine seedlings are available for natural rust-resistant type selection because all other competing species are removed (Hoff et al. 1994).

   
  Author
Katherine C. Kendall
U.S. Geological Survey
Biological Resources Division
Glacier Field Station Science Center
West Glacier, Montana 59936-0128

References


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