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The Whitebark Pine (Pinus albicaulis; family Pinaceae) is a species of pine tree that occurs in the mountains of the Western United States and Canada, specifically the subalpine areas of the Sierra Nevada, the Cascade Range, the Pacific Coast RangesThe Pacific Coast Ranges are the series of mountain ranges that stretch along west coast of North America from Alaska to Mexico. The character of the ranges varies considerably, from the record-setting tidewater glaciers in the ranges of Alaska, to the lo, and the Rocky MountainsThe Rocky Mountains often called the Rockies, are a broad mountain range in western North America. The Rocky Mountains stretch more than 3000 miles (4800 km) from Mexico, through the continental United States, into Canada and Alaska. The highest peak is M (including the Greater Yellowstone EcosystemGreater Yellowstone is the last large, nearly intact ecosystem in the northern temperate zone of the Earth and is partly located in Yellowstone National Park. Conflict over management has been controversial, and the area is a flagship site among conservat). The Whitebark Pine is typically the highest-elevation pine tree of these mountains, marking the tree-lineTree-line or timberline is the edge of the habitat at which trees are capable of growing. Beyond the tree-line, they are unable to grow due to inappropriate environmental conditions. There are several types: Arctic tree-line The furthest north in the Nort. Thus, they are often found as krummholzTree-line or timberline is the edge of the habitat at which trees are capable of growing. Beyond the tree-line, they are unable to grow due to inappropriate environmental conditions. There are several types: Arctic tree-line The furthest north in the Nort, trees dwarfed by exposure and growing close to the ground. In more favourable conditions, it makes a tree to 20 m, rarely 27 m tall.

Whitebark Pine is a member of the white pine group, Pinus subgenus Strobus , and like all members of that group, the leaves ('needles') are in fascicles (bundles) of five, with a deciduous sheath. This distinguishes it from the Lodgepole PineLodgepole Pine Pinus contorta is a common tree in western North America. Like all pines, it is evergreen. There are three subspecies, one of them with two varieties: Pinus contorta subsp. contorta Shore Pine Pacific Coast, southern Alaska to California Pi, with two needles per fascicle, and Ponderosa Pine and Jeffrey Pine, which both have three per fascicle; these three all also have a persistent sheath at the base of each fascicle.

Distinguishing Whitebark Pine from the related Limber Pine, also a white pine, is very much more difficult, and can only easily be done by the cones. In Whitebark Pine, the cones are 4-7 cm long, dark purple when immature, and do not open on drying, but are fragile and are pulled apart by birds (see below) to release the seeds. In Limber Pine, the cones are 6-12 cm long, green when immature, and open to release the seeds; the scales are not fragile. A useful clue resulting is that Whitebark Pines almost never have intact old cones lying under them, whereas Limber Pines usually do.

Whitebark Pine can also be hard to tell from Western White Pine in the absence of cones. The most useful clue here is that Whitebark Pine needles are entire (smooth when rubbed gently in either direction), whereas Western White Pine needles are finely serrated (feeling rough when rubbed gently from tip to base). Whitebark Pine needles are also usually shorter, 4-7 cm long, to Western White Pine's 5-10 cm (though note the overlap).

The Whitebark Pine is an important source of food for several species, including most importantly the Clark's Nutcracker, the major seed disperser of the pine. Clark's Nutcrackers cache huge numbers of Whitebark Pine seeds (up to 100,000 seeds per bird each year) in small, widely scattered caches usually on bare ground: ideal situations for regeneration of the pine, for the many caches which never get used. One consequence of this is that Whitebark Pines often grow as clumps of several trees, originating from a single cache of 5-10 seeds; this assists tree survival at tree-line, with the cluster of trees providing shelter for each other where a single tree might not survive.

Unfortunately, Whitebark Pines in Washington, Idaho, Montana, British Columbia, and Alberta are afflicted with White Pine Blister Rust (Cronartium ribicola), a fungus that was introduced from Europe. Whitebark Pine mortality in some areas is exceeding 60%. The blister rust has also devastated the commercially valuable Western White Pine in these areas. However, there is no known way of controlling the blister rust in existing trees. Research is under way, locating and breeding from the occasional naturally resistant Whitebark Pines, and by studying the resistance mechanisms of the European and Asian white pines (e.g. Swiss Pine, Macedonian Pine), which are strongly resistant to the disease.