Rocky Mountain Bristlecone Pine (Pinus aristata) is a pine in the bristlecone pine group, native to the United States, where it occurs in the Rocky Mountains in Colorado and northern New Mexico, with an isolated population in the San Francisco Mountains in Arizona. It occurs at very high altitudes, from 2500-3700 m, in cold, dry subalpine climate conditions, often at the tree-line.
It is a medium-size tree, reaching 5-15 m tall and with a trunk diameter of up to 1.5 m. The bark is grey-brown, thin and scaly at the base of the trunk. The leaves ('needles') are in fascicles of five, stout, 2.5-4 cm long, deep green to blue-green on the outer face, with stomata confined to a bright white band on the inner surfaces. The cones are ovoid-cylindrical, 5-10 cm long and 3-4 cm broad when closed, purple at first, ripening yellow-buff when 16 months old, with numerous thin, fragile scales, each scale with a bristle-like spine 4-8 mm long. The cones open to 4-6 cm broad when mature, releasing the seeds immediately after opening. The seeds are 5 mm long, with a 10-20 mm wing; they are mostly dispersed by the wind, but some are also dispesed by Clark's Nutcrackers, which pluck the seeds out of the opening cones. The nutcrackers uses the seeds as a food resource, storing many for later use, and some of these stored seeds are not used and are able to grow into new plants.
It differs most conspicuously from the other bristlecone pines in that the needles usually have only one, (only rarely two) resin canals, and these are commonly interrupted and broken, leading to highly characteristic small white resin flecks appearing on the needles. This character, which looks a bit like 'dandruff' on the needles, is diagnostic of Pinus aristata; no other pine shows it (though sometimes, scale-insect infestations can look superficially similar).
It is a long-lived tree, though not so markedly so as Great Basin Bristlecone Pine (Pinus longaeva), not exceeding 3,000 years and only very rarely over 1,500 years.
Rocky Mountain Bristlecone Pine is by far the most successful of the bristlecone pines in cultivation, where is a very attractive slow-growing small tree suitable for small gardens in cold climates. Even so, it is never as long-lived as in the wild, typically living less than 100 years before it succumbs to root decay in the warmer, moister conditions prevalent in most inhabited places.
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