Rocky Mountain bristlecone pine

Pinus aristata is a medium-size tree, commonly reaching 15 meters (49 ft) in height and occasionally as much as 20 m (66 ft) in their natural habitat. In favorable conditions they are straight and upright trees, but they become increasingly stunted, short, and twisted the closer they grow to timberline. The crown of the tree is flattened, irregular, or round in shape when fully mature. The trunk can be very substantial, commonly 75 cm (2 ft) and up to 1 m (3.3 ft) in diameter, and tapers very noticeably towards the top. The bark is fairly smooth and white-gray on young trees, but becomes dark and cracked with a scaly texture on old trees. The color of older bark varies from red-brown to gray. Young trees that are not stunted have evenly spaced whorls of branches. On older trees the branches are crooked and twisting and may be partially dead. Twigs begin as red-brown, but become gray with age. Twigs and very young branches are covered in needles that last for 10 to 17 years, giving them a shaggy bottle brush appearance. The needle like leaves are bundles of five and are 2–4 centimeters long, but usually longer than 3 cm. The fascicle, the base that holds the needles together, breaks down more quickly in Pinus aristata than in Pinus longaeva, giving a more even appearance to its branches. …

The Colorado bristlecone grows in three western US states, Colorado, New Mexico, and Arizona. The Rocky Mountain bristle cone reaches its northern limit at approximately 40 degrees north latitude. They are widely distributed in the Southern Rocky Mountains, but are limited to small areas within its range. The elevation range where it grows is widest, from 2,100 to 4,000 meters (6,900 to 13,100 ft), in the state of Colorado. The Arizona population is only found above 2900 m and only as high as 3700 m. In New Mexico the trees start at a similarly high elevation of 3000 m and have the same maximum as in Arizona. Rocky Mountain bristlecone will grow in krummholz habitat at the edge of the alpine tundra, where it has the typical dwarfed and gnarled growth of these "crooked woods". Its habitat extends downward into the subalpine forests and even lower in the driest parts of the middle elevations. The bristlecones and limber pines (Pinus flexilis) will grow into full trees at elevations where silver spruces (Picea engelmannii) and subalpine firs (Abies lasiocarpa) are reduced to stunted, ground hugging shrubs. Their greater tolerance of dry conditions is what enables them to grow more successfully in challenging mountain habitats.

In Arizona this species of bristlecone pine is only found on the San Francisco Peaks, four volcanic cinder cones in north-central Arizona. …

The seeds of the Rocky Mountain bristlecone are eaten by Clark's nutcracker (Nucifraga columbiana) and may also be stored by them in years where crops of seeds are particularly large. Unlike the seeds of the Colorado pinyon (Pinus edulis) the seeds of show the same speckled patterns for camouflage on soil as other species and are not adapted to be primarily animal dispersed. The Rocky Mountain bristlecone is quite vulnerable to fire. After forest fires stands show very little regeneration after thirty years and a lower abundance compared with other species tree species that are more able to colonize newly open areas.

Formerly it was used for fuel, electric utility poles, signposts, and as support timbers in mines.

Cultivation. Rocky Mountain bristlecone is the most common of the bristlecone pines in cultivation. It is a very attractive slow-growing small tree suitable for small gardens in cold climates. It was brought into cultivation by Parry at the Arnold Arboretum soon after he collected the materials for its scientific description in 1861. It has been cultivated since at least 1863 in the United Kingdom, but is a rarely planted tree there. In gardens it will range from 2–6 meters (8–20 ft) high and 3–4.5 m (10–15 ft) wide. However, due to its slow growth rare it is rare to see one taller than 10 m in landscaping. It requires a well drained soil, one that is rocky and poor in organic material being particularly suitable. They also tolerate alkaline soils. Trees are tolerant of windy conditions, but will show evidence damage from the wind. They have an extreme resistance to cold, being hardy in USDA zones 3–7. They can be encouraged to grow more quickly with supplemental water, but overwatering resulting in deformed growth or death is a frequent problem in landscaping. The Colorado bristlecone is quite flammable and is also susceptible to air pollution. Though symmetrical and bushy when young, as the trees grow they become more open in structure and irregular in shape.

'Blue Bear'. …