Pinus aristata, the Rocky Mountain bristlecone pine or Colorado bristlecone pine is a long-lived species of bristlecone pine tree native to the United States. It is found in the Rocky Mountains in Colorado and northern New Mexico, with an isolated population in the San Francisco Peaks of Arizona. It is found at very high altitudes, from in cold, dry subalpine climate conditions, often at the tree line, although it also forms extensive closed-canopy stands at somewhat lower elevations.
Description
Pinus aristata is a medium-size tree, commonly reaching in height and occasionally as much as 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 and up to in diameter, and tapers very noticeably towards the top. On older trees the branches are crooked and twisting and may be partially dead. Twigs begin as red-brown, but become gray with age. Seed cones are dark purple when immature and turn brown as they ripen. At full size they are 6–11 centimeters in length. Large crops of cones are produced only once per century on average.
Longevity
Though they do not obtain the extreme age of the Great Basin bristlecone pines, Colorado bristlecones still reach quite advanced ages. There are 13 trees that are known to be older than 1600 years by reliable dating, including one on Mount Goliath dated to the year 403. Four of these trees exceed 2100 years of age and one is years old (see Notable trees). Based on size and appearance scientists estimate there are 30 more trees in the same general area older than 1600 years that have not yet had core samples taken.
The Rocky Mountain bristlecone is known both for having stripes of dead wood on the trunks of older trees and for the extreme lengths of time that dead trees will remain sound. In this it is very much like the Great Basin bristlecone.
Taxonomy
thumb|The branches and cones of Pinus aristata as illustrated by [[Charles Edward Faxon in The Silva of North America]]
Pinus aristata was scientifically described in 1862 by George Engelmann with its present scientific name. but was rejected by others such as Edward James Ravenscroft. It is listed as an accepted species with no subspecies by Plants of the World Online, and World Plants. Prior to work by Dana K. Bailey published in 1971, Pinus longaeva trees growing in California, Nevada, and Utah were not identified as a separate species or subspecies. A distance of separates the populations of P. aristata from the closest groves of P. longaeva. As the first bristlecone identified, Pinus aristata is still occasionally referred to simply as bristlecone pine, however Pinus longaeva is also sometimes called bristlecone pine in English. To distinguish it from its relative the name Rocky Mountain bristlecone pine has been used since at least 1971 and the similar Colorado bristlecone has been in use since at least 1976. The common name hickory pine is used, most often in Colorado, due to the hardness and density of the wood being like that of a hickory.
Range and habitat
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 , 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.
thumb|upright=1.8|Bristlecone pine (Pinus aristata) at timberline, the summit of the [[San Francisco Peaks, 1926]]
In Arizona this species of bristlecone pine is only found on the San Francisco Peaks, In both New Mexico and Colorado they mostly grow on steep, south-facing slopes that are quite dry.
In Colorado there are more than 50 areas with notable populations of bristlecone pine found in the southern and central mountain ranges of the state. On the Spanish Peaks in southern Colorado bristlecone pines grow together with ponderosa pines and Rocky Mountain white oaks at relative low elevations, an unusual habitat type. Though they also grow in nearly pure stands higher up in the subalpine zone of the Spanish Peaks. In the southern part of the range around Pikes Peak they occur on the south facing slopes while limber pines grow on the north facing areas. In areas around Mount Blue Sky bristlecone pines more often mix with limber pines, though bristlecones tend to be more common on fine textured soils while rocky ridges have more limber pines.
Ecology
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.
Notable trees
The oldest tree in Colorado is a Rocky Mountain bristlecone that was dated by F. Craig Brunstein and David Yamaguchi to 442 BCE. Designated in the 1992 paper as CB-90-11, it grows on Black Mountain in the Pike National Forest west of Pike's Peak. At the time core samples were taken by researchers the trees showed many signs of old age including dead wood that had been sculpted by wind erosion, crowns that had died back, only strips of living bark, and a radius greater than . Three other trees in the same area were dated to earlier than 100 BCE.
The oldest known tree on Mount Goliath was discovered by Dr. Paula Krebs in 1970 while working on her doctoral dissertation. The tree, which she named Great Grandma, was dated to 403 CE making it years old.
The American National Register of Champion Trees records a tree with a very large diameter of Huerfano County, Colorado. This tree also has a height of and a crown spread of when it was measured in 2018.
Uses
Formerly it was used for fuel, electric utility poles, signposts, and as support timbers in mines. 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. In gardens it will range from high and 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. The Colorado bristlecone is quite flammable and is also susceptible to air pollution.
'Sherwood Compact'
This is a very dwarfed variety. It grows with a somewhat more treelike form when grafted onto Pinus strobus as a rootstock, but remains very small. It was introduced to the horticultural trade in 1983.