Photo by Brewbook, Wikimedia Commons
(Click on the link to listen to an audio version of this blog … Bristlecone pine
There are three species of pines that are referred to as “bristlecone pines:” the Great Plains bristlecone pine (Pinus longaeva) (found in Utah, Nevada and eastern California), the foxtail bristlecone pine (P. balfouriana) (found in the southern Sierra Nevada Mountains of California) and the Rocky Mountain (or “Colorado”) bristlecone pine (P. aristate) (found in Colorado, New Mexico and Arizona).
All of these bristlecone pine species are extremely long-lived. The Great Plains bristlecone pine, in fact, is the longest lived, non-clonal organism on Earth! In 2010 a tree in the White Mountains of California was determined to be 5062 years old! This specimen and its age, though, was never confirmed. Another White Mountain bristlecone pine, though, was found to be 4853 years old, and this specimen, although its location is kept secret, was tested and its age confirmed by other researchers. The oldest Colorado bristlecone pine (growing on Black Mountain, Colorado) was found to be “only” 2480 years old! A typical age for a Colorado bristlecone pine is around 1500 years.
The distribution of the Colorado bristlecone pine is quite limited. It is a tree of high elevations (7000 to 13,000 feet). It is most commonly found on dry, steep, wind-swept, south or west-facing slopes. It is most abundant on sites east of the Continental Divide (although it is found on the western slopes of the San Juan Mountains of southwestern Colorado and northwestern New Mexico). The northern end of its distribution is just to the south of Rocky Mountain National Park in Colorado, and the southern end is in the northern New Mexico. Two, isolated populations of this tree are found in northern Arizona.
Photo by Brewbook, Wikimedia Commons
The growth habits of the Colorado bristlecone pine is very dependent on elevation. At tree-line it grows in a twisted, ground-hugging, stunted form that resembles a natural bonsai plant. This growth form is called “krummholz” (German for “twisted wood”) and the miniaturized forest mix within which it grows (often with common juniper (Juniperas communis), Engleman spruce (Picea engelmannii) and limber pine (Pinus flexilis) is called a “krummholz forest.” At subalpine elevations, however, the bristlecone pine grows as a small tree while at lower elevations it can grow as a 40’ tree with a 30” width (dbh). Throughout its Rocky Mountain range at all elevations, the Colorado bristlecone pine is most frequently found growing with Engleman spruce and limber pine.
The tree form of the Colorado bristlecone pine has a twisted, tapering trunk with a dense, irregular crown of spreading branches. It usually grows with a single trunk but can, occasionally, form multiple trunks. Its bark is gray and smooth on younger trees but becomes more furrowed, darker and more reddish-brown on older trees. Its needles are dark green, short (1 to 2” long), in bundles of five that grow in densely bunched masses concentrated on the ends of branches (an arrangement that is sometimes referred to as a “fox tail” growth pattern). The needles are very resinous and are often covered with “dandruff-like” flakes of dried resin. Needles are retained for very long periods of time (10 to 15 years!). This “needle-economy” reflects the slow-growing, highly efficient energy physiology of this species!
Photo by J. Witcosky, USDA, Wikimedia Commons
Pollen (male) cones are quite small (0.4” long) and concentrated on lower branches of the tree. Seed (female) cones are larger (2 to 4.3” long) and concentrated on the upper branches. Again, this arrangement helps to ensure that a given tree will not readily self-pollinate. The cones are tightly attached to the branches of the tree with almost no visible stalk at all! The seed cones have the prominent, eponymous bristles on their scales and as they mature change from purple to yellow over 16 months following pollination. Seeds are immediately released from the ripe cones and either fall directly to the ground or are spread via wind dispersal. The seeds are small (3/16” long) with a ½ to ¾” wing to assist wind transport). The small size of the seeds makes them a fairly unfavorable food source for birds and mammals, although Clark’s nutcracker, in spite of its preference for the larger seeds of limber pine and pinyon pine, is known to wrest seeds from maturing bristlecones and cache them in bunched trenches on selected wind-swept ridge sites.
Adaptations of this tree to living in extreme environments include: 1. Very slow growth rates (on average a bristlecone pine adds 1/100” to its girth each year, 2. Growth cessation during drought (no growth rings are seen at all when moisture is very limiting), 3. Its wood (and needles and cones) are rich with resins (this protects the tree from insects and disease, although it does increase its fire susceptibility), 4. It retains its needles for decades (the efficient energy economy mentioned above!), and 5. It can grow where very few other tree species can survive (greatly reducing competition for this very slow-growing, shade-intolerant species!).
Photo by S. Rae, Wikimedia Commons
The very slow growth rate of the bristlecone pine makes regeneration of sites difficult, but it also make regrowth of the bristlecone pine forest seem almost inevitable. The trees can “wait” for many hundreds of years to pass before ideal growth conditions are realized. The bristlecone pine is quite susceptible to fire and is also under stress by the destabilized populations of mountain pine beetles and by the invasion of the exotic, invasive fungus that causes white pine blister rust. Climate change (causing the warming and drying of the bristlecone pine’s high elevation habitats) is threatening the survival of the bristlecone pine both directly and indirectly through the spread of insects and pathogens.
Interestingly, bristlecone pine trees can be purchased both at local nurseries and from on-line sources. These trees can then be planted in gardens or in suburban landscapes where they grow (although slowly!) into attractive, well-shaped trees. These “domesticated” bristlecones, though, only live a tiny fraction of their potential, natural life spans. Growing in the warmer, wetter environments of human-maintained landscapes. greatly stresses the tree. Most of these “garden” bristlecones are only expected to live for 100 years.
There is a tree preserve in the mountains just west of Denver called the “Windy Ridge Bristlecone Pine Scenic Area.” It has a number of ancient bristlecone pines growing in a relatively open, almost savanna-like environment. I have added this site to my local, “must see” list for the coming spring and summer. I will send along pictures as soon as I can!