GROWTH CHARACTERISTICS OF ANCIENT ROCKY MOUNTAIN BRISTLECONE PINES, SOUTHERN ROCKY MOUNTAINS, COLORADO, USA

The growth forms of 126 Rocky Mountain bristlecone pines (Pinus aristata) ranging in age from about 25 to 2,500 years were studied at sites on Almagre Mountain, Black Mountain, Mount Princeton, Mount Big Chief, Goliath Peak, Stanley Mountain, and North Peak, Colorado. Typical young bristlecone pines <150 years old have circular, bark-enclosed trunks, no crown dieback, and as many as 80 or more living limbs. In contrast, bristlecone pines 1,000-2,500 years old have trunks that have exposed, erosion-sculptured wood; extensive crown dieback; a prominent dead, erosion-sculptured pith spike; usually only one bark strip covering as little as 5-10 percent of the circumference of the trunk; and usually only one living limb. However, some of the studied >1,000-year-old trees have as many as three bark strips and four living limbs. Each of the five bristlecone pines in this study that are about 2,000-2,500 years old has only one bark strip and one living limb. The width, location, orientation, and number of living bark strips and the number, location, and attachment heights of living limbs determine the forms of most of the studied bristlecone pines 1,000-2,500 years old. Forms of 43 of the 1,000-2,500-year-old trees were classified as follows: slab (30 trees), twisted-slab (4 trees), irregular-shape trunk (3 trees), massive roughly circular trunk (3 trees), double-bark-strip slab (2 trees), and multi-stem (1 tree). (Twisted slab and multi-stem forms are based on long axis view of tree; other forms are based on inferred cross section of trunk.) The maximum age at which the trunks of most of the studied bristlecone pines remain enclosed by bark ranges between about 400 and 600 years. Eighty-five percent of the studied trees 1,000-2,500 years old have a bark strip on the upslope side of the trunk. For trees 1,000-2,500 years old, average bark-strip width is 38 cm (range 7.6-122 cm); average tree height is 5.5 m (range 1.8-9.1 m); and average attachment height of living limbs is 1.9 m (range 0.3-6.1 m). Growth-form-altering damage to some living bristlecone pines by falling or tumbling rocks and boulders, gnawing animals, forest fire, and lightning strikes was observed to cause loss of varying amounts of cambial tissue and loss of some living limbs and branches.