BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pinus aristata | Rocky Mountain Bristlecone Pine 



GENERAL BOTANICAL CHARACTERISTICS : 
Rocky Mountain bristlecone pine is a long-lived, ectomycorrhizal, native
species, which may be nitrogen fixing.  Trees attain heights of 15 to 30
feet (5-9 m) with diameters between 12 and 18 inches (31-46 cm).
Needles are retained in excess of 20 years.  Trees often have a
contorted and multistemmed appearance.  The sparse crowns of twisted or
contorted trees are supported by narrow strips of cambium.  The effects
of sand- and ice-blasting by wind are seen in older trees with a double
"pick-a-back" growth form.  Here the windward trunk is dead, but a
narrow strip of cambium supports branches on the leeward side of the
tree, the "piggy back" [10,28].

Rocky Mountain bristlecone pine exhibits slow growth rates and dense,
highly resinous wood.  Bristlecones are known for their longevity.
Although trees of Rocky Mountain bristlecone pine may reach 1,500 years
in age, some Great Basin bristlecones (Pinus longaeva) have been
estimated to live 5,200 years.  Rocky Mountain bristlecone pine and
foxtail (Pinus balfouriana) grow more rapidly than the Great Basin
bristlecone pine because of greater moisture availability.  However,
they also are more subject to heart rot and do not reach the longevity
of Great Basin bristlecone [2,13].  Compared with other white pines,
Rocky Mountain bristlecone pine is very resistant to white pine blister
rust.

Rocky Mountain bristlecone, Great Basin bristlecone, and foxtail pine
share the following characteristics [10,24]:

(1)  occurrence at the upper limits of tree growth and a reduced rate of
       growth
(2)  a capacity for long life
(3)  continued growth after loss of large areas of tissue from drought,
       stress, wind injury, and soil erosion
(4)  heavy, dense, resin-impregnated wood with small, closely arranged
       water-conducting cells (tracheids) which provide resistance to
      moisture and decay
(5)  survival in an environment of drought, low temperatures, short
       growing seasons, daily temperature extremes, and poor soils
(6)  retention of needles for several years, which reduces the need for
       moisture and nutrients, as well as carrying a tree over
       several years of stress
(7)  relative safety from ground fire due to sparse ground cover and
       litter scarcity.


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
The environmental and physiological factors contributing to the
longevity of Rocky Mountain bristlecone pine also work to lower its
regeneration potential.  Trees are intolerant of shade, grow slowly, and
are locally scattered in distribution.  Reproduction is sparse.
Bristlecones characteristically occur in old to moderately old, open
stands, and form the vegetative climax.  However, this species may also
act like a pioneer in recently disturbed areas where under favorable
conditions tree growth is vigorous and fast [5,28].  Recovery from
logging, road construction, and other disturbances, though, is generally
extremely slow, often taking hundreds of years [12].

With 75 percent seed germination within 8 days, researchers concluded
that the seeds of bristlecone pines do not exhibit dormancy.  In an
effort to understand the limited occurrence of Rocky Mountain
bristlecone pine, the germinability of seeds was studied in Clear Creek,
Colorado.  Rapid germination, scarification mortality, and lack of
seedling cold hardiness were identified as contributing factors working
against Rocky Mountain bristlecone pine reproduction.  Seeds had a
germination rate of 80 percent and germinated within 10 days (with or
without light).  Infected with fungus, 90 percent of the scarified seeds
died.  A gradual decrease in temperature to 14 degrees Fahrenheit (-10
deg C) killed 10- and 180-day-old seedlings [28].


SITE CHARACTERISTICS : 
Rocky Mountain bristlecone pine grows in subalpine areas.  Stands are
usually pure, even-aged, and open.  They are found from 9,200 to 11,800
feet (2,800-3,600 m) in elevation in Colorado and New Mexico, usually on
rocky outcrops of southern or southwestern slopes with limited soil,
intense sunlight, and rapid runoff.  In the San Francisco Peaks Research
Natural Area in Arizona, bristlecone pines occur from 10,500 to 11,500
feet (3,200-3,500 m) on south-facing ridges and talus slopes [2,32].

Rocky Mountain bristlecone pines occur most often on igneous and
volcanic substrates.  Short growing seasons and little available
moisture characterize their habitat.  Trees are intolerant of shading
and occur in areas of sparse ground cover.  Precipitation ranges from 16
to 40 inches (400-1,000 mm) [2,31,32].

Above Sante Fe, New Mexico, limber pine dominates on xeric sites between
7,550 to 10,170 feet (2,300 and 3,100 m) but is replaced on the driest
sites by Rocky Mountain bristlecone pine.  To the north Rocky Mountain
bristlecone pine increases in importance, and in the area of the Spanish
Peaks of Colorado, it is found from treeline down to ponderosa
pine-Gambel oak (Quercus gambelii) woodlands.  North of this area Rocky
Mountain bristlecone pine is replaced by limber pine.  At Pikes Peak in
Colorado, it is important on the south slopes from middle to high
elevations; however, on north slopes it is restricted to high-elevation
sites.  On Mt. Evans in Colorado, bristlecone and limber pine occur at
high elevations and are partitioned by substrates [2,25,30].


SUCCESSIONAL STATUS : 
Rocky Mountain bristlecone pine is a climax species and persists at
treeline for hundreds to thousands of years in the absence of
disturbance and competition.  It is highly adapted to its habitat of
very shallow soils, slow primary succession, short growing season, and
avalanches.  This tree is intolerant of shading and competition.  Below
its lower elevational limits, it is outcompeted and replaced by other
conifers on moist sites.  Although its upper limits of growth are
temperature sensitive, its lower growth limits are precipitation
sensitive [2,25,32].  There is no evidence for the expansion or
recession of the range of Rocky Mountain bristlecone pine, so it is
considered a senescent species [27].


SEASONAL DEVELOPMENT : 
Bristlecone pines flower from July to August, and cones start ripening
about mid-September.  The period of flowering and cone opening is
uniform for Rocky Mountain and Great Basin bristlecone pines and foxtail
pine [5,10].  Phenological data for Rocky Mountain bristlecone pine in
the San Francisco Peaks area of Arizona are as follows:

Plant Activity                         1918-23            1969
------------------------------------------------------------------------
Vegetative buds swelling               June 1-20          June 1
Vegetative buds elongating or opening  June 20-30         June 15
Shoots making rapid growth             July 1-30          ----
Male buds appearing                    July 1-10          ----
Female and male buds mature            ----               July 22
Pollen falling                         July 20-Aug. 20    July 22-27
Cones full grown                       Sept. 10-20        ----
Seeds mature                           Sept. 20-Oct. 10   Sept. 24-Oct. 2
Cones opening                          ----               Sept. 27-Oct. 10
Leaves falling                         Oct. 1-30          ----
Period of active growth                June 20-Sept. 20   ----

Related categories for Species: Pinus aristata | Rocky Mountain Bristlecone Pine