BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pinus edulis | Colorado Pinyon 



GENERAL BOTANICAL CHARACTERISTICS : 
Colorado pinyon is a long-lived, slow-growing, monoecious, cone-bearing,
evergreen tree.  The needles are 1.2 to 2 inches (3-5 cm) long and
typically two per fasicle.  The trunk is short and branching; lower
branches often reach to the ground.  Crowns in young trees are compact
and conical; in mature trees crowns are rounded or irregular, becoming
flat-topped and open with age.  In relatively open stands, crown width
often equals crown height.  Closed stands produce trees with short,
narrow crowns.  Although trees may live to be approximately 1,000 years
old [56], life spans of 350 to 450 years are more common.  Age at
maturity can range from 75 to 100 years.  Trees at this stage are
typically 10 to 30 feet (3-9 m) tall and about 12 inches (30 cm) in
trunk diameter.  Height growth is 2 to 4 inches (5-10 cm) per year;
diameter growth is roughly 1 inch (2.5 cm) per decade [10,18].

Growth is dependent upon soil moisture stored from winter snows.
Pinyons have both verticle taproots and lateral roots; both are capable
of active absorption.  In shallow soils root systems extend well beyond
the crown.  The alleopathic effects of needle litter reduce plant
establishment under tree crowns [25].  Losses from extreme, prolonged
drought and frost have been recorded.


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Colorado pinyon regenerates entirely by seed.  Trees begin bearing cones at
age 25 when they are 3 to 4 feet (1 m) tall, but do not produce
significant quantities of cones until they are 75 to 100 years old [18].
Cones are then produced in increasing numbers in the upper half of the
crown.  Cone crops are produced sporadically.  Large crops occur every 4
to 7 years; small localized crops or crop failure may occur in the
interim.  Although larger trees can produce a few cones every year, most
seeds are not filled.

Colorado pinyon has an average of 12 to 20 seeds per cone.  Conelets emerge
in June from buds that have overwintered; cones are not mature until the
September of the second year [32].  Seed dispersal takes place from
September through October.  Many seeds are not viable due to embryo
abortion and disease.

Small mammals and birds are the primary dispersal agents of the heavy,
wingless pinyon seeds.  Seeds need to be buried 0.8 to 1.2 inches (2-3
cm) deep in the soil or litter in order to germinate, and the neglected
food caches of rodents and birds provide suitable sites.  Clark's
nutcrackers, pinyon jays, Stellar's jays, and scrub jays play a major
role in pinyon seed disperal.  Interdependence between pinyons and these
members of the "food caching" guild is so strong that numerous
coadaptive traits have evolved [4,53].

Most seeds germinate the spring following dispersal, although some may
remain viable for 2 to 3 years.  Optimal germination temperature is 70
degrees Fahrenheit (21 deg C).  Germinative capacity of filled seeds is
83 percent.  Seedlings require a nurse plant for successful
establishment [17].  Survival is dependent upon ample summer
precipitation.  Growth is extremely slow, with 2-year-old seedlings
averaging only 2 inches (5 cm).


SITE CHARACTERISTICS : 
The woodland mosaic formed by Colorado pinyon occurs primarily on the high
plains, plateaus, mesas, canyons, foothills, and lower mountain slopes
of the Colorado Plateau.  Sites are intermediate between ponderosa pine
and submontane scrub above, and semiarid grassland below.  Colorado pinyon
occurs most commonly at elevations between 4,500 and 7,500 feet (1,370
and 2,286 m) where annual precipitation ranges from 12 to 18 inches
(30-46 cm) [18].  Where summer (July-September) rainfall is less than 3
inches (80 mm), Colorado pinyon is gradually replaced by singleleaf pinyon
along the eastern edge of the Great Basin [51].

The distribution of Colorado pinyon is primarily a function of climate.  Its
lower limits are determined by lack of moisture; upper limits by biotic
competion, low temperatures, and excessive soil moisture [43].
Therefore, the elevational zones it occupies vary considerablly
depending on local topography and geographical location.  Colorado pinyon
usually grows on the higher elevation sites in the pinyon-juniper
woodlands it occupies [1,49].

Colorado pinyon has a wide ecological amplitude and is able to occupy a
broad range of soil types.  Soils are mostly in the orders Aridisols and
Mollisols, with Argixerolls and Haploxerolls being well represented.
Pinyon grows in soils that are ususally alkaline, mostly calcareous, and
rocky; overall productivity is low.  The best Colorado pinyon stands occur
on coarse gravel, gravelly loam, or coarse sand where soil depths exceed
5 feet [50].  Trees on dry sites may be 4 to 6 inches (10-15 cm) in
diameter at 80 to 100 years, while those on deeper soils may be 10 to 12
inches (26-31 cm) at 150 to 160 years.

Within the pinyon-juniper woodland, pinyon composition increases with
increasing elevation.  Stand composition can range from essentially pure
pinyon to stands where pinyon is codominant with one, and sometimes two,
juniper species.  These codominates include alligator juniper (Juniperus
deppeana), one-seed juniper (J. monosperma), Utah juniper (J.
osteosperma), and Rocky Mountain juniper (J. scopulorum).  Blue grama
(Bouteloua gracilis) is a common understory associate throughout much of
the Southwest.

Elevational ranges for some western states are as follows [11,49]:

	From     4,500 to 6,500 feet (1,372-1,982 m) in AZ
	         4,000 to 9,000 feet (1,220-2,744 m) in CO
	         5,000 to 7,000 feet (1,524-2,134 m) in NM
		 6,000 to 10,000 feet (1,830-3,049 m) in UT
		 6,100 to 7,100 feet (1,860-2,134 m) in WY


SUCCESSIONAL STATUS : 
Colorado pinyon is the climax species in most pinyon-juniper woodlands
[48,51].  It competitively confines juniper to the more marginal,
low-elevation sites.  Pinyon is responsible for most of the increases in
tree dominance and density within the pinyon-juniper woodland over the
past 150 years [56].  Pinyon effectively outcompetes juniper on
sagebrush grasslands; it grows faster and lives approximately as long
and can therefore more successfully invade such sites.


SEASONAL DEVELOPMENT : 
Detailed phenolgical observations are not available for pinyons due to
the lack of easily observed, periodic phenophases [50].  Seed production
in Colorado pinyon follows a 2-year generalized cycle.  Leader growth begins
in early spring.  Male and female conelets emerge from winter buds in
June, and pollination occurs soon afterwards.  Cone growth ceases in
mid-August when cones are brown and approximately a 0.5 inch (1.3 cm) in
diameter.  The following May growth resumes, at which time pollen grains
are reactived and fertilization takes place.  Cones are brilliant green
and 3 inches (7.6 cm) in diameter by August; cones are ripe and turn
brown by September.  Seeds are dispersed in September and October.  Most
cones fall from the tree during the winter [32,37].

Related categories for Species: Pinus edulis | Colorado Pinyon