Colorado pinyon often grows as a low, bushy tree [166] with an irregularly rounded, spreading crown [26,85,109,133]. Crowns of young trees are broadly conical, and those of old trees are spreading or flat-topped [5,64]. The trunk is generally short and crooked [26,64,85,109,133], with several large, crooked branches [64]. It may grow to 40 inches (1 m) in diameter [5,26,64,126]. Height is typically 26 to 56 feet (8-17 m) [5,26,64,85,109,133,147,164].
The needles are 2.7 to 4.3 inches (6-11 cm) long [167] and in fascicles of 2 [126]. Needles remain on the tree for approximately 9 years. Bark thickness of Colorado pinyon ranges from 0.5 to 0.87 inch (1.3-2.2 cm) [64], with young trees having smoother and thinner bark than older trees. Cones are 1.5 to 2 inches (3.5-5) cm long and are borne singly or in groups of 2 to 4 [126]. The average cone contains 10 to 20 soft-shelled seeds [59,61,108,133,139]. The Flora of North America provides a morphological description and identification key for Colorado pinyon.
Colorado pinyon's root system consists of a taproot and shallow lateral roots occurring more than 1 inch (3 cm) below the soil surface [59,71]. Taproots extend to soil depths of at least 20 feet (6 m) [50]. Laterals are generally found at depths of 6 to 16 inches (15-40 cm) and can extend from the tree up to twice the crown radius [59,139,159].
Colorado pinyon is a slow-growing, long-lived tree [59,64,105]. It can survive more than 500 years [9,32,44,64,155] and may reach 800 to 1,000 years of age [118,139]. The density of Colorado pinyon in woodland communities ranges from none or few to several hundred stems per hectare [139].
Fire opens pinyon-juniper stands, increases diversity and productivity in understory species, and creates a mosaic of stands of different sizes and ages across the landscape. In addition, fire maintains the boundaries between the woodlands and adjacent shrub- or grasslands [14,25].
Fire adaptations: Mature Colorado pinyon trees are short with open crowns, but they do not self-prune their dead branches [14,25,117]. The accumulated fuel in the crowns, thin bark, and the relative flammability of the foliage make individual trees susceptible to fire [14,86,117]. Stand structure also impacts fire susceptibility; open stands of trees with large amounts of fine grass fuel or dense, mature trees capable of carrying crown fire during dry, windy conditions are the most flammable [14,67,117]. With sparse fuels, Colorado pinyon survives fire because it is seldom exposed to lethal heat [67].
Where stand-replacing fires do occur and potential seed sources are removed, dispersal of Colorado pinyon seeds by animals becomes particularly important in the reestablishment of tree seedlings. Birds in particular may cache seeds at "considerable distances" from the seed source [68]. Seeds cached within the shade of shrubs or trees are more likely to germinate and establish seedlings [59,62,71,86]. For more information on seed dispersal and Colorado pinyon succession following fire, please see the "Biological and Ecological Characteristics" section of this FEIS species summary.
FIRE REGIMES: Pinyon-juniper stands can support stand-replacing fires [63], though presettlement fire regimes were likely a mixture of surface and crown fires with intensities and frequencies dependent on site productivity [125]. Natural fires may be infrequent due to a sparseness of vegetation combined with an infrequency of lightning in some areas [25,62,86]. Floyd and others [44] estimated the "natural" fire turnover times of pinyon-juniper woodlands in southern Colorado at approximately 400 years, with fires largely the result of lightning strikes. Keeley [86] estimated the natural fire frequency of pinyon-juniper woodlands at 100 to 300 years. The woodlands are described as "resilient" with a minimum fire-free interval of 100 years and an unlimited maximum fire-free interval [86]. However, of 10 fire-scarred Colorado pinyon trees collected from 3 locations in New Mexico, multiple fire scars reflected a mean point fire interval for the trees of 27.5 years, with a range of intervals from 10 to 49 years [16]. A 208-year fire chronology of an eastern California pinyon-juniper woodland (based on fire scarred trees) suggests that fires burned somewhere within the <100-acre study area every 8 years [67]. Gottfried and others [62] estimate fire intervals ranging from 10 to 50 years for surface fires and >200 years for crown fires in the Middle Rio Grande Basin. Other studies report surface fire intervals of 20 to 30 years, and standwide fires occurring at 15 to 20 year intervals in New Mexico [125]. The variation in fire intervals in Colorado pinyon is the result of differences in fuel loading and composition; where vegetation is sparse and unable to carry fire, fire-free intervals are much longer than in areas with a well-developed understory or greater tree density.
The amount of fine fuels varies with habitat type, stand history, and climatic conditions. Fuel loadings of more than 11 tons per acre (25,000 kg/ha) are considered heavy [125]. Fine fuels in many open pinyon-juniper stands range from 600 to 1,000 pounds per acre (635-998 kg/ha), and approximately 600 to 700 pounds per acre (635-726 kg/ha) are required to sustain surface fires [14]. Open pinyon-juniper stands (average canopy cover 12.4 to 21.8%) at Los Alamos, New Mexico, contained an average of 17,666 pounds per acre (20,033 kg/ha) of downed woody fuels and 22,347 pounds per acre (25,342 kg/ha) of total surface fuels [122]. Stands of moderate tree density where overstory competition reduces the herbaceous fuel and the trees are widely spaced are less likely to burn. Closed pinyon-juniper stands do not have understory shrubs to carry a surface fire, and do not burn until conditions are met to carry a crown fire [14]. Key conditions for crown fires include sufficient canopy closure to promote fire spread between trees, abundance of dead woody fuels on the surface and as standing snags, and extreme weather conditions (low humidity and high winds) [62,125].
Fire intervals in Colorado pinyon are difficult to quantify because living fire-scarred trees are rare: Colorado pinyon is often killed directly by fire or indirectly due to increased susceptibility to heart rot [16,125]. Though fire-scarred Colorado pinyon verify fire occurrence in pinyon-juniper communities, they are not a reliable indicator of fire frequency. Localized stand-replacing fires do occur in pinyon-juniper woodlands, and the absence of frequent fire in pinyon-juniper communities likely results in increased tree cover and tree density, encouraging crown fires rather than surface fires [67].
Due to the slow establishment and growth of Colorado pinyon, repeated fires maintain earlier seral stages in these communities [25,31,80,86]. Repeated burning every 20 to 40 years may eventually replace pinyon-juniper woodlands with shrub communities because shrubs colonize areas much faster than trees can re-establish [86,87], while the absence of fire eventually allows Colorado pinyon to replace extensive shrub vegetation [87]. Frequent fire may prevent the expansion of Colorado pinyon into grasslands, based on the perception that periodic fires burned these grasslands often enough to kill tree seedlings while they are most susceptible to fire. In the absence of frequent fire, seedlings become established in the grassland, eventually converting it to a woodland or savanna community. The effectiveness of fire in restricting the spread of Colorado pinyon (and juniper) depends on fire frequency and intensity of the fire, with the time required for seedlings to reach 4 feet (1.2 m) tall a critical determinant of the effective fire interval [130].
Fire return intervals for plant communities and ecosystems in which Colorado pinyon occurs are summarized below. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 plains grasslands Bouteloua spp. 125] curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [4,142] mountain-mahogany-Gambel oak scrub C. ledifolius-Quercus gambelii Arizona cypress Cupressus arizonica Rocky Mountain juniper Juniperus scopulorum 125] blue spruce* Picea pungens 35-200 pine-cypress forest Pinus-Cupressus spp. 3] pinyon-juniper Pinus-Juniperus spp. 125] Mexican pinyon P. cembroides 20-70 [119,154] Colorado pinyon* P. edulis 10-400+ [44,62,86,125] interior ponderosa pine* P. ponderosa var. scopulorum 2-30 [3,7,103] Arizona pine P. ponderosa var. arizonica 2-10 [3] galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea 125] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [3] oak-juniper woodland (Southwest) Quercus-Juniperus spp. 125] *fire return interval varies widely; trends in variation are noted in the species summaryColorado pinyon is found on level or gently rolling uplands [39] to moderately steep and very steep slopes (27-75%) [72]. It also occurs in riparian areas in the Southwest [42] and on slopes adjacent to river drainages [72]. Colorado pinyon sites include dry foothills, plateaus, mesas, mountain slopes, and canyon sides [2,64,72,98,112,166]. The distribution of Colorado pinyon in pinyon-juniper woodlands may be limited by Colorado pinyon's lack of tolerance for water stress on low elevational, xeric sites [8,174]. At high elevations, distribution may be limited by low temperatures or competition with ponderosa pine [8]. Moisture is likely the most critical factor controlling the distribution, composition, and density of pinyon-juniper woodlands [58], though the distribution of Colorado pinyon may also be affected by soil characteristics [174].
Elevation: Pinyon-juniper woodlands occur in the foothills above desert shrub or grassland vegetation but below ponderosa pine forest [25,101,127]. Colorado pinyon occurrence ranges from 4,500 to 9,000 feet (1,400-2,700 m) elevation [26,64,112,127,139]. In pinyon-juniper woodland, Colorado pinyon tends to increase in abundance with increasing elevation, while junipers decrease [25,34,123,127]. Colorado pinyon is not generally affected by topographic position (aspect or steepness of slope), other than its prevalence relative to juniper [129,131]. The following table presents information on the elevational distribution of Colorado pinyon by state:
State Elevation range References Arizona 4,000-7,500 feet (1,220-2,280 m); upper limit of 6,500 feet (1,980 m) on north-facing slopes [38,39,69,85,96,115,174] California 4,200-8,850 feet (1,280-2,700 m) [73,156] Colorado Occurs from 5,200 to 9,000 feet (1,580-2,750 m); abundant from 7,000 to 7,900 feet (2,100-2,400 m) [72,96,166,166,174] New Mexico 5,000-8,850 feet (1,520-2,700 m) [2,109] Texas >6000 feet (1,830 m) [147] Utah 5,000-8,400 feet (1,520-2,560 m) in Utah; upper limit of 8,400 feet (2,560 m) on south-facing slopes [82,107,174]Climate: Colorado pinyon occurs in the warm, semiarid climate of the Southwest (Arizona, New Mexico) and in the cold, semiarid climate of the mountainous west (Nevada, Utah, Colorado) [34]. Summers in the pinyon-juniper zone are hot and winters relatively cold. A high percentage of clear days, intense solar radiation, and windy conditions favor high evapotranspiration rates [139], and precipitation generally exceeds evapotranspiration only during December, January, and February [39]. Growth is limited primarily by low precipitation in the Southwest, while in the mountainous west it is limited by both freezing temperatures and low precipitation [34].
Temperature and precipitation in the pinyon-juniper zone vary in relation to elevation and geographic location [131]. Colorado pinyon occurs on sites experiencing approximately 120 frost-free days and 4 to 20 inches (102-520 mm) of annual precipitation, with variable seasonal distribution [39,108,139,168,176]. In the southern portion of the pinyon-juniper woodland distribution, precipitation peaks occur during the summer fed by moisture from the Gulf of Mexico. In the more northern areas, precipitation from convection storms occurs in July and August, and winter storms from the Pacific coast provide moisture during the cool season [131,139,176]. Colorado pinyon is mostly dependent on soil moisture stored from winter precipitation. Much of summer rainfall is ineffective due to runoff after heavy thunderstorms and high evaporation [159]. Colorado pinyon occurs in zones that are generally 6 degrees warmer than in the vegetation zone above and 5 degrees cooler than the zone below [39].
Colorado pinyon is tolerant of cold and drought [101,176]. According to field studies using simulated rainfall events, Colorado pinyon can respond effectively to both monsoon precipitation and small rainfall events [170].
Soils: Colorado pinyon occurs on a wide range of soil types and is not limited by the character or geologic origin of soils [139,176]. Soils of these communities may be shallow to moderately deep and are often rocky, well drained, and low in fertility [34,64,72,127]. Colorado pinyon growing in deeper soils generally grow faster than those in shallow soils [159]. Colorado pinyon occurs on a range of parent materials, including sandstone, limestone, shale, basalt, granite, and mixed alluvium [34,39,69,127,147].
Soils under well-developed pinyon-juniper stands are completely occupied by tree roots, limiting understory growth [112,139]. The lateral roots of the tree species efficiently access interspaces in these communities for soil water and nutrients, further impacting herbaceous species [15,90]. In addition, understory vegetation is reduced by shading and potentially by allelopathic effects [35]. Colorado pinyon accumulates nutrients beneath the tree canopy [34,157]. Organic carbon and nitrogen are greater under pinyon-juniper canopies than in interspaces, especially under mature canopies as compared to younger or more recently disturbed stands [89,90,123]. In addition to accumulations of organic matter, concentrations of soluble salts (Na, Ca, Mg, and K) are significantly higher (p<0.05) under Colorado pinyon canopies than in adjacent shrub-dominated areas. Accrual of nitrate and sulfate is also evident under Colorado pinyon trees, as is higher average concentration of phosphorus and boron, which may be phytotoxic to some herbaceous species [9]. Colorado pinyon litter is specifically associated with a reduction of blue grama production [79].
Livestock grazing is an important use of pinyon-juniper woodlands [34]. Pinyon-juniper communities provide food and shelter for deer, elk, pronghorn, wild horses, small mammals, and both game and nongame bird species [112,134]. They also provide habitat for coyotes, mountain lions, and bobcats [134], and are important winter habitat for goshawks [63].
Pinyon-juniper woodlands are important winter ranges for mule deer [34,98], providing cover, shelter, and understory forage [34]. Colorado pinyon provides browse for mule deer, though it is not substantially utilized [10,17,94]. It may constitute 1 to 5% of mule deer winter diets [91].
The seeds of Colorado pinyon are an important food source for birds, particularly Clark's nutcracker [18,21], scrub jays, and pinyon jays [21]. Clark's nutcracker preferentially harvest seed from trees with large cone crops. Cones chosen for seed removal also tend to have more seeds as well as more viable seeds, potentially resulting in differential reproductive success of Colorado pinyon [22]. Seeds are an important food for small mammals, primarily chipmunks and squirrels [21].
Palatability/nutritional value: Colorado pinyon browse is unpalatable to domestic cattle, sheep, and, horses [28]. The seeds are rich in protein and unsaturated fats, containing essential amino acids, carbohydrates, fats, vitamins, and minerals [81,98].
Cover value: Colorado pinyon provides good cover for elk, mule deer, white-tailed deer, pronghorn, upland game birds, small nongame birds, and small mammals [28,39]. Pinyon-juniper woodlands also provide important cover for coyotes [54].
Colorado pinyon forms a characteristic woodland community with Utah juniper (Juniperus
osteosperma) known as the pinyon-juniper woodland
[26,73,98,104,167]. In this community, Utah juniper often extends to lower elevations without the Colorado pinyon component, while
Colorado pinyon grows at elevations above Utah juniper. The pinyon-juniper woodland often forms large
continuous stands, as for example in the western part of the Uinta Basin [26]. Other pinyon (Pinus spp.) and juniper (Juniperus spp.)
species occurring in these woodlands
include singleleaf pinyon (P. monophylla), Parry pinyon (P.
quadrifolia), Mexican pinyon (P. cembroides), alligator juniper (J. deppeana), Rocky Mountain juniper (J.
scopulorum), and California juniper (J. californica) [74,104,153,156,163].
Colorado pinyon is generally replaced by singleleaf pinyon in pinyon-juniper woodlands on the western edge of its
distribution [167].
Shrub species occurring as understory dominants with Colorado pinyon are pointleaf
manzanita (Arctostaphylos pungens) [102,153], big sagebrush (Artemisia
tridentata) [102,153,169], true mountain-mahogany (Cercocarpus montanus) [11,72,74,102,153,169], rubber
rabbitbrush (Chrysothamnus nauseosus) [11,102,153], Stansbury cliffrose (Purshia
mexicana var. stansburiana), antelope bitterbrush (Purshia
tridentata) [102,153],
Gambel oak (Quercus gambelii) [51,64,72,74,102,153], gray oak (Q. grisea) [74], wavyleaf oak
(Q. undulata) [102,153], blackbrush (Coleogyne ramosissima) [102,153], Nevada
ephedra (Ephedra nevadensis), broom snakeweed (Gutierrezia
sarothrae) [169], and plains prickly-pear (Opuntia polyacantha) [51]. Herbaceous
species occurring as understory dominants with Colorado pinyon include blue grama
(Bouteloua gracilis) [51,74,102,153], Arizona fescue (Festuca arizonica) [102,153],
mountain muhly (Muhlenbergia montana) [74],
New Mexico muhly (M. pauciflora) [153], mutton grass (Poa
fendleriana) [102,153], galleta (Pleuraphis jamesii) [51], Columbia needlegrass
(Achnatherum nelsonii) [102,153], and sand bluestem (Andropogon
gerardii var. paucipilus) [102].
Classifications identifying Colorado pinyon as a plant community dominant are
listed below:
The edible seeds of Colorado pinyon are gathered from native stands and marketed commercially [26,34,49,105,112,133,147,167]. Traditionally, these seeds were an important dietary supplement for Native Americans in the Southwest [19,30,81,98,116]. Traditional Native American uses for Colorado pinyon pitch include medicinal purposes and waterproofing of baskets and clay water bottles [30,81,116]. Colorado pinyon is used for Christmas trees [34,40,49,105] and landscaping [49].
Wood Products: The wood of Colorado pinyon is narrow-ringed, hard, and very brittle [64]. Wood products from Colorado pinyon include fuelwood, charcoal, mine timbers, railroad crossties, lumber, fenceposts, and pulpwood [30,34,49,64,98,105,112,116,167].
Colorado pinyon regenerates solely from seed; asexual regeneration has not been documented [5]. The natural reproduction of Colorado pinyon is limited due to unfavorable climate, infertility of the seed, rapidly declining germination of seed produced, and loss of seed to vertebrates and insects [159].
Breeding system: Colorado pinyon is generally monoecious [34,48,59,97,139,167]. The upper crown of Colorado pinyon tends to bear more ovulate than staminate cones and the opposite is true of the lower crown, though there may be broad overlap [97]. Some dioecious individuals do occur [48,59]. Dioecy may be more prevalent in younger age classes, perhaps as an adaptation to arid environments or other stress [47,48,59]. Dioecy may also occur in response to site factors, with male trees potentially predominating on south-facing slopes; however, clear connections between site conditions and Colorado pinyon breeding systems have not been established [46]. In 1 study, monoecious trees produced significantly more (p<0.001) empty seeds than female dioecious trees, and seedlings from monoecious trees were shorter and less vigorous than seedlings from dioecious trees [48]. Laboratory experiments have found 57% survival of self-pollinated cones and 83% survival of outcrossed cones. Outcrossed cones produced more seeds per cone than self-pollinated cones [97].
Pollination: The pollen of Colorado pinyon is carried for miles by the wind [34,97].
Seed production: Colorado pinyon may start bearing cones at 25 years. Good seed production occurs on trees that are 75 to 100 years old, with maximum seed production occurring on trees 160 to 200 years of age [34,59,139,159]. Large seed crops are produced every 3 to 7 years and are adversely impacted by water stress [40,61,112,139]. The periodicity of seed crops is related to the drain of nutrients required to produce a large crop and the time required for nutrients to be replenished [40]. Cones require 3 years to mature [59,61,108,139]. Colorado pinyon cones and seeds are attacked by a variety of insect species, which may destroy large portions of seed crops [121,139].
Seed dispersal: The wingless seeds of Colorado pinyon are dispersed by birds [5,20,34,68,98,101,112,166] and small mammals, primarily squirrels and chipmunks [5,20,21,34,68,112]. Four species of birds cache Colorado pinyon seeds: Clark's nutcrackers, scrub jays, pinyon jays, and Steller's jays [34,68,139]. However, most seeds are cached outside the elevational range of Colorado pinyon by Clark's nutcracker and to some extent, pinyon and Steller's jays. Scrub jays that live permanently in pinyon-juniper woodlands cache substantial numbers of seeds, making these birds locally important in Colorado pinyon regeneration [68].
Failure of Colorado pinyon cones to open, possibly due to a wetter spring moisture regime, renders seeds more difficult to access and reduces seed dispersal, particularly dispersal by small mammals [43].
On sloping sites, Colorado pinyon seeds may be washed "some distance away" by runoff [64].
Seed banking: In general, Colorado pinyon has short-lived seeds. As a result, seeds form only a temporary seed bank, with most seeds germinating the spring following dispersal. The potential for a large temporary seed bank is high following years of good seed production, while in other years the seed bank is likely sparse [20].
Germination: Colorado pinyon seeds generally germinate in the shade of a tree or shrub rather than in open grassland [34,58,105]. Germination occurs in response to moisture and moderate temperatures of 65 to 75 degrees Fahrenheit (18-24 oC). It occurs in the spring after snowmelt and/or warming temperatures [32,34,58,59,112]. If moisture conditions are not suitable, Colorado pinyon seeds may not germinate until the summer monsoon season [59]. Floyd [47] found that germination of Colorado pinyon occurs optimally at a mean temperature of 68 degrees Fahrenheit (20 oC) and 15 hours of light, with germination rates ranging from 25 to 65% [47]. Cold stratification may result in more rapid seed germination [60]. Floyd [45] found that germination and establishment of Colorado pinyon may be enhanced under a Gambel oak canopy, possibly due to increased moisture retention by litter, shading, decreased evapotranspiration in oak stands, and reduced seed predation. One study found higher Colorado pinyon germination rates when scrub jays cached seeds under junipers or near/under bushes, while very few seeds germinated if cached in the open [68].
Viability of fresh seeds varies between 85 and 95%. Seed viability decreases rapidly in 1 year or less, and the rate of germination is low [34,112].
Seedling establishment/growth: Reproduction of Colorado pinyon is generally sparse and scattered due to removal of seeds by birds and mammals [64], and seedling establishment is dependent on chance dispersal to favorable sites and ample rainfall [86]. More seedlings establish under trees or shrubs than away from them [59,71,86]. Colorado pinyon seedlings require extra moisture or shade until their elongating taproots reach deeper substrates [117]. Taproots of 1-year-old seedlings in northern Arizona averaged 8 inches (20.5 cm) long with a range of 6.7 to 10.6 inches (17-27 cm). Height of 1-year-old Colorado pinyon seedlings averaged 2 inches (5 cm) on northern Arizona sites [71]; growth was estimated at 1 inch per year for the first 10 years [34,105]. Biomass of 1-year-old seedlings is distributed evenly between shoot and root growth [71]. Seedlings growing in partial shade until they reach about 12 inches (30 cm) in height experience better early growth than those in complete shade under mature trees [71,112]. Water is the primary limiting factor in seedling survival and growth [112]. Competition for moisture usually results in the suppression of smaller trees, though they gradually resume normal growth when released from severe competition [59,112]. Saplings grow 4 to 6 inches (10-15 cm) in height annually, and mature trees grow 2 to 4 inches (5-10 cm) annually [59,139]. Mean annual diameter growth of Colorado pinyon is approximately 0.7 inch (1.8 cm) per decade when trees are about 50 years old [139].
Pinyon-juniper stands have slow succession rates [44,80]. Colorado pinyon occurs as an early to late-seral or climax species [31,34,172]. Following a fire in a pinyon-juniper stand in southern Colorado, Colorado pinyon began establishing in postfire year 25 [31]. Successional pathways of pinyon-juniper stands are indeterminate, and conditions after disturbance are generally less stable than the late seral, tree-dominant communities [63]. Factors that influence the pattern of succession after fire include past use history, site factors, moisture regime, stand age when disturbed, fire severity, presence of residual trees, and the presence of animal dispersal agents [14,141]. Seedlings may appear as early as the 1st postfire year [172], potentially the result of effective seed dispersal by animals, a wet moisture regime, and suitable shady sites provided by residual trees.
The general successional recovery after fire in dense stands of pinyon-juniper begins with the establishment of annuals, a stage that may peak in the 2nd and 3rd postfire years. A perennial grass stage follows, in which perennials are more abundant than annuals, with a shrub stage developing soon after. The re-establishment of trees during the shrub stage then leads to the pinyon-juniper climax, presuming no further fire occurrence [5,14,25,32,57,131,141]. The suppression of shrubs by mature trees may take up to 100 years [31,32], and climax stands may require 300 years to develop [25]. Frequent disturbance in these woodlands maintains earlier seral stages (e.g. the open shrub stage) [25,63,80]. As pinyon-juniper crown cover increases, cover, productivity, and density of understory species decrease. The understory is generally most productive, diverse, and responsive to disturbance when pinyon-juniper crown cover is at or below 20%. When crown cover exceeds 20 to 30%, understory thinning accelerates [77]. "Old-growth" stands of pinyon-juniper are fairly open and contain a cohort of dominant old, slow-growing trees with little or no understory of grass or shrubs. Down dead material is common, as is dead material on the live trees [113].
Colorado pinyon is intolerant of shade in all but the seedling stage of its growth [58,64,139,141,159]. "Nurse plants" are required during this stage to protect the seedlings from excessive drying and heating [14].
The currently accepted scientific name of Colorado pinyon is Pinus edulis
Engelm. (Pinaceae) [26,41,73,84,85,109,133,166,167].
Where the ranges of both species overlap, Colorado pinyon frequently
hybridizes with singleleaf pinyon (P. monophylla)
[27,95,96,98,99,100,106]. The eastern edge of the Great Basin is the rough boundary
between Colorado and singleleaf pinyons, and forms a "zone of
hybridization" [95]. Colorado pinyon also hybridizes
with Mexican pinyon (P. cembroides) [106].
Colorado pinyon is used to rehabilitate mined areas and critical habitats that have been damaged by fire [61,173].
Artificial regeneration: Dormancy of Colorado pinyon seeds is broken by soaking the seed for 1 to 2 days, followed by cold stratification at temperatures between 33 and 41 degrees Fahrenheit for 0 to 60 days [61,92].
