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	Twoneedle pinyon on the rim of the Grand Canyon. Wikimedia Commons image by Antoine Taveneaux.

INTRODUCTORY

• AUTHORSHIP AND CITATION
• FEIS ABBREVIATION
• SYNONYMS
• NRCS PLANT CODE
• COMMON NAMES
• TAXONOMY
• LIFE FORM
• FEDERAL LEGAL STATUS
• OTHER STATUS

AUTHORSHIP AND CITATION:
Anderson, Michelle D. 2002. Pinus edulis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.usda.gov /database/feis/plants/tree/pinedu/all.html [ ].

Revisions: On 28 August 2018, the common name of this species was changed in FEIS
from: Colorado pinyon
to: twoneedle pinyon. Images were also added.

Where the ranges of both species overlap, twoneedle 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 twoneedle and singleleaf pinyons, and forms a "zone of hybridization" [95]. Twoneedle pinyon also hybridizes with Mexican pinyon (P. cembroides) [106].

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• ECOSYSTEMS
• STATES
• BLM PHYSIOGRAPHIC REGIONS
• KUCHLER PLANT ASSOCIATIONS
• SAF COVER TYPES
• SRM (RANGELAND) COVER TYPES
• HABITAT TYPES AND PLANT COMMUNITIES

Distribution of twoneedle pinyon. Map courtesy of USDA, NRCS. 2018. The PLANTS Database. National Plant Data Team, Greensboro, NC. [2018, August 28] [160].

Shrub species occurring as understory dominants with twoneedle 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 twoneedle 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 twoneedle pinyon as a plant community dominant are listed below:

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

The needles are 0.6 to 2.0 inches (1.5-5 cm) long [167] and in fascicles of 2 [126]. Needles remain on the tree for approximately 9 years. Bark thickness of twoneedle 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 twoneedle pinyon.

Twoneedle 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].

Twoneedle 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 twoneedle pinyon in woodland communities ranges from none or few to several hundred stems per hectare [139].

Open twoneedle pinyon cone with seeds. Wikimedia Commons image by S. King, USDI, NPS.

Breeding system: Twoneedle pinyon is generally monoecious [34,48,59,97,139,167]. The upper crown of twoneedle 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 twoneedle 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 twoneedle pinyon is carried for miles by the wind [34,97].

Seed production: Twoneedle 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]. Twoneedle 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 twoneedle 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 twoneedle 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 twoneedle 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 twoneedle pinyon regeneration [68].

Failure of twoneedle 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, twoneedle pinyon seeds may be washed "some distance away" by runoff [64].

Seed banking: In general, twoneedle 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: Twoneedle 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, twoneedle pinyon seeds may not germinate until the summer monsoon season [59]. Floyd [47] found that germination of twoneedle 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 twoneedle 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 twoneedle 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 twoneedle 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]. Twoneedle 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 twoneedle 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 twoneedle pinyon is approximately 0.7 inch (1.8 cm) per decade when trees are about 50 years old [139].

Elevation: Pinyon-juniper woodlands occur in the foothills above desert shrub or grassland vegetation but below ponderosa pine forest [25,101,127]. Twoneedle 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, twoneedle pinyon tends to increase in abundance with increasing elevation, while junipers decrease [25,34,123,127]. Twoneedle 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 twoneedle pinyon by state:

Climate: Twoneedle 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]. Twoneedle 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]. Twoneedle 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]. Twoneedle 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].

Twoneedle pinyon is tolerant of cold and drought [101,176]. According to field studies using simulated rainfall events, twoneedle pinyon can respond effectively to both monsoon precipitation and small rainfall events [170].

Soils: Twoneedle 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]. Twoneedle pinyon growing in deeper soils generally grow faster than those in shallow soils [159]. Twoneedle 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]. Twoneedle 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 twoneedle pinyon canopies than in adjacent shrub-dominated areas. Accrual of nitrate and sulfate is also evident under twoneedle pinyon trees, as is higher average concentration of phosphorus and boron, which may be phytotoxic to some herbaceous species [9]. Twoneedle pinyon litter is specifically associated with a reduction of blue grama production [79].

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].

Twoneedle 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].

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Fire adaptations: Mature twoneedle 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, twoneedle 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 twoneedle 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 twoneedle 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 twoneedle 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 twoneedle 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 twoneedle pinyon are difficult to quantify because living fire-scarred trees are rare: twoneedle pinyon is often killed directly by fire or indirectly due to increased susceptibility to heart rot [16,125]. Though fire-scarred twoneedle 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 twoneedle 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 twoneedle pinyon to replace extensive shrub vegetation [87]. Frequent fire may prevent the expansion of twoneedle 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 twoneedle 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 twoneedle 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".

FIRE EFFECTS

• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

Prescribed burning to establish and maintain open areas, early seral plant communities, plant diversity, and herbage production in pinyon-juniper woodlands may provide habitat for wildlife, particularly deer, elk, and bighorn sheep [33,65,125,145]. Fire in open pinyon-juniper stands with even a scattered understory of perennial grasses will increase forage production through a reduction in competition for moisture and light [78]. Mule deer may utilize burned pinyon-juniper woodlands more heavily as winter range, as long as some unburned areas remain close to the burned sites [110]. 

Burning after herbicide application may be more effective in reducing twoneedle pinyon and increasing forage production than either treatment alone [172]. Burning in dense stands with little or no understory is difficult due to lack of fine fuels [145,172]. Allowing the herbaceous understory to recover from herbicide treatment may increase the fine fuels that carry fire, thus improving the effectiveness of fire treatments [172].

Prescribed burning of large slash piles is not recommended because of soil degradation. However, small piles of slash may be burned in low-severity fires to encourage herbaceous vegetation. Slash burning is less desirable in partially harvested stands, where selection or shelterwood methods have been used to sustain tree production because burning tends to damage residual trees, especially where slash has accumulated at the base [125].

Twoneedle pinyon seedlings growing in severely burned soils are generally smaller than those in lightly burned or unburned soils. Elevated concentrations of extractable nitrogen and phosphorus may be present for up to a year following severe burning, and these concentrations may be high enough to reduce pinyon seedling growth [70]. Beyond these initial effects, fire may result in a sizeable loss of nitrogen and other nutrients from the floor of pinyon-juniper woodlands, mostly due to loss of the litter layer [23,88,112,157]. Burning may result in short-term increases in ammonium, which in combination with soil heating, affects the processes of nitrogen mineralization, nitrification, and nitrogen immobilization. One study in Arizona found immediate increases in soil ammonium and nitrate after burning. However, within 90 days of burning the ammonium levels approximated those in unburned controls while nitrate levels in controls were 5 to 15 times the levels on burned sites [88]. Other studies also demonstrate that prescribed burning of pinyon-juniper slash following fuelwood harvest results in short-term increased soil nitrogen. Immediate increases in ammonium following burning have been observed 1 year after fire, nitrate concentrations increase while ammonium concentrations decrease, and by postfire year 5, no increased levels of nitrogen in either form remain detectable [24]. Postfire nitrification rates appear to be related to the degree of soil heating, which affects nitrogen-fixing microbes [88]. Slash burning following chaining may also increase soil moisture after the 1st postburn year [55], though adverse effects include temporarily reduced infiltration rates and a subsequent increase in erosion and runoff [56].

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER USES
• OTHER MANAGEMENT CONSIDERATIONS

Pinyon-juniper woodlands are important winter ranges for mule deer [34,98], providing cover, shelter, and understory forage [34]. Twoneedle 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 twoneedle 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 twoneedle pinyon [22]. Seeds are an important food for small mammals, primarily chipmunks and squirrels [21].

Palatability/nutritional value: Twoneedle 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: Twoneedle 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].

Artificial regeneration: Dormancy of twoneedle 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].

Wood Products: The wood of twoneedle pinyon is narrow-ringed, hard, and very brittle [64]. Wood products from twoneedle pinyon include fuelwood, charcoal, mine timbers, railroad crossties, lumber, fenceposts, and pulpwood [30,34,49,64,98,105,112,116,167].

Two years after a chaining treatment in a pinyon-juniper stand in Utah, trees were reduced to 6% of the total cover (from 26% pretreatment), while herbaceous plants increased from 2 to 13% and shrubs increased from 2 to 8%. After 24 years, trees and shrubs comprised 84% of total cover [148]. Another study in Utah found that 26 years after chaining, the density of twoneedle pinyon remained 26% lower than on untreated sites [161]. Small trees left after mechanical treatment may grow faster after release from the dominance of the larger overstory trees, reducing or eliminating any gains in herbage yield [6]. A study in New Mexico found that 20 years after a cabling treatment, density of twoneedle pinyon was almost the same as on uncabled control sites, though basal and aerial cover were lower than on controls. Herbage production on these sites did not increase appreciably following mechanical treatment [138]. Another study in New Mexico found that twoneedle pinyon increased coverage 28 years after chaining [141]. Most trees that remain alive on a chained area will increase in size and become more visually apparent; some recruitment of new seedlings will also occur [151]. Twoneedle pinyon may be slow to re-establish following the mechanical removal of trees, but once tree density reaches 48 trees per acre (120 trees/ha) the rate of re-establishment may increase. This rate increase is likely the result of suppression of competing herbaceous growth and the accumulation of moist shady litter to benefit twoneedle pinyon seedlings [144].

The effects of chaining on infiltration rates are inconclusive, though chaining followed by seeding to grassland likely reduces sediment yield [171].

An increased use of cabled or chained areas by mule deer in spring and summer may be related to an increase in forage, though treatments may also reduce the amount of hiding or escape cover [75,76]. Small mammals may also prefer mechanically treated areas due to increased forage production and the potential increased cover availability in the form of debris and slash piles [75,76,124,143]. However, conversion projects (e.g. chaining and cabling) often fail to increase livestock carrying capacity for more than a few years [112], and use of mechanically treated areas by birds is often less than on control plots [143].

Though chaining and cabling are often used to increase forage production, the result may be invasions of weedy species. On a chained site in Utah, bur buttercup (Ranunculus testiculatus) and cheatgrass (Bromus tectorum) occurred in high numbers following chaining and seeding of herbaceous species. However, these weedy annuals decreased 85% and 88%, respectively, by the 3rd posttreatment year as the seeded perennials became more established [13].

Twoneedle pinyon may be effectively controlled with applications of tebuthiuron and picloram [111,162,172]. Medium-sized and large saplings are more readily defoliated by tebuthiuron than seedlings and small saplings [162], though trees shorter than 4 feet (1.2 m) may experience higher mortality if herbicide treatment is followed by fire [172]. High levels of trampling may also damage twoneedle pinyon, reducing seedling density [158]. 

Forage production in pinyon-juniper stands is substantially increased by protection from grazing; ungrazed sites in New Mexico produced up to twice as much forage as grazed sites after about 20 years [132]. Heavy grazing, on the other hand, may result in an increase in the density and extent of twoneedle pinyon stands [136,137]. Grazing hastens establishment of twoneedle pinyon trees by reducing competition from the herbaceous understory [144,168]. The resulting scattered understory reduces the opportunity for fire in pinyon-juniper communities [168].

Pinus edulis: References

1. Alexander, Robert R. 1987. Classification of the forest vegetation of Colorado by habitat type and community type. Res. Note RM-478. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 14 p. [9092]

2. Allen, Rogert B.; Peet, Robert K. 1990. Gradient analysis of forests of the Sangre de Cristo Range, Colorado. Canadian Journal of Botany. 68: 193-201. [11231]

3. Arno, Stephen F. 2000. Fire in western forest ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 97-120. [36984]

4. Arno, Stephen F.; Wilson, Andrew E. 1986. Dating past fires in curlleaf mountain-mahogany communities. Journal of Range Management. 39(3): 241-243. [350]

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