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

GENERAL BOTANICAL CHARACTERISTICS:

The needles of singleleaf pinyon are solitary, rigid, and 1 to 1.4 inches (2.5-3.5 cm) long. The sheaths deciduous [232]. The single needle (leaf) is unique among pines of the world [121]. Singleleaf pinyon needles are long-lived (5-12 years) [83,139]. Their longevity is considered an extreme expression of "evergreenness," giving the tree the ability to conserve nutrients to take advantage of short favorable conditions within a generally unfavorable period [139]. The allelopathic effects of singleleaf pinyon needle litter on germination and growth of herbaceous plants has been documented [60], and is attributed to terpene hydrocarbons [241]. These compounds also impede decomposition, are highly flammable, and are readily volatilized by fire [239].

Singleleaf pinyon bears ovulate cones, 1.4 to 2.2 inches (3.5-5.5 cm) long, with thick scales. Cones bear large (0.4-0.7 inch (10-17 mm) long), heavy (0.01 oz (400 mg)), moderately thin-shelled, edible, wingless seeds [137,211,232]. Cones that dry and open in the fall drop to the ground in winter or spring and may form a conspicuous litter layer [83,124,142]. 

Singleleaf pinyon is long- lived. On fire-safe sites, large trees can monopolize site resources over a life span of 350 years or more [63]. Dominant pinyons are often 400 years old and have been known to reach 800 to 1000 years [105,174]. Mature singleleaf pinyon trees (over 200 years) are relatively uncommon [21,98]. Of the 20 million acres of pinyon-juniper stands in Nevada and Utah, about half are estimated to be between 40 and 120 years old, with almost 20% in Utah and 9% in Nevada over 200 years of age [162,163]. Biomass estimates for singleleaf pinyon are available for Nevada [36,37,146].

Hybrids are diagnosed by frequency of monophylly and leaf resin canal number [75,119,127]. Hybrids of singleleaf and Colorado pinyon have a mixture of 1- and 2-needle fascicles on the same branchlets, fewer resin ducts than singleleaf pinyon but more than Colorado pinyon, and intermediate cone size [121].

RAUNKIAER [170] LIFE FORM:

REGENERATION PROCESSES:

Singleleaf pinyon generally begins bearing cones at about 35 years of age, begins producing good seed crops at about 75 to 100 years, and reaches maximum production at about 160 to 200 years [58,142]. Singleleaf pinyon exhibits region-wide synchrony in cone production, masting every 2 to 3 years [35,215]. Some seeds may be produced every year [99], and good seed crops occur somewhere over a geographical area (e.g. the Great Basin) nearly every year [142]. A 5-year study reported per acre cone production as follows: 1975, 765 cones; 1976, 0 cones; 1977, 2,560 cones; 1978, 2,325 cones; 1979, 585 cones [99]. Mast years in singleleaf pinyon may be related to the polar front jet stream [161]. Singleleaf pinyon is said to be more productive and predictable than Colorado pinyon, with seed production being predicted fairly accurately 2 years in advance and more accurately 1 year in advance [99]. Seed production and survival may be affected by several insect pests of cones and seeds. For example, the pinyon cone beetle can destroy more than 50% of the crop of its host pinyon, while coneworms tunnel in cones and shoots but are of minor importance [38]. For other pests, such as the pinyon cone borer, the magnitude of the effects is unknown [147].

Because singleleaf pinyon seeds are totally wingless, seed dispersal is dependent on vertebrate dispersers that store seeds in food caches, where unconsumed seeds germinate. This dispersal mechanism is a good example of a co-evolved, mutualistic, plant-vertebrate relationship [58,123,213]. The cone scales of singleleaf pinyon have a membranous tissue that holds the seed in place after the cones open, protecting them from ground-foragers and keeping them available to avian dispersers. Several corvids are responsible for dispersal of singleleaf pinyon seeds over distance. Scrub and Steller's jays forage alone or in pairs, carrying 1 to a few seeds less than a mile before burying them. Pinyon jays forage in flocks of hundreds, carrying about 40 nuts each up to 5 miles (8.3 km) before caching them in the soil. Clark's nutcrackers forage in somewhat smaller flocks, each carrying several dozen seeds a distance of up to 13 miles (22 km) and burying 1 to 15 seeds per cache, 1 to 3 cm deep in gravelly soil, mineral soil, or duff [122,123,211]. Seed caching by Clark's nutcrackers begins late August to early September [211]. In a good seed crop year, an individual Clark's nutcracker may scatter-hoard 17,900 singleleaf pinyon seeds [35,211]. The large range of singleleaf pinyon may be attributable, in part, to seed dispersal by these birds [137]. Seed dispersal by humans in the past has also been suggested [126]. Chipmunks, squirrels, deer mice, pinyon mice, Great Basin pocket mice, and Panamint kangaroo rats all scatter-hoard singleleaf pinyon seeds locally [35,121]. These animals consume most of the seed, but some is left to germinate. Quantitatively, these rodents are less effective than avian dispersers, since it is only in mast years that large numbers of seeds fall to the ground and become available to rodents. The seed characteristics and the microhabitats in which seeds are placed are important in determining their fate after dispersal. Rodents are qualitatively effective dispersers of singleleaf pinyon seed since they tend to bury seeds under and adjacent to shrubs, whereas avian dispersers tend to cache seeds in interspace environments, a less suitable environment for singleleaf pinyon seedlings [35,180]. 

In general, pinyon seeds are short-lived with little innate dormancy and thus form only a temporary seed bank [35,58,143]. Fresh seeds have 85 to 95% viability, but this decreases in a year or less [58]. Most seeds germinate the spring following dispersal [35], requiring 28 to 90 days of cold stratification for germination [125]. Germination and establishment are most likely when favorable moisture conditions follow a mast year. Seeds may germinate in the open, but seedling establishment in the open is rare [63,143]. 

Singleleaf pinyon seedling establishment is episodic. Population age structure is affected by drought, which differentially reduces seedling and sapling recruitment more than other age classes [185]. Top-growth of seedlings is slow (1.0 inch (2.5 cm) per year in height, and 0.012 inch (0.3 mm) per year in diameter). Root growth is more rapid, with the taproot reaching 6 inches (15 cm) 10 days after germination [142]. Seedlings can thereby withstand soil water below the wilting point for about 2 weeks. However, field drought conditions are often more severe than this, and so seedlings only survive in the most favorable microenvironments [58]. Singleleaf pinyon seedlings survive best in the microhabitat provided by nurse plants, where organic matter, nutrient concentrations, relative humidity, water infiltration, and water holding capacity tend to be higher, and irradiance and soil temperatures tend to be lower [31,32,35,63,105,143]. However, nurse plants also compete for water and nutrients, so the trade-off is slower seedling growth rate [32,35]. Seedlings maintain a more favorable water status and have greater drought avoidance than shrub nurse plants [44,50]. The complex interaction between seedlings and nurse plants is a balance between facilitation and competition on moisture and light gradients [33]. 

The ecotones between singleleaf pinyon woodlands and adjacent shrublands and grasslands provide favorable microhabitats for singleleaf pinyon seedling establishment since they are active zones for seed dispersal, nurse plants are available, and singleleaf pinyon seedlings are only affected by competition from grass and other herbaceous vegetation for a couple of years. This facilitates expansion of woodlands along these ecotones, with singleleaf pinyon seedlings eventually overtopping and shading out the shrubs. Conversely, singleleaf pinyon seedlings establishing under adult trees have little chance of maturing unless the adult tree is removed or dies [35,44,50].

Singleleaf pinyon is slow growing. A dominant tree requires about 60 years to reach 6.6 feet (2 m) in height, and about 150 years to attain 28 feet (8.5 m) in height and a stump height diameter of about 12 inches (30 cm) [142,143]. Average annual height and diameter growth of immature dominants is about 2 inches (5 cm), and 0.04 to 0.20 inch (1 to 5 mm), respectively. Growth rates vary considerably even among trees on identical sites, and are greatly influenced by competition for severely limited water supplies. Dominant trees may maintain constant diameter growth rates for more than 200 years. Observed reductions in growth rates with age are likely caused by increasing competition as stands develop, and no definite age of culmination of growth has been determined [141,142].

SITE CHARACTERISTICS:

Elevational range of singleleaf pinyon is generally 3,280 to 9,186 feet (1000-2800 m). In Baja and parts of southern California it may be found below 3,280 feet (1000 m). The lower elevational limit in the high desert of the Great Basin is just above the elevation of the adjacent valleys, varying from 4,987 to 6,988 feet (1520-2130 m). The upper limit of singleleaf pinyon varies with climate and competing tree species, but it has been found as high as 10,000 feet (3050 m) in the White Mountains of California [124]. The frequency of monophylly was found to be inversely related to elevation in Zion National Park, with a narrow elevational range up to about 5,600 feet (1860 m); Colorado pinyon occurred at higher elevations and hybrids of the 2 species were distributed throughout the elevational ranges of both [75,127]. The relationship of monophylly to elevation was also observed in the New York Mountains in California and was interpreted as a variation of character along a moisture-temperature gradient [243], with Colorado pinyon occupying higher elevation sites and singleleaf pinyon lower sites [214].

Singleleaf pinyon is the most xeric pine in the United States. Its mean annual precipitation range is 8 to 18 inches (200-460 mm), with most precipitation falling December through April. Its mean annual air temperature is 50 degrees Fahrenheit (10 °C), ranging from 21 degrees Fahrenheit (-6 °C) in January to 86 degrees Fahrenheit (30 °C) in July [142]. Annual stem growth, needle length and percentage of double-needled fascicles all had significant positive correlations (p<0.05) with annual precipitation received prior to the completion of each year's growth [202]. Mature singleleaf pinyon is not shade tolerant; however, water rather than light is the limiting factor in survival and growth of this species [143]. Results of a Nevada study suggest that the ability of singleleaf pinyon to exist on a wide range of environmental conditions is not a function of variable ecophysiological responses, but an opportunistic response to the availability of resources and conditions suitable for growth [97].

Elevation and precipitation ranges are as follows:

Pinyon-juniper woodlands occur on many types of soils and parent materials. Singleleaf pinyon typically grows on shallow, well-drained, low fertility soils, although it has been found on more productive soils as well [58,80,81]. Singleleaf pinyon in shallow soils tend to grow more slowly than those in deeper soils [215]. Surface soil pH usually is between 6.0 and 8.0 [80]. A study in the Great Basin found singleleaf pinyon was most common on granitic parent material (33% cover), followed by alluvial parent material (9.5% cover), and finally limestone parent material (0.3% cover). It was not found on quartzite or sandstone. Singleleaf pinyon was most commonly found on soils that contained 15-35% skeletal material by volume [89]. 

While singleleaf pinyon can grow on a wide variety of soil types and environmental conditions, composition and distribution of associated understory species may be driven by the plants' position relative to the tree crown [69], soil type [59,237], the seasonality and effectiveness of precipitation [234], and/or the distribution of nutrients [209]. Summerfield and others [191] found that soils supporting singleleaf pinyon stands in western Nevada commonly had mollic epipedons, argillic horizons, shallow depth to bedrock, mesic temperature regimes, and low available water capacities. These soils are well suited for producing woodlands, but have low potential for forage production. A study in the Great Basin in Nevada found that singleleaf pinyon was absent from sites with hydrothermally altered andesite parent material. Researchers concluded that the absence of singleleaf pinyon was more likely due to the absence of big sagebrush nurse plants than to substrate-induced nutrient limitations, since it was able to grow on this soil in the greenhouse [32,45,177]. 

SUCCESSIONAL STATUS:

A general successional pattern in pinyon-juniper after overstory removal may be as follows: grasses and forbs dominate for the first 10 years; shrubs are well established within 20 years and dominate at 30 years; between 10 and 20 years, tree seedlings appear, their presence becoming important after about 50 years; site is again woodland with low understory cover after 70 to 80 years [200,201]. A fire chronosequence study in the Great Basin followed this general pattern for grasses, forbs, and shrubs over time, although grass and shrub cover was maintained as late as 115 years following fire, and singleleaf pinyon had less than 10% cover 115 years after fire [186]. After fire disturbance, several successional pathways from annual to shrub dominance are possible, including initial postfire dominance by shrubs [61,69].

Successional stages in pinyon-juniper woodlands often follow the model of initial floristics, having the same species present in different amounts and dominance on the landscape over time [58,68]. Singleleaf pinyon may be present in early to mid-succession, but slow growth and establishment preclude early dominance [69,112]. Postfire succession may be to native species present in the preburn community or seed bank, or may be to invasive exotics such as cheatgrass that subsequently inhibit shrub and tree establishment [60,142]. Early successional stages following fire or tree harvesting are often dominated by several weedy annuals and sprouting shrubs. Disturbed sites may also see the establishment of non-sprouting shrubs [142]. Later successional stages from shrub to tree dominance have been studied in a few specific sites [5,60,69,203,205]. Variability in tree- and shrub-dominated communities complicates extrapolation of these results to sites of different growing conditions [196].

Once trees establish, they continually increase their dominance of the site. Trees start competing with the understory when they are approximately double the size of the shrub nurse plant [60], and can exclude the understory within a 100-year period [60,113]. The litter of singleleaf pinyon inhibits establishment of understory species through allelopathy, especially affecting Idaho fescue and Sandberg bluegrass, with cheatgrass and bottlebrush squirreltail being the least affected. An understory may be maintained by periodic fire, reducing tree density [60].

Because a number of variables may affect succession, following a standard successional model to predict stand development may be unwise [60]. Most successional projections come from stands that have been grazed [5,60,69], and response on grazed and ungrazed sites will be floristically different [61]. Seed reserves in soil under singleleaf pinyon may also decrease in number and species diversity from early to late succession [113]. Koniak [112] found that aspect and elevation influenced postfire community after fire in a Great Basin singleleaf pinyon stand, with north and east slopes supporting a higher cover and occurrence of shrubs, perennial grasses and perennial forbs, while south and west slopes generally had high cover and occurrence of annual forbs and annual grasses. Goodrich and others [77] provide a general overview of succession in pinyon-juniper woodlands, and its application to management for different uses based on successional stage.

SEASONAL DEVELOPMENT:

Related categories for SPECIES: Pinus monophylla | Singleleaf Pinyon