Pinus monophylla Torr. & Frem.
Pinaceae -- Pine family
R. 0. Meeuwig, J. D. Budy, and R. L. Everett
Singleleaf pinyon (Pinus monophylla), also called pinyon, nut pine, one-leaf pine, and piñon (Spanish), is a slow-growing, low, spreading tree that grows on dry, low mountain slopes of the Great Basin. One large tree near Reno, NV, is about 112 cm (44.2 in) in d.b.h., 16.2 m (53 ft) tall, and has a crown spread of about 20 m (66 ft). Principal uses of the tree include fuel, fenceposts, Christmas trees, and edible seeds.
Singleleaf pinyon is the predominant tree species in the Great Basin. It dominates extensive areas in the dry mountain ranges of Nevada, southern and eastern California, and western Utah. Some stands are in Baja California, northwestern Arizona, and southeastern Idaho.
- The native range of singleleaf pinyon.
Singleleaf pinyon grows under more xeric conditions than any other pine in the United States. Its climate is similar to that of pinyon (Pinus edulis), but during the growing season, relative humidity and precipitation are even lower and potential evapotranspiration is greater.
Average annual precipitation ranges from about 200 mm. (8 in) to about 460 mm (18 in) but precipitation varies widely from year to year. Most of the precipitation occurs during the winter months (December to April), usually as snow. Mean annual temperature is about 10° C (50° F). The mean maximum temperature in July, the hottest month, is about 30° C (86° F). The mean minimum temperature in January, the coldest month, is about -6° C (21° F).
Tree growth usually starts in April and usually ceases in September or October. During most of this time, the trees depend on soil moisture stored by winter precipitation.
Singleleaf pinyon grows on pediments, slopes, and ridges. It grows best on coarse textured, well drained Mollisols and is rarely found on valley floors. Soils are both residual and alluvial and are derived from granite, rhyolite, andesite, limestone, and a variety of other parent materials. Soil depth and age are highly variable. Surface soil pH usually is between 6.0 and 8.0. Trees impact soils by changing soil nutrient distribution both laterally and vertically in the soil profile and concentrating nutrients under the crowns (5).
The elevational range of singleleaf pinyon is usually between 1000 m (3,280 ft) and 2800 m (9,200 ft), depending on local conditions. In the high desert of the Great Basin, the lower limit is somewhat above the elevation of the adjacent valleys, varying from about 1520 m to 2130 m (5,000 to 7,000 ft). In Baja California and parts of California, singleleaf pinyon can be found below 1000 m (3,280 ft) (9). The upper elevational limit varies with local climate and presence of competing tree species. Singleleaf pinyon has been reported at 3050 m (10,000 ft) in the White Mountains of California (21).
Throughout its range, singleleaf pinyon is the major component of the forest cover type Pinyon-Juniper (Society of American Foresters Type 239) (6). Utah juniper (Juniperus osteosperma) is present in varying amounts in most singleleaf pinyon stands. Pinyon is usually the dominant species, but juniper tends to be dominant along the northern geographical limit of pinyon and near the lower elevational limit of pinyon in many areas. Utah juniper is absent from some pinyon stands in western and southern Nevada and from most pinyon stands in southern California. California juniper (J. californica) replaces Utah juniper in some southern California pinyon woodlands (9).
Curlleaf mountain-mahogany (Cercocarpus ledifolius) is associated with singleleaf pinyon in many areas, particularly near the upper elevational limit of pinyon. In the Sierra Nevada, singleleaf pinyon can be found growing with Jeffrey pine (Pinus jeffreyi), ponderosa pine (P. ponderosa), and western juniper (Juniperus occidentalis). Singleleaf pinyon has been found growing with bristlecone pine (Pinus aristata var. longaeva) in the White Mountains of California (21), on several mountain ranges in Nevada, and in the San Francisco Mountains of southwestern Utah (14).
There is a great diversity of undergrowth vegetation associated with singleleaf pinyon woodlands, and coverages vary with site quality and successional stage. Early successional stages following fire or tree harvesting are often dominated by several weedy annuals: coyote tobacco (Nicotiana attenuata), ground smoke (Gayophytum ramosissimum), stickseed (Lappula redowskii) and root sprouting shrubs: rabbitbrush (Chrysothamnus spp.), ephedra (Ephedra spp.), snowberry (Symphoricarpos spp.), and desert peach (Prunus andersonii) (3). The invasion of Cheatgrass (Bromus tectorum) on disturbed pinyon sites can create a fire disclimax community that inhibits shrub and subsequent tree establishment. Other shrubs that often establish from soil seed reserves or immigrate rapidly to disturbed sites include mountain big sagebrush (Artemisia tridentata var. vaseyana), basin big sagebrush (A. tridentata var. tridentata), Wyoming big sagebrush (A. tridentata var. wyomingensis), low sagebrush (A. arbuscula), black sagebrush (A. nova), sulfur eriogonum (Eriogonum umbellatum), and antelope bitterbrush (Purshia tridentata). Associated grasses include Sandberg blue grass (Poa sandbergii), bottlebrush squirreltail (Sitanion hystrix), Nevada bluegrass (Poa nevadensis), Idaho fescue (Festuca idahoensis), bearded bluebunch wheatgrass (Agropyron spicatum), Great Basin wildrye (Elymus cinereus), and needle-and-thread grass (Stipa comata) (24). All perennial understory species decline during stand development.
Flowering and Fruiting- Singleleaf pinyon is monoecious and requires two seasons between flowering and seed ripening. Male and female strobili emerge from buds in late spring or early summer. The staminate cones usually begin to shed pollen by mid-June when the young ovulate cones are receptive to pollination. Growth and development of the cones are slow during the first growing season. By September the cones are about one-fourth their mature length. Fertilization occurs the following spring, about 1 year after pollination. The cones grow rapidly during the second growing season. Three to four cones may mature on each branchlet. As the cones mature in early September they turn from green to brown. Cone-opening may last from September to early November depending on environmental conditions and elevation.
Mature cones range in length from 3.7 to 8.3 cm (1.5 to 3.3 in) and average 5.5 cm (2.2 in). The number of sound seeds per cone varies from 2 to 60 with an average of 20. The number of seeds per kilogram ranges from 1,200 to 3,400 (540 to 1,540 seeds/lb), with an average of 2,000 seeds per kilogram (900/lb). The viability of fresh seeds is high, but shelf life is short. The germination of some seed lots decreased from 90 percent before storage to 10 percent after 6 months of cold storage. Because the average moisture content of freshly collected seeds is 30 percent (wet weight basis), seeds should be dried before storing.
Seed Production and Dissemination- Seed production by singleleaf pinyon is highly variable. It varies from tree to tree, from year to year, and from place to place. The interval between good seed crops in any particular area varies from 3 to 7 years, yet good seed crops occur somewhere in the Great Basin woodlands nearly every year. Generally, singleleaf pinyon trees do not begin bearing cones before they are 35 years old and do not begin producing good seed crops earlier than 100 years. A productive tree yields about 5 kg (11 lb) of seeds in a good year, but some trees never yield good crops.
The heavy wingless seeds are not adapted to wind dispersal but tend to fall to the duff under the parent tree. The seeds are disseminated locally by rodents and over distances by birds. These animals consume most of the seed but leave some to germinate in place or in caches, especially when seed production is above average.
Seedling Development- Pinyon depends upon a standing crop of seedlings for species perpetuation. Seedlings require a nurse crop; thus, most seedlings are found under shrubs in mid succession and under the tree crown in late succession (4). Germination is epigeal. Although pinyon seedlings germinate in the open, few survive without some degree of protection from direct solar radiation. Top growth during the seedling stage is extremely slow. Seedling height growth usually is less than 2.5 cm (1.0 in) per year and diameter growth usually is less than 0.3 mm (0.012 in) per year. Root growth, however, is quite rapid. The thick taproot can reach 15 cm (6 in) in 10 days after germination. Seedlings growing under low shrubs usually have greater diameter growth and less height growth than those under tree canopies. Seedlings maintain a more favorable water status and have greater drought avoidance than the shrub nurse plant (2). This phenomena facilitates seedling survival and tree encroachment into adjacent shrub communities.
Vegetative Reproduction- Single leaf pinyon does not reproduce naturally by vegetative means.
Growth and Yield- Singleleaf pinyon is one of the slowest growing conifers. It usually requires about 60 years to attain a height of 2 m (6.6 ft). Average annual height growth of immature dominants is about 5 cm (2 in). Height growth rates vary considerably among individual trees, even among those on identical sites. Heights of mature dominants range from about 6 in to 12 m (20 to 40 ft), depending on site quality and genotype (17). One pinyon 14 m (46 ft) tall has been reported (19).
Because singleleaf pinyon tends to have large branches or multiple stems at breast height, diameter and basal area measurements are taken more conveniently near ground level, usually at 15 cm (6 in) above the ground (stump height). Diameter growth rates are greatly influenced by competition for severely limited water supplies. Average annual diameter growth of dominant trees is 1 to 5 mm (0.04 to 0.20 in). The average dominant tree takes about 150 years to reach a stump height diameter of 30 cm (12 in). There is no definite age of culmination of diameter growth; pinyon trees in strongly dominant positions can maintain essentially constant diameter growth rates for more than 200 years. Reductions in diameter growth rates are caused by increasing competition as stands develop, rather than by tree age (17).
Average annual basal area growth of fully stocked stands is about 0.25 m²/ha (1.1 ft²/acre) on typical sites, as low as 0.09 m²/ha (0.4 ft²/acre) on poor sites, and as high as 0.55 m²/ha (2.4 ft²/acre) on good sites. Average annual above-ground biomass accumulation rates of fully stocked stands varies from about 0.2 to about 2.2 t/ha (0.1 to 1.0 ton/acre), depending on site quality. On average sites it is about 0.9 t/ha (0.4 ton/acre) (17). Total above-ground biomass averages 150 t/ha (67 ton/acre) of which 60 percent is in the tree and 40 percent is in the forest floor.
Singleleaf pinyon trees more than 300 years old are fairly common on poor sites but rare on good sites. It appears that all the better sites were either burned in the past 300 years or have been cut over in the past century or so. The poorer sites are virtually fireproof because their sparse vegetation will not carry fire, and these sites were not cut because of the small size and poor form of their trees. The maximum life-span of singleleaf pinyon is probably greater than 600 years. A healthy tree 433 years old and 100 cm (40 in) diameter at stump height was found in the Sweetwater Mountains in east-central California (18). Trees slightly older than 600 years have been reported in the Toiyabe Range in central Nevada and in the Egan Range in eastern Nevada.
Rooting Habit- Little information is available on the rooting habit of pinyon. A few excavated trees indicate root systems are comprised of a short, stunted taproot, a fibrous "feeder" root system under the tree crown, and an extensive lateral root system that extends for at least three times tree height in all directions. Complete crown closure is rare in most stands, but where the stands are well developed the soils are usually completely occupied by tree roots resulting in suppression of the understory vegetation.
Reaction to Competition- Although pinyon is intolerant of shade, water rather than light is usually the limiting factor in survival and growth. The competition for available soil moisture among trees is so intense that younger trees are usually suppressed and some eventually die. Although older trees usually survive, growth rates are greatly reduced. Suppressed trees may resume normal growth rates when released from severe moisture stress (17). Light competition does not appear to be a factor because overstory foliage is rarely dense enough to reduce light intensities below tolerance levels.
Damaging Agents- Susceptibility to fire depends on the stage of development of the pinyon stand. In young stands, enough shrubby and herbaceous vegetation often exists to carry fire over extensive areas. As the stand develops, understory vegetation becomes too sparse to carry fire, and the trees generally are too widely spaced to carry a crown fire except with the aid of extremely high winds (1). Thus, fire is ordinarily confined to younger stands and to a few individual lightning-struck trees in older stands.
Pinyon ips (Ips confusus) is endemic over the range of singleleaf pinyon. It occasionally attacks and kills trees weakened by other agents but usually causes little damage (8). Minor epidemics can occur in areas where uprooted trees or slash accumulation permit the population to build up and successfully attack healthy trees. Mountain pine beetle (Dendroctonus ponderosae) attacks singleleaf pinyon on rare occasions.
Pinyon needle scale (Matsucoccus acalyptus) weakens trees by repeated feeding, which makes them more vulnerable to attack by Ips confusus and may kill small trees directly. The pinyon sawfly (Neodiprion edulicolus), a defoliator, reduces tree vigor and renders small trees unfit as Christmas trees (15). Pinyon foliage also is attacked by a number of less damaging insects including an aphid (Pineus coloradensis), a beetle (Glyptoscelis aridis), a scale (Matsucoccus monophyllae), a stinkbug (Dendrocoris pini), and a sawfly (Zadiprion rohweri) (8).
Singleleaf pinyon nut production is sporadic, varying considerably from place to place and from year to year. Weather variations are partially responsible, but cone insects probably reduce nut production substantially. The most damaging cone insects are two moth species (Eucosma bobana and Dioryctria albovittella) and the singleleaf pinyon cone beetle (Conophthorus monophyllae) (7,11).
Although singleleaf pinyon is attacked by a number of diseases, only three are known to cause serious injury. Pinyon dwarf mistletoe (Arceuthobium divaricatum) is a widespread parasite found on both singleleaf and Colorado pinyon; it causes extensive damage (10). It rarely kills trees directly but renders them more susceptible to insect attack.
Pinyon blister rust (Cronartium occidentale) occurs extensively on Ribes spp. in most western states but attacks singleleaf and Colorado pinyon only in localized areas (22). It occasionally kills small trees but rarely becomes epidemic.
A root disease (Verticicladiella wageneri) kills singleleaf pinyon in a few localized areas, notably in the San Bernardino Mountains of California (20,25). It spreads by root contact and is confined to the xylem in the roots and lower trunk.
Because of its small size and lack of self-pruning, singleleaf pinyon is not suitable for lumber, but it is used extensively for firewood. Because most singleleaf pinyon above-ground biomass and nutrient capital is in the slash (less than 7.6 cm, -1 in, in dia.) nutrient loss from fuelwood removal is not a significant nutrient drain on the site (23). Pinyon makes a desirable Christmas tree because of its fragrance and strong branches. Pinyon nuts were a staple in the diet of the Indians before white settlement and are still harvested extensively.
At present, the greatest values of the pinyon-juniper woodlands are the shelter and forage they provide numerous species of wildlife. As the demand for energy increases, the highest use will shift toward fuel production.
The wide geographic and elevational ranges of singleleaf pinyon suggest a great deal of genetic variation. Difference in growth form, foliage color, and cone production among trees growing on identical sites are commonly observed. Little research has been done, however, on the genetics of intraspecific variation of singleleaf pinyon. Singleleaf pinyon hybridizes with Colorado pinyon (Pinus edulis) in Utah and Arizona where the ranges of the two species overlap (12), and with Parry pinyon (P. quadrifolia) in southern California and Baja California (13).