The pinyon-juniper woodland is a characteristic woodland community formed by Colorado pinyon (Pinus edulis) and Utah juniper (Juniperus osteosperma). In this community, Utah juniper often extends to lower elevations without the Colorado pinyon component, while Colorado pinyon grows at elevations above Utah juniper.

Colorado pinyon often grows as a low, bushy tree with an irregularly rounded, spreading crown. The trunk is generally short and crooked, with several large, crooked branches. It may grow to 40 inches (1 m) in diameter. Height is typically 26 to 56 feet (8-17 m). The needles are 2.7 to 4.3 inches (6-11 cm) long and in fascicles of 2. Needles remain on the tree for approximately 9 years, and are an important repository for nutrients accumulated by the plant. In Arizona pinyon is found from 4,000-7,500 feet (1,220-2,280 m); upper limit of 6,500 feet (1,980 m) on north-facing slopes. Colorado pinyon is a slow-growing, long-lived tree. It can survive more than 500 years and may reach 800 to 1,000 years of age.

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. Large seed crops are produced every 3 to 7 years and are adversely impacted by water stress. 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. Cones require 3 years to mature. 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.

Utah juniper is a short tree that may live as long as 650 years. Utah junipers grow less than 26.4 feet (8 m) and are often as short as 9.9 to 14.85 feet (3-4.5 m), with a trunk 4 to 7.5 inches (10-30 cm) thick. Sometimes the tree has multiple stems. In Arizona, Utah juniper occurs at 3,000 to 7,500 feet (909-2273 m), more commonly above 5,000 feet (1515 m).

Utah junipers begin to produce seed only when they are about 30 years old. Utah juniper seeds are long-lived. In one study, 17% of Utah juniper seeds germinated after 45 years. In general, about 8 to 49% of Utah juniper seeds germinate.

Growth Factors

Soils of these communities may be shallow to moderately deep and are often rocky, well drained, and low in fertility. Colorado pinyon occurs on a wide range of soil types and is not limited by the character or geologic origin of soils. Colorado pinyon growing in deeper soils generally grow faster than those in shallow soils. Colorado pinyon occurs on a range of parent materials, including sandstone, limestone, shale, basalt, granite, and mixed alluvium.

Soils under well-developed pinyon-juniper stands are completely occupied by tree roots, limiting understory growth. The lateral roots of the tree species efficiently access interspaces in these communities for soil water and nutrients, further impacting herbaceous species. In addition, understory vegetation is reduced by shading and potentially by allelopathic effects. Colorado pinyon accumulates nutrients beneath the tree canopy. 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. In addition to accumulations of organic matter, concentrations of soluble salts (Na, Ca, Mg, and K) are significantly higher 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. Colorado pinyon litter is specifically associated with a reduction of blue grama production.

Annual precipitation is typically from 10 to about 15 inches in pinyon-juniper woodlands, and tree species in these communities have evolved both drought and cold resistance. Pinyons dominate at higher elevations, and tend to form more closed-canopied stands that exhibit forestlike dynamics and species composition, commonly including a significant shrub component of oaks and alderleaf, mountain mahogany and limited grasses. Juniper tends to grow at lower elevations and in more arid areas as its scaled foliage allows it to conserve water more effectively than pinyon pine. Juniper-dominated woodlands tend to include open savannas of scattered trees without a significant shrub component.

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. At high elevations, distribution may be limited by low temperatures or competition with ponderosa pine. Moisture is likely the most critical factor controlling the distribution, composition, and density of pinyon-juniper woodlands, though the distribution of Colorado pinyon may also be affected by soil characteristics.

Water is the primary limiting factor in seedling survival and growth of pinyon.

Utah juniper responds to low nutrient levels in the soil by developing extensive networks of fine roots at the base of the tree and at the end of lateral roots.

Utah juniper thrives on very dry sites. Precipitation patterns in juniper communities vary, but Utah juniper is generally found in areas of 12 to 18 inches (305-457 mm) of precipitation, with extremes of below 10 inches (254 mm)/year to highs of 20 inches (508 mm).

Utah juniper commonly grows on alluvial fans and dry, rocky hillsides, with shallow, alkaline soils. Utah juniper is considered a "sodium-sensitive" species. In west-central Utah, Utah juniper is found on a range of soil textures, but most often on gravelly loams and gravelly clay loams with a pH range of 7.4 to 8.0.

Species Interactions

Besides pinyon and juniper, there are other plants associated with this vegetation type. Shrub species occurring as understory dominants are pointleaf manzanita (Arctostaphylos pungens), big sagebrush (Artemisia tridentata), true mountain-mahogany (Cercocarpus montanus), rubber rabbitbrush (Chrysothamnus nauseosus), Stansbury cliffrose (Purshia mexicana var. stansburiana), antelope bitterbrush (Purshia tridentata), Gambel oak (Quercus gambelii), gray oak (Q. grisea), wavyleaf oak (Q. undulata), blackbrush (Coleogyne ramosissima), Nevada ephedra (Ephedra nevadensis), broom snakeweed (Gutierrezia sarothrae), and plains prickly-pear (Opuntia polyacantha). Herbaceous species occurring as understory dominants include blue grama (Bouteloua gracilis), Arizona fescue (Festuca arizonica), mountain muhly (Muhlenbergia montana), New Mexico muhly (M. pauciflora), mutton grass (Poa fendleriana), galleta (Pleuraphis jamesii), Columbia needlegrass (Achnatherum nelsonii), and sand bluestem (Andropogon gerardii var. paucipilus).

Colorado pinyon cones and seeds are attacked by a variety of insect species, which may destroy large portions of seed crops

The wingless seeds of Colorado pinyon are dispersed by birds and small mammals, primarily squirrels and chipmunks. Four species of birds cache Colorado pinyon seeds: Clark's nutcrackers, scrub jays, pinyon jays, and Steller's jays. 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.

Colorado pinyon seeds generally germinate in the shade of a tree or shrub rather than in open grassland. Reproduction of Colorado pinyon is generally sparse and scattered due to removal of seeds by birds and mammals, and seedling establishment is dependent on chance dispersal to favorable sites and ample rainfall. Competition for moisture usually results in the suppression of smaller trees, though they gradually resume normal growth when released from severe competition.

Juniper's roots are in part responsible for its competitiveness versus understory species. Junipers compete more efficiently for soil moisture than do herbaceous understory plants; therefore, over time, junipers are more likely to maintain a stable population, while understory plants decrease.

Juniper litter has an allelopathic effect on some understory species, especially Idaho fescue (Festuca idahoensis), Sandberg bluegrass, and blue grama. Cheatgrass does not appear to suffer from allelopathic effects, and fourwing saltbush (Atriplex canescens) growth increases under juniper canopies.

Utah juniper is colonized by soil fungi, forming vesicular-arbuscular mycorrhizae.