Physiognomy and Structure: Vegetation included in this alliance has an open to moderately dense tree canopy that is typically 3-10 m tall. Stands are either solely dominated by evergreen needle-leaved trees or may be codominated by scale-leaved evergreen trees. An open to moderate ground layer dominated by perennial graminoids is usually present. Perennial forbs and cacti are often scattered throughout the stands. Annual forbs and grasses may be seasonally present. Scattered shrubs may be present, but do not form a layer (<10% cover).

Floristics: This alliance is characterized by a very open to moderately dense tree canopy typically 3-12 m tall. The diagnostic tree species is Pinus edulis often with Juniperus osteosperma or, less frequently, Juniperus monosperma within the range of Juniperus osteosperma in northern Arizona. Juniperus osteosperma may also dominate stands as long as there is significant presence (>5% cover) of Pinus edulis to characterize the stand as a pinyon-juniper stand and not the more xeric, typically lower elevation Juniperus osteosperma woodland and savanna. At higher elevations and relatively mesic sites, such as along drainages, Juniperus scopulorum may be present and sometimes dominant. Other conifers are absent or accidental with very low cover. The understory is characterized by an open to dense herbaceous layer. Scattered shrubs may be present, such as Artemisia spp., Cercocarpus montanus, Ephedra viridis, Ericameria nauseosa, Fraxinus anomala, Gutierrezia sarothrae, Opuntia polyacantha, and Rhus aromatica, but do not form a layer and do not exceed cover of the herbaceous layer. Perennial grasses typically dominate the herbaceous layer, although diverse forbs species are often present, but with low cover. Characteristic species include Achnatherum hymenoides, Bouteloua gracilis, Elymus elymoides, Hesperostipa comata, Hesperostipa neomexicana, Leymus salinus, Muhlenbergia pungens, Poa fendleriana, Poa secunda, Pleuraphis jamesii, and Pseudoroegneria spicata. Commonly present forbs include species of Arenaria, Artemisia, Eriogonum, Heterotheca, Hymenoxys, Mirabilis, Oxytropis, Penstemon, Phlox, Senecio, Stenotus, and Zinnia. Annual grasses and forbs are seasonally present. The non-native, invasive annual grass Bromus tectorum becomes abundant in disturbed stands and may dominate the understory of highly disturbed stands. Some stands included in this alliance have been seeded with non-native perennial grasses such as Psathyrostachys juncea or Agropyron cristatum to control soil erosion or increase forage production and now have a semi-natural understory.

Dynamics:  Pinus edulis is extremely drought-tolerant and slow-growing (Little 1987, Powell 1988b, Muldavin et al. 1998c). It is also non-sprouting and may be killed by fire (Wright et al. 1979). The effect of fire on a stand is largely dependent on the tree height and density, fine-fuel load on the ground, weather conditions, and season (Dwyer and Pieper 1967, Wright et al. 1979). Trees are more vulnerable in open stands where fires frequently occur in the spring, when the relative humidity is low, wind speeds are over 10-20 mph, and there are adequate fine fuels to carry fire (Wright et al. 1979). Under other conditions, burns tend to be spotty with low tree mortality. Large trees are generally not killed unless fine fuels, such as tumbleweeds, have accumulated beneath the tree to provide ladder fuels for the fire to reach the crown (Jameson 1962). Closed-canopy stands rarely burn because they typically do not have enough understory or wind to carry a fire (Wright et al. 1979).

Although Pinus edulis is drought-tolerant, prolonged droughts will weaken trees and promote mortality by secondary agents. Periodic die-offs of pinyon pine caused by insects, such as the pinyon Ips beetle (Ips confusus), or fungal agents, such as blackstain root-rot (Leptographium wageneri), tend to be correlated with droughts (Anhold 2005). These mortality events may be localized or widespread but can result in 50 to 90% mortality of Pinus edulis (Harrington and Cobb 1988).

Climatic and other factors have resulted in denser and expanded pinyon-juniper stands throughout the Colorado Plateau and Great Basin. Denser stands are more susceptible to attack by insects and disease (Anhold 2005). In addition, altered fire regimes, cutting trees for fencing or firewood, and improper grazing by livestock have significant impacts on the quality of sites. Grazing by livestock can modify the fire regime by removing the fine fuels that carry fire. Fire, livestock grazing, and trampling by recreationalists and vehicles disturb cryptogamic soil crusts that help maintain soil structure, reduce soil erosion, provide habitat for plants and preserve biological diversity (Ladyman and Muldavin 1996). More study is needed to understand and manage these woodlands ecologically.