Physiognomy and Structure: Vegetation included in this alliance has a moderately sparse 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 broad-leaved or scale-leaved evergreen trees. An open to moderately dense shrub layer (0.5-3 m tall) is present and ranges from a single species to a diverse mix of broad-leaved and microphyllous deciduous or evergreen shrubs that are usually less than 3 m tall. A sparse 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. This alliance also includes sparse understory stands often with scattered shrubs and grasses.

Floristics: These woodlands are characterized by diagnostic tree species Pinus edulis that forms an open to dense tree layer often with Juniperus monosperma. Juniperus monosperma may also dominate stands as long as there is significant presence of Pinus edulis (not accidental) to characterize the stand as a pinyon-juniper stand and not the more xeric, typically lower elevation Juniperus osteosperma woodland. At higher elevations and relatively mesic sites, such as along drainages, Juniperus scopulorum may be present and sometimes dominant. In southern transitional areas with ~Madrean Pinyon - Juniper Woodland Group (G200)$$ in central New Mexico, Juniperus deppeana may be present but not dominant. Other conifers are absent or accidental with very low cover. The understory is characterized by shrubs that typically form an open to moderately dense layer. Characteristic shrubs are many and include Artemisia bigelovii, Artemisia tridentata, Atriplex canescens, Cercocarpus montanus, Chrysothamnus viscidiflorus, Ericameria nauseosa, Fallugia paradoxa, Gutierrezia sarothrae, Holodiscus dumosus, Quercus gambelii, Quercus x pauciloba, Ribes cereum, Rhus trilobata, and Symphoricarpos oreophilus. Similar scattered shrubs frequently characterize sparse and rocky understory woodland stands and so those types are included in this alliance. The herbaceous layer is sparse to moderate and composed of grasses and often with diverse but low cover of forbs. Common species may include Achnatherum hymenoides, Achnatherum nelsonii, Achnatherum scribneri, Andropogon hallii, Achnatherum hymenoides, Bouteloua gracilis, Festuca arizonica, Hesperostipa comata, Hesperostipa neomexicana, Koeleria macrantha, Muhlenbergia montana, Piptatheropsis micrantha (= Piptatherum micranthum), Pleuraphis jamesii and Sporobolus spp. Many forb species occur, but few have much cover. Commonly present forbs include species of Artemisia, Eriogonum, Heterotheca, Mirabilis, Penstemon, Phlox, Senecio, and Zinnia. Annual grasses and forbs are seasonally present. Adjacent vegetation at higher elevations is typically woodland or forest dominated by Pinus ponderosa. Adjacent vegetation at lower elevations is often Juniperus spp.-dominated woodland and savanna, Artemisia spp.-dominated shrubland, or grassland.

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.