Physiognomy and Structure: Vegetation included in this alliance has a sparse to moderately dense tree canopy that is typically <3 m tall, but may extent to 10 m on favorable sites. Stands are either solely dominated by evergreen needle-leaved trees or may be codominated by scale-leaved evergreen trees. A sparse to moderately dense shrub layer (0.5-3 m tall) may be present. If present, the shrub layer 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.

Floristics: This vegetation of this open woodland and scrub alliance is sparse to moderately dense (10-80% total vegetation cover) and is dominated by dwarfed (usually <3 m tall) Pinus edulis and/or Juniperus osteosperma trees that form extensive tall shrublands or scrublands in the canyon country of the Colorado Plateau region. Both tree species are present with 3-25% cover. The understory may be lacking or be characterized by an open to moderately dense layer of shrubs (>10% cover) or, if less, then exceeding cover of grasses. Shrubs, if present, often include Artemisia bigelovii, Atriplex canescens, Cercocarpus intricatus, Cercocarpus montanus, Chrysothamnus viscidiflorus, Coleogyne ramosissima, Ephedra torreyana, Ephedra viridis, Fendlera rupicola, Glossopetalon spinescens var. aridum (= Glossopetalon nevadense), Petradoria pumila, Purshia stansburiana, Quercus havardii var. tuckeri, or Quercus turbinella. Shrub mixes often occur on steeper talus slopes. Herbaceous layers are typically sparse and composed of xeric grasses and diverse forbs, especially cushion plants. Opuntia fragilis or cushion plants, such as Arenaria eastwoodiae, Enceliopsis nudicaulis, Erigeron compactus, Erigeron pumilus, Eriogonum alatum, Frasera albomarginata, Heterotheca villosa, Hymenopappus filifolius, Paronychia sessiliflora, Polygala subspinosa, Stenotus armerioides, Tetraneuris acaulis, Tetraneuris torreyana (= Tetraneuris depressa), Townsendia aprica, and Townsendia incana, may characterize the understory and exceed shrub cover. Common perennial grasses include Achnatherum hymenoides, Aristida purpurea, Bouteloua gracilis, Pleuraphis jamesii, and Poa fendleriana. Other common forbs, such as Arenaria fendleri, Artemisia ludoviciana, Descurainia pinnata, Eriogonum ovalifolium, Heterotheca villosa, Lappula occidentalis, Machaeranthera grindelioides, Oenothera pallida, Penstemon linarioides, and Streptanthus cordatus, may be present with low cover. Stands of this community sampled at higher elevation have northern aspects and more dense vegetation cover. Biological soil crusts may have significant cover in sites derived from loess.

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). Many xeric stands in this alliance are too sparse to burn. However, 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.