Physiognomy and Structure: Vegetation included in this alliance has an open tree canopy that is typically 5-10 m tall, but may reach 15 m. A sparse to moderately dense tree subcanopy (3-10 m tall) is typically present. The canopy is dominated by evergreen needle-leaved trees with the subcanopy dominated by broad-leaved or scale-leaved evergreen trees. An open to dense (>10% cover) shrub layer may be present and is typically dominated by broad-leaved evergreen shrubs that are usually less than 5 m tall. A sparse to moderately dense layer dominated by perennial graminoids is usually present. Perennial forbs have sparse cover. Annual forbs and grasses may be seasonally present.

Floristics: This Madrean alliance is characterized by open to moderately dense woodlands with 10-60% cover of pinyon and juniper trees with shrubs in the understory forming an open to dense short-shrub layer with greater than 10% cover. Perennial graminoid cover is typically low and patchy because of high shrub cover on rocky sites. The presence of Madrean pinyons Pinus cembroides, Pinus discolor, or Pinus remota is diagnostic of this alliance. Junipers may be absent to dominant in the tree canopy with Juniperus coahuilensis, Juniperus deppeana, or Juniperus flaccida. Wide-ranging species Pinus edulis and Juniperus monosperma may be present to dominant when diagnostic Madrean species are present. Evergreen oaks such as Quercus arizonica, Quercus emoryi, Quercus gravesii, or Quercus grisea are often present in the tree canopy but have lower total cover than conifers. The shrub layer is composed of a variety of shrubs and rosettes such as Agave lechuguilla, Arctostaphylos pungens, Cercocarpus montanus var. paucidentatus, Garrya wrightii, Mimosa dysocarpa, Ptelea trifoliata, Quercus emoryi, Quercus graciliformis, Quercus gambelii, Quercus hypoleucoides, Quercus mohriana, Quercus x pauciloba, Quercus pungens, Quercus rugosa, Quercus toumeyi, Quercus turbinella, Rhus trilobata, and Salvia regla. All of these species are diagnostic except Cercocarpus montanus var. paucidentatus, Quercus gambelii, Quercus x pauciloba, Quercus turbinella, and Rhus trilobata. A sparse to moderately dense layer dominated by perennial graminoids is usually present. Characteristic species are typical of the desert grasslands or Madrean foothills and include Bouteloua eriopoda, Eragrostis intermedia, Muhlenbergia dubia, Muhlenbergia emersleyi, Muhlenbergia pauciflora, Muhlenbergia setifolia, Piptochaetium fimbriatum, Piptochaetium pringlei, and Schizachyrium cirratum. Other common graminoids include Bouteloua curtipendula and Bouteloua gracilis. Forbs are typically sparse. Some of the more common forbs include Ageratina herbacea (= Eupatorium herbaceum), Artemisia carruthii, Artemisia ludoviciana, Cheilanthes fendleri, Dysphania graveolens (= Chenopodium incisum), Geranium caespitosum, Hedeoma hyssopifolia, Packera neomexicana (= Senecio neomexicanus), Thalictrum fendleri, and Viguiera spp. Annual forbs and grasses may be seasonally present.

Dynamics:  Stands are typically small and naturally patchy, and reflect the late-successional stage of Sierra Madrean woodlands and savannas. This is a highly diverse alliance with a preponderance of Sierra Madrean elements that have a restricted distribution in the U.S. Grazing and, to a lesser degree, fuelwood harvest continue to impact stands. Altered fire regimes (failed fire suppression followed by increased intensity and extent of fire) may be reducing the number of stands.

Pinyon trees are drought-tolerant and slow-growing (Little 1987, Powell 1988b, Muldavin et al. 1998a). They are 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, 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 et al. 1962). Closed-canopy stands rarely burn because they typically do not have enough fine fuels in the understory or wind to carry a fire (Wright et al. 1979).

Although pinyon trees are 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).

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. Climatic and other factors have resulted in denser and expanded pinyon-juniper stands that have likely become denser in much of the Madrean region. Denser stands are more susceptible to attack by insects and disease (Anhold 2005).