Physiognomy and Structure: Vegetation included in this alliance has an open tree canopy that is typically up to 5-15 m tall. The canopy is dominated by evergreen needle-leaved trees. Sparse to moderately sparse shrub layers may be present. If present, the shrub layer is typically dominated by broad-leaved deciduous or evergreen scale-leaved shrubs usually less than 2 m tall. A sparse to moderately dense herbaceous layer dominated by perennial graminoids is often present. Perennial forbs have sparse cover. Annual forbs and grasses may be seasonally present.

Floristics: Stands have an open to moderately closed canopy typically 5-15 m tall that is solely dominated or codominated by the long-lived evergreen needle-leaved tree Pinus aristata. Individual trees may reach 20 m. In some stands trees are clumped with grassy patches interspersed. Other tree species that may be present to codominant vary geographically within its range. Picea engelmannii and Pseudotsuga menziesii are the most common. Picea pungens is common in the southern extent where stands occur at high elevations, and Pinus flexilis has also been reported as an associate in some stands. Abies lasiocarpa and Populus tremuloides may be scattered in some stands, but are generally restricted to more mesic sites and are typically absent in the drier southern extent of this alliance. The understory vegetation ranges from moderately dense to typically sparse because sites are dry and often have large amounts of rock cover. The shrub layer is generally sparse or absent. If present, shrubs are generally scattered, except for Juniperus communis, which often occurs in clumps. Other shrub species may include Dasiphora fruticosa ssp. floribunda (= Pentaphylloides floribunda), Ribes cereum, Ribes montigenum, Rosa woodsii, Saxifraga bronchialis, Symphoricarpos oreophilus, and immature trees. The sparse to moderately dense herbaceous layer often dominates the understory. The most common species are typically graminoids, especially species of Festuca. Other common species may include Calamagrostis purpurascens, Carex spp., Danthonia parryi, Festuca arizonica, Festuca brachyphylla, Festuca idahoensis, Festuca thurberi, Koeleria macrantha, Muhlenbergia filiculmis, Muhlenbergia montana, Poa fendleriana, and Trisetum spicatum. Forbs are generally sparse, but Trifolium dasyphyllum may reach 10% cover. Other scattered forbs may include species of Achillea, Antennaria, Artemisia, Arenaria, Arnica, Astragalus, Campanula, Erigeron, Hymenoxys, Penstemon, Polemonium, Sedum, Senecio, and Thalictrum.

Dynamics:  Pinus aristata is a slow-growing, extremely long-lived tree (Brunstein and Yamaguchi 1992). Several individuals over 2000 years old have been found in Colorado. DeVelice et al. (1986) observed it occurs in open park-like stands and on steep rocky slopes where Picea engelmannii - Abies lasiocarpa stands are excluded by drought. Fire is important in the grass-dominated stands but is rarely intense enough to result in tree-killing crown fires (DeVelice et al. 1986). In some stands, suppression of grass fires has allowed encroachment of the trees into meadows (Larson and Moir 1987). Forage production is good in some stands, but rarely utilized by livestock because stands are steep and generally remote which makes access difficult. Peet (1978, 1981) noted that Pinus aristata is dominant at higher elevations in much of the southern Rocky Mountains, where Pinus flexilis is restricted to lower elevations. This is attributed to apparent competitive exclusion, because Pinus flexilis is dominant at high elevations in northern Colorado, Wyoming and Montana.