Physiognomy and Structure: These are typically moderately dense to open (15-40% cover) microphyllous evergreen (or late cold-deciduous) shrublands with the dominant shrubs varying from 0.5-3 m in height. Maximum shrub height generally decreases with latitude and altitude, reaching a minimum in subalpine stands. A second tier of microphyllous evergreen or cold-deciduous shrubs may be present. Cespitose graminoids are typical herbaceous associates and may strongly dominate the ground layer. Some regional variants may have a substantial forb component as well. Lichens and mosses may cover the ground surface in good-condition stands.

Floristics: Shrublands included in this alliance are characterized by a Purshia tridentata- or Ribes cereum-dominated shrub canopy. Canopies are 1-2 m tall and total cover ranges from 15-40%. Other shrubs are poorly represented, but Artemisia frigida is the most constant shrub associate. Arctostaphylos uva-ursi, Artemisia tridentata ssp. vaseyana, Ceanothus fendleri, Chrysothamnus viscidiflorus, Ericameria nauseosa (= Chrysothamnus nauseosus), Jamesia americana, Juniperus communis, Prunus virginiana, Rhus trilobata, and Rubus deliciosus are sometimes present. Scattered trees such as Juniperus scopulorum, Pinus ponderosa, and Pseudotsuga menziesii may be present at low covers. The herbaceous layer is dominated by perennial graminoids ~0.5-m tall, the most constant species being Hesperostipa comata, Leymus ambiguus, and Muhlenbergia montana. Other common herbaceous associates include Achillea millefolium, Achnatherum scribneri (= Stipa scribneri), Antennaria sp., Artemisia ludoviciana, Astragalus sp., Bouteloua gracilis, Chenopodium leptophyllum, Cryptantha virgata, Elymus albicans, Erigeron spp., Geranium caespitosum, Harbouria trachypleura, Helianthus pumilus, Heterotheca villosa, Penstemon virens, Phacelia heterophylla, Potentilla fissa, Scutellaria brittonii, Symphyotrichum falcatum.

Dynamics:  Sawyer and Keeler-Wolf (1995) report that stands of Purshia tridentata can reach 125 years of age on deep, well-drained sites, but more commonly become decadent at 30 years, and die at 40-50 years of age. Stands appear to result from either a disturbance event (such as fire), or from rare years when many seedlings survive. This results in even-aged stands (Sawyer and Keeler-Wolf 1995). Purshia tridentata displays considerable plasticity in growth across its range. The broad range in height and form of different populations appears to be related to ecotypic variation (Mozingo 1987). Although mycorrhizae are considered important in establishment and growth of individual plants, Purshia tridentata is one of the first species to colonize barren volcanic substrates following eruption. The species is valuable as winter browse for native ungulates and livestock and is used extensively. Moderate livestock utilization (<60% of the year''s current growth) has been reported to stimulate twig growth the following spring (Mueggler and Stewart 1980).