Physiognomy and Structure: No Data Available

Floristics: This type is consistently dominated by Picea engelmannii and Abies lasiocarpa. Late-successional stands may have a dominant overstory of Picea engelmannii possibly resulting, in part, from a reproductive advantage over Abies through exposure of mineral soil seedbeds by heavy grazing (Roe et al. 1970). Steele et al. (1981) note that Pinus albicaulis codominates with Abies lasiocarpa, with Picea becoming increasingly prevalent to the south and east. Both Abies and Picea are found layering in the understory (Cooper and Pfister 1985, Youngblood and Mauk 1985). Although an important early-seral associate, Populus tremuloides is usually present in mature stands only as remnant stems of poor vigor. Pinus contorta is a minor seral species. This association is generally above the cold limits of Pseudotsuga, but the species does occur at lower elevations of the type.

The shrub layer is sparse, with scattered clumps of Ribes montigenum usually confined to near the bases of large trees. Other shrubs, which may occur in openings or on recently disturbed areas, include Sambucus racemosa, Symphoricarpos oreophilus, and Shepherdia canadensis. The undergrowth is characteristically depauperate. Forbs with high constancies are Aquilegia coerulea, Arnica cordifolia, Arnica latifolia, Achillea millefolium, Astragalus miser, Chamerion angustifolium (= Epilobium angustifolium), Ligusticum porteri, Lupinus argenteus, Mertensia arizonica, Thalictrum fendleri, Thalictrum occidentale, Fragaria virginiana, Polemonium pulcherrimum, Pseudocymopterus montanus, Orthilia secunda (= Pyrola secunda), Frasera speciosa, Osmorhiza berteroi (= Osmorhiza chilensis), and Osmorhiza depauperata. Graminoids are weakly represented. Carex rossii is the only sedge encountered with any regularity. Under disturbed conditions, or in seral stands with incomplete conifer canopies, Lupinus argenteus, Lathyrus lanszwertii, Hymenoxys hoopesii (= Helenium hoopesii), Chamerion angustifolium, Aquilegia coerulea, and Achillea millefolium are expected to increase.

Youngblood and Mauk (1985) recognize two phases of Abies lasiocarpa/Ribes montigenum for Utah, the Ribes montigenum phase, representing the modal conditions of the type, at elevations between 2895-3200 m (9500-10,500 feet) on gentle to steep straight slopes, and the Mertensia arizonica phase, representing the most hydric extent of the type at the highest elevations, usually above 3050 m (10,000 feet) on gentle to moderate slopes or swales. It has a dense, relatively lush ground cover with numerous forbs as a result of abundant ground moisture from snowpack accumulations on lee slopes and numerous seeps and small springs.

Pfister (1972) recognizes three phases of this type for Utah, a Ribes montigenum phase (modal type), a Thalictrum fendleri phase representing the moist mesic extent of the type and characterized by the presence of Thalictrum fendleri and a much more diverse herb layer, and a Lonicera involucrata phase, a geographic isolate found in the LaSal and Abajo mountains of Utah. In addition to the three phases of Pfister, Mauk and Henderson (1984) recognize a Trisetum spicatum phase occupying the cool, dry, and windswept exposures in the timberline zone from 3200-3415 m (10,500-11,200 feet), and a Pinus contorta phase occurring only in the south-central Uinta Mountains near 3140 m (10,300 feet) on gentle to moderately steep slopes and ridges with well-drained soils.

Steele et al. (1983) recognize two phases of this type in Wyoming, the Ribes montigenum modal type phase and the Pinus albicaulis phase representing the upper extremes of Abies lasiocarpa/Ribes montigenum between 2950-3110 m (9680-10,200 feet) and characterized by the presence and frequent dominance of Pinus albicaulis.

Dynamics:  Evidence from stands burned by wildfire indicate that this type will be difficult to reforest and may persist as Artemisia tridentata ssp. vaseyana- and bunchgrass-dominated communities (Cooper and Pfister 1985). However, according to Youngblood and Mauk (1985), following a major stand-destroying fire, remnant Populus tremuloides root systems resprout, creating clones. Invasion of Abies lasiocarpa follows, and saplings may develop quickly under this nurse crop of Populus tremuloides. Invasion of Picea is much slower and may be restricted to areas of bare soil. Pfister (1972) suggests this progression to a pure Abies - Picea climax, without further disturbance, may require at least 1000 years.

The uneven-aged climax stand usually has abundant Abies lasiocarpa seedlings and saplings in addition to vegetative layering. Picea engelmannii is found on raised root mounds and depressions in the understory, resulting from windthrow of the shorter lived Abies lasiocarpa following root rot (Youngblood and Mauk 1985). Abies lasiocarpa is almost uniformly susceptible to decay by Fomes annosus, which increases the chances of windthrow. Stands of Abies may also become infected with yellow witches'' broom rust (Melampsorella caryophyllacearum). Windthrow continually removes individuals or small groups of large-crowned old trees, allowing replacement of Populus, Abies, and Picea in the resultant openings. Devastation of old-growth Picea engelmannii overstories by the Engelmann spruce bark beetle (Dendroctonus rufipennis) creates sufficient disturbance to allow regeneration and release of all three tree species. The combination of these factors creates the characteristic all-aged structure of this type, in dynamic equilibrium with present environmental conditions and showing periodic fluctuations due to natural small-scale perturbations (Pfister 1972).