Physiognomy and Structure: These forests are dominated by needle-leaved evergreen trees up to 45 m in height and of high cover (60-100%). Although cold-deciduous trees are relatively rare, they can be prominent in some regional variants or seral stands. Stands may be so tightly stocked that little light reaches the forest floor and understory layers are depauperate. In stands with somewhat more open canopies, a moderately dense shrub layer may be present and dominated by ericaceous or cold-deciduous species. The herbaceous layer is dominated by perennial forbs or sod-forming graminoids, and herbaceous cover increases with increasing light availability and/or soil moisture. There is often significant cover of mosses and sometimes lichens on the forest floor and on downed woody material.

Floristics: Despite the wide distribution of these subalpine forests, their tree canopy characteristics are remarkably similar across their range of distribution. Abies lasiocarpa and/or Picea engelmannii are dominant singly or together. Both species are typically present in most stands. In some stands, Picea engelmannii may be absent altogether, or Abies lasiocarpa may only occur as seedlings and saplings. Picea engelmannii will often be prominent on more moist sites or in more mature stands. Populus tremuloides is a major seral species, and Pinus contorta occurs less frequently. Other common canopy associates may include Abies concolor, Picea pungens, and Pseudotsuga menziesii. The shrub and herbaceous layers of these forest associations are highly variable. Forest understories can be dominated by grasses or dry sedges in the more open forests, or by ericaceous shrubs or moist forbs in closed-canopy, moist sites. Shrub layers where they do occur are typically less than 1 m tall. Important to dominant shrubs include Acer glabrum, Jamesia americana, Juniperus communis, Mahonia repens, Physocarpus monogynus, Ribes spp., Shepherdia canadensis, Vaccinium cespitosum, Vaccinium myrtillus, and Vaccinium scoparium. Forb cover is often sparse, but diverse with species including Arnica spp., Aquilegia spp., Caltha leptosepala, Cardamine cordifolia, Chamerion angustifolium, Erigeron eximius, Lupinus argenteus, Mertensia spp., Mitella pentandra, Orthilia secunda, Osmorhiza berteroi, Oxypolis fendleri, Packera spp., Pyrola spp., and Stellaria spp. Graminoid cover is generally low but may include Achnatherum lettermanii (= Stipa lettermanii), Bromus carinatus, Carex geyeri, Carex rossii, Festuca idahoensis, Luzula parviflora, and Trisetum spicatum.

Dynamics:  Picea engelmannii can be very long-lived, reaching 500 years of age. Abies lasiocarpa decreases in importance relative to Picea engelmannii with increasing distance from the region of Montana and Idaho where maritime air masses influence the climate. Fire is an important disturbance factor, but fire regimes have a long return interval and so are often stand-replacing. Picea engelmannii can rapidly recolonize and dominate burned sites, or can succeed to other species such as Pinus contorta or Populus tremuloides. Due to great longevity, Pseudotsuga menziesii may persist in stands of this alliance for long periods without regeneration. Old-growth characteristics in Picea engelmannii forests will include treefall and windthrow gaps in the canopy, with large downed logs, rotting woody material, tree seedling establishment on logs or on mineral soils unearthed in root balls, and snags. Picea engelmannii is susceptible to infestations by spruce beetle (Dendroctonus rufipennis) or spruce budworm (Choristoneura occidentalis), which can cause high mortality during outbreaks. In the Southwest, dwarf mistletoe Arceuthobium microcarpum is a common cause of mortality for the species.

Abies lasiocarpa forests develop on sites with limited, short growing seasons and relatively deep winter snowpacks. Tree growth is very slow in these habitats, and forests are rapidly colonized by much more rapidly growing shade-intolerant species, such as Pseudotsuga menziesii, Pinus contorta, or Populus tremuloides, following fire, clearcut logging, or windthrow disturbance. Abies lasiocarpa is among the most shade-tolerant trees in the Rocky Mountains, but seedlings compete poorly in greater than 50% full sunlight (Burns and Honkala 1990a). Fire is important in many of the more open sites, as well as those on steep slopes. Snow avalanches occur frequently at upper elevations, and can result in a mosaic of varying stand ages on sites affected by this disturbance type.