Physiognomy and Structure: Associations in this alliance are characterized by a closed tree canopy of tall coniferous trees which can attain 50 m in height. Canopy cover ranges from 60-100%. A subcanopy is often present, composed of broad-leaved cold-deciduous shrubs or shade-tolerant small trees. Herbaceous cover is highly variable, ranging from 0-60% cover depending on location, precipitation, fire history, and stand age. Generally the abundance of subcanopy and herbaceous cover decreases with increasing stand age and as available light decreases.

Floristics: Canopies are dominated by Abies grandis and/or Pseudotsuga menziesii. Several other conifers may be present in the canopy, typically as seral species, including Larix occidentalis, Pinus contorta, or Pinus monticola. Picea engelmannii and Taxus brevifolia become increasingly common towards the eastern edge of the range, and Tsuga heterophylla and Thuja plicata may be associates at moister sites. Lillybridge et al. (1995) report that Pinus monticola was probably more important in the canopy prior to the introduction of white pine blister rust, a pathogen which has severely reduced populations of Pinus monticola. The shrub and herbaceous components of these forests are highly variable across the range of the alliance, and Cooper et al. (1987) describe these forests as being very species-diverse due to the moderate climate regime. Cold-deciduous or ericaceous shrubs can be may dominant in the understory in some stands. Shrubs can include Acer glabrum, Linnaea borealis, Menziesia ferruginea, Physocarpus malvaceus, Spiraea betulifolia, Symphoricarpos albus, Symphoricarpos occidentalis, Taxus brevifolia, Vaccinium cespitosum, and Vaccinium membranaceum. The composition of the herbaceous layer is related to local moisture conditions and the degree of canopy closure. The herbaceous layer can be sparse or, if the shrub layer is not abundant, can be relatively species-rich. Forbs and ferns commonly dominant include Calamagrostis rubescens, Carex geyeri, Clintonia uniflora, Coptis occidentalis, Cornus canadensis, Heracleum maximum, Linnaea borealis, Trautvetteria caroliniensis, and Xerophyllum tenax. Other species include Adenocaulon bicolor, Aralia nudicaulis, Arnica cordifolia, Balsamorhiza sagittata, Goodyera oblongifolia, Gymnocarpium dryopteris, Osmorhiza berteroi (= Osmorhiza chilensis), Pteridium aquilinum, Pyrola asarifolia, Thalictrum occidentale, Trientalis borealis ssp. latifolia (= Trientalis latifolia), Trillium ovatum, and Valeriana sitchensis. Graminoids that can be present include Bromus vulgaris, Carex concinnoides, Festuca idahoensis, Festuca occidentalis, Koeleria macrantha, and Melica bulbosa.

Dynamics:  Abies grandis forests include many sites dominated by Pseudotsuga menziesii and Pinus ponderosa which were formerly maintained by wildfire, and may now be dominated by Abies grandis (a fire-sensitive, shade-tolerant species) (Lillybridge et al. 1995, Chappell et al. 1997). Pre-European settlement fire regimes were typically of frequent, low-intensity surface fires, maintaining relatively open stands of a mix of fire-resistant species. With the advent of vigorous fire suppression, longer fire-return intervals are now the rule, and mixed-stature stands with Abies grandis in various size classes now provide fuel "ladders," making these forests more susceptible to high-intensity, stand-replacing fires (Cooper et al. 1987, Lillybridge et al. 1995). This alliance also includes montane forests along rivers and slopes, and in mesic "coves" which were historically protected from wildfires. They are very productive forests which have been priorities for timber production. Successional relationships in this alliance are complex. Pseudotsuga menziesii is less shade-tolerant than many northern or montane trees such as Abies concolor, Picea engelmannii, Thuja plicata, or Tsuga heterophylla, and seedlings compete poorly in deep shade. At drier locales, seedlings may be favored by moderate shading, such as by a canopy of Pinus ponderosa, which helps to minimize drought stress. In some locations, much of these forests have been logged or burned during European settlement, and present-day stands are second-growth forests dating from fire, logging, or other stand-replacing disturbances (Mauk and Henderson 1984, Chappell et al. 1997). Pseudotsuga menziesii forests were probably subject to a moderate-severity fire regime in pre-settlement times, with fire-return intervals of 30-100 years. Many of the important tree species in these forests are fire-adapted (Larix occidentalis, Pinus contorta, Pinus ponderosa, Populus tremuloides) (Pfister et al. 1977), and fire-induced reproduction of Pinus ponderosa can result in its continued codominance in Pseudotsuga menziesii forests (Steele et al. 1981). Seeds of the shrub Ceanothus velutinus can remain dormant in forest stands for 200 years (Steele et al. 1981) and germinate abundantly after fire, competitively suppressing conifer seedlings. Some stands may have higher tree-stem density than historically, due largely to fire suppression. Fire suppression has also led to the succession of Pinus ponderosa woodlands or Quercus spp. woodlands to Pseudotsuga menziesii forests.