Physiognomy and Structure: Stands of this alliance typically contain a dense canopy of tall conifers approaching 50 m or more in height. There is often a sparse to dense layer of cold-deciduous or evergreen shrubs. The herbaceous layer is usually a dense layer of shade-tolerant forbs and ferns, but in some cases can be sparse.

Floristics: These forests have a closed tree canopy less than 50 m tall that is dominated or codominated by Thuja plicata and/or Tsuga heterophylla. Other important and occasionally codominant conifers may include Abies grandis, Larix occidentalis, Pseudotsuga menziesii, or Taxus brevifolia. Broad-leaved cold-deciduous trees, including Acer macrophyllum, Betula papyrifera, and Populus balsamifera ssp. trichocarpa, may form a subcanopy in stands of this alliance. The woody understory of these forests is highly varied across the range of the alliance and may have sparse to dense cover. Broad-leaved deciduous and evergreen shrubs may include Acer circinatum, Alnus viridis ssp. sinuata, Amelanchier alnifolia, Lonicera utahensis, Mahonia aquifolium, Menziesia ferruginea, Paxistima myrsinites, Rosa gymnocarpa, Spiraea betulifolia, Spiraea betulifolia, Symphoricarpos albus, Vaccinium membranaceum, or Vaccinium parvifolium. The species composition and structure of the herbaceous layer in these forests are reflective of local climate regime and the degree of canopy closure. The most common dominant herbaceous species include Aralia nudicaulis, Clintonia uniflora, and Xerophyllum tenax. Other common herbaceous associates include Arnica latifolia, Asarum caudatum, Bromus vulgaris, Clintonia uniflora, Coptis occidentalis, Galium triflorum, Goodyera oblongifolia, Gymnocarpium dryopteris, Maianthemum stellatum, Prosartes hookeri (= Disporum hookeri), Pyrola asarifolia, Tiarella trifoliata, Trillium ovatum, and Viola orbiculata.

Dynamics:  Forest associations of this alliance are dominated by moderately fast-growing, shade-tolerant conifers, which require moist conditions and a moderate temperature regime for establishment and growth. Following disturbance, a variety of other conifer species can become established and dominate sites previously supporting stands of this forest alliance. Pseudotsuga menziesii, in particular, can become established and dominate sites for many years following disturbance. In the northern Rocky Mountains, stand-replacing disturbance can result in conversion to communities dominated by either Larix occidentalis or Pinus monticola. Typically, stand-replacement fire-return intervals are 150-500 years with moderate-severity fire intervals of 50-100 years. Specific fire influences vary with site characteristics. Generally, wetter sites burn less frequently and are older stands with more Tsuga heterophylla and Thuja plicata. Drier sites are younger and have more diverse canopies. These are the moistest mid-montane environments on the east side of the Cascades and in northeastern Washington.