This ecological system occurs in the northern Rockies of western Montana west into northeastern Washington and southern British Columbia. These are vegetation types dominated by Tsuga heterophylla and Thuja plicata in most cases, found in areas influenced by incursions of mild, wet, Pacific maritime air masses. Much of the annual precipitation occurs as rain, but where snow does occur, it can generally be melted by rain during warm winter storms. Occurrences generally are found on all slopes and aspects but grow best on sites with high soil moisture, such as toeslopes and bottomlands. At the periphery of its distribution, this system is confined to moist canyons and cooler, moister aspects. Generally these are moist, non-flooded or upland sites that are not saturated yearlong. Along with Tsuga heterophylla and Thuja plicata, Pseudotsuga menziesii commonly shares the canopy, and Pinus monticola, Pinus contorta, Abies grandis, Taxus brevifolia, and Larix occidentalis are major associates. Mesic Abies grandis associations are included in this system, and Abies grandis is often the dominant in these situations; Tsuga heterophylla and Thuja plicata can both be absent. Cornus nuttallii may be present in some situations. Picea engelmannii, Abies lasiocarpa, and Pinus ponderosa may be present but only on the coldest or warmest and driest sites. Linnaea borealis, Paxistima myrsinites, Alnus incana, Acer glabrum, Spiraea betulifolia, Symphoricarpos hesperius (= Symphoricarpos mollis ssp. hesperius), Cornus canadensis, Rubus parviflorus, Menziesia ferruginea, and Vaccinium membranaceum are common shrub species. The composition of the herbaceous layer reflects local climate and degree of canopy closure; it is typically highly diverse in all but closed-canopy conditions. Important forbs and ferns include Actaea rubra, Anemone piperi, Aralia nudicaulis, Asarum caudatum, Clintonia uniflora, Coptis occidentalis, Thalictrum occidentale, Tiarella trifoliata, Trientalis borealis, Trillium ovatum, Viola glabella, Gymnocarpium dryopteris, Polystichum munitum, and Adiantum pedatum. Typically, stand-replacement, fire-return intervals are 150-500 years, with moderate-severity fire intervals of 50-100 years.
Information Source: http://www.natureserve.org/explorer/