This ecological system occurs throughout the mountains of the Pacific Northwest, from the southern Cascades of Oregon to the mountains of southeastern Alaska bordering British Columbia. It occurs at the transition zone of forest to alpine, forming a subalpine forest-meadow ecotone. Mountain hemlock forests, as they approach treeline, become open patches of mature-height trees surrounded by mesic and wet meadows rich in dwarf-shrubs and forbs. Clumps of trees to small patches of forest interspersed with low shrublands and meadows characterize this system. Krummholz often occurs near the upper elevational limit of this system where it grades into alpine vegetation. Associations include woodlands, forested, and subalpine meadow types. It occurs on the west side of the Cascade Mountains and is a transitional open forest into the true alpine on the interior side of the Coastal Mountains of British Columbia where deep, late-lying snowpack is the primary environmental factor. Major tree species are Tsuga mertensiana, Abies amabilis, Chamaecyparis nootkatensis, and Abies lasiocarpa. This system includes British Columbia Hypermaritime and Maritime Parkland (Tsuga mertensiana). Dominant dwarf-shrubs include Phyllodoce empetriformis, Cassiope mertensiana, and Vaccinium deliciosum. Dominant herbaceous species include Lupinus arcticus ssp. subalpinus, Valeriana sitchensis, Carex spectabilis, and Polygonum bistortoides. There is very little disturbance, either windthrow or fire. The major process controlling vegetation is the very deep long-lasting snowpacks (deepest in the North Pacific region) limiting tree regeneration. Trees get established only in favorable microsites (mostly adjacent to existing trees) or during drought years with low snowpack. It is distinguished from more interior dry parkland primarily by the presence of Tsuga mertensiana or Abies amabilis and absence or paucity of Pinus albicaulis and Larix lyallii.
Information Source: http://www.natureserve.org/explorer/