This system of the northern Rockies, Cascade Mountains, and northeastern Olympic Mountains is typically a high-elevation mosaic of stunted tree clumps, open woodlands, and herb- or dwarf-shrub-dominated openings, occurring above closed forest ecosystems and below alpine communities. It includes open areas with clumps of Pinus albicaulis, as well as woodlands dominated by Pinus albicaulis or Larix lyallii. In the Cascade Mountains and northeastern Olympic Mountains, the tree clump pattern is one manifestation, but these are also woodlands with an open canopy, without a tree clump/opening patchiness to them; in fact, that is quite common with Pinus albicaulis. The climate is typically very cold in winter and dry in summer. In the Cascades and Olympic Mountains, the climate is more maritime in nature and wind is not as extreme. The upper and lower elevational limits, due to climatic variability and differing topography, vary considerably; in interior British Columbia, this system occurs between 1000 and 2100 m elevation, and in northwestern Montana it occurs up to 2380 m. Landforms include ridgetops, mountain slopes, glacial trough walls and moraines, talus slopes, landslides and rockslides, and cirque headwalls and basins. Some sites have little snow accumulation because of high winds and sublimation. Larix lyallii stands generally occur at or near upper treeline on north-facing cirques or slopes where snowfields persist until June or July. In this harsh, often wind-swept environment, trees are often stunted and flagged from damage associated with wind and blowing snow and ice crystals, especially at the upper elevations of the type. The stands or patches often originate when Picea engelmannii, Larix lyallii, or Pinus albicaulis colonize a sheltered site such as the lee side of a rock. Abies lasiocarpa can then colonize in the shelter of the Picea engelmannii and may form a dense canopy by branch layering. Major disturbances are windthrow and snow avalanches. Fire is known to occur infrequently in this system, at least where woodlands are present; lightning damage to individual trees is common, but sparse canopies and rocky terrain limit the spread of fire. These high-elevation coniferous woodlands are dominated by Pinus albicaulis, Abies lasiocarpa, and/or Larix lyallii, with occasional Picea engelmannii. In the Cascades and Olympics, Abies lasiocarpa sometimes dominates the tree layer without Pinus albicaulis, though in this dry parkland Tsuga mertensiana and Abies amabilis are largely absent. The undergrowth is usually somewhat depauperate, but some stands support a near sward of heath plants, such as Phyllodoce glanduliflora, Phyllodoce empetriformis, Empetrum nigrum, Cassiope mertensiana, and Kalmia polifolia, and can include a slightly taller layer of Ribes montigenum, Salix brachycarpa, Salix glauca, Salix planifolia, Vaccinium membranaceum, Vaccinium myrtillus, or Vaccinium scoparium that may be present to codominant. The herbaceous layer is sparse under dense shrub canopies or may be dense where the shrub canopy is open or absent. Vahlodea atropurpurea (= Deschampsia atropurpurea), Luzula glabrata var. hitchcockii, and Juncus parryi are the most commonly associated graminoids.
Information Source: http://www.natureserve.org/explorer/