Engelmann spruce and subalpine fir forests comprise a substantial part of the subalpine forests of the Cascades and Rocky Mountains from southern British Columbia east into Alberta, and south into New Mexico and the Intermountain region. They also occur on mountain "islands" of north-central Montana. They are the matrix forests of the subalpine zone, with elevations ranging from 1275 m in its northern distribution to 3355 m in the south (4100-11,000 feet). They often represent the highest elevation forests in an area. Sites within this system are cold year-round, and precipitation is predominantly in the form of snow, which may persist until late summer. Snowpacks are deep and late-lying, and summers are cool. Frost is possible almost all summer and may be common in restricted topographic basins and benches. Despite their wide distribution, the tree canopy characteristics are remarkably similar, with Picea engelmannii and Abies lasiocarpa dominating either mixed or alone. Pseudotsuga menziesii may persist in occurrences of this system for long periods without regeneration. Pinus contorta is common in many occurrences, and patches of pure Pinus contorta are not uncommon, as well as mixed conifer/Populus tremuloides stands. In some areas, such as Wyoming, Picea engelmannii-dominated forests are on limestone or dolomite, while nearby codominated spruce-fir forests are on granitic or volcanic rocks. Upper elevation examples may have more woodland physiognomy, and Pinus albicaulis can be a seral component. What have been called "ribbon forests" or "tree islands" by some authors are included here; they can be found at upper treeline in many areas of the Rockies, including the central and northern ranges in Colorado and the Medicine Bow and Bighorn ranges of Wyoming. These are more typically islands or ribbons of trees, sometimes with a krummholz form, with open-meadow areas in a mosaic. These patterns are controlled by snow deposition and wind-blown ice. Xeric species may include Juniperus communis, Linnaea borealis, Mahonia repens, or Vaccinium scoparium. In the Bighorn Mountains, Artemisia tridentata is a common shrub. More northern occurrences often have taller, more mesic shrub and herbaceous species, such as Empetrum nigrum, Rhododendron albiflorum, and Vaccinium membranaceum. Disturbance includes occasional blowdown, insect outbreaks and stand-replacing fire. Mean return interval for stand-replacing fire is 222 years as estimated in southeastern British Columbia.
Information Source: http://www.natureserve.org/explorer/