Vegetation of the Big Horn Mountains, Wyoming, in Relation to Substrate and Climate

The vegetation of the Big Horn Mountains is typical of the Central Rocky Mountain region; a lower Juniperus osteosperma zone is followed by Pinus ponderosa, Pseudotsuga menziesii, Pinus contorta, and Picea engelmannii—Abies lasiocarpa zones. Rock or geologic substrate type has a strong influence on forest vegetation. On sedimentary areas forests cover less that 50% of the area and are composed of Pseudotsuga menziesii or Picea engelmannii—Abies lasiocarpa. On granitic substrates forests cover over 80% of the surface and are composed chiefly of Pinus contorta with Picea engelmannii—Abies lasiocarpa at higher elevations. Populus tremuloides is present but unimportant as a vegetation type. Most of the rainfall comes during March, April, and May; precipitation is less than 30 mm per month during summer. Thus soil—water relationships are important in determining vegetation pattern. Precipitation comes from the east, which allows forest vegetation to reach lower elevations on the east than on the west slope.