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 forest cover less than 50% of the area and are composed of Pseudotsuga menziesii or Picea engelmannii-Abies lasiocarpa. On granitic substrates forest 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.© by the Ecological Society of America. Abstract reproduced by permission.