With elevation gain, average daily temperature drops at the rate of 5° per 900 meters. Precipitation, wind velocity, and evaporation loss increase. Soil thins. These factors, along with others such as fire frequency, north or south exposure, and availability of moisture, combine to determine the range of each community. In the forest community below 1,800 meters, Douglas-fir, lodgepole pine, and western larch predominate. In the valleys, Engelmann spruce and subalpine fir are found. The somewhat lower and much better watered western valleys of the park support western redcedar and western hemlock. Treeline is the upper limit to which the tolerances of trees to environmental conditions permit them to grow. Because there are so many controlling factors (wind, temperature, exposure to sunlight, snow cover, etc.) treeline in the diagram is only approximate. In Glacier it averages 2,000 meters. Avalanche chutes or sheer cliff walls may suppress it to below 1,500 meters; on protected slopes it may be as high as 2,150 meters. At the eastern edge of the park below 1,200 meters, the forest gives way to the prairie community, composed mostly of soft-stemmed plants adapted to the conditions of low precipitation that prevail here in the rainshadow of the mountain range. Clumps of aspen, found in the prairie in sheltered spots, occur here in the transition zone between prairie and forest.] [Illustration: A Mountain Profile This diagram represents the eastward-facing slope of a hypothetical mountain near the eastern boundary of Glacier National Park. Its life communities are somewhat different from those of mountain slopes at the western edge, chiefly because of the differential in annual precipitation. Illustration: Here, above approximately 2,750 meters, in a realm of ice, snow, and barren rock, there is little life. Alpine tundra Below 2,750 meters and above 2,000 meters, depending on other factors such as exposure to sun and wind and steepness of terrain, exists the alpine tundra community, with vegetation similar to that of the vast, essentially level, treeless zones of the Arctic. Scrub-forest Roughly between 1,800 and 2,000 meters, the dominant vegetation is scrub-forest. Trees here are stunted; except in sheltered spots they are more or less prone rather than upright. Net growth is slow, not only because of the short growing season but also because of the pruning effect of icy mountain winds. Very few tree species can survive in this harsh habitat. Coniferous forest