The eastern valleys, however, deprived of abundant annual moisture and exposed to the wind and temperature ravages of the prairie’s continental climate, support a dramatically different kind of forest. Here Engelmann spruce and subalpine fir are the climax trees, contrasted with such trees as the western redcedar and western hemlock of the mild and moist McDonald valley. Elevation exerts an additional restriction on the distribution of tree species. Since climatic conditions vary with change in elevation—lower temperatures resulting in shorter growing seasons, and increased wind exposure resulting in greater loss of moisture through evaporation—we would expect to find the forest composition change as we ascend a mountain slope. In Glacier, eastern valleys average 240 meters higher than western, and thus even if they had more moisture they would not sustain the redcedars and hemlocks. All plants have range limits, some narrow, some broad; and they excel where their particular set of preferences as to moisture, soil, sunlight, and wind exposure are best met. On sites that do not meet their optimum requirements, they face being crowded out by species better adapted to the prevailing conditions. Physical features of the land determine vegetation also. Certain trees prefer the moist areas along a streambed—the great black cottonwoods, for example. And on steep hillsides, avalanches prevent the growth of climax trees, permitting instead only shrubby, pliant growth—mountain-ash, mountain maple, alder, menziesia. Forest communities are named for their dominant tree species. Thus, an area in which Douglas-fir dominates is called a “Douglas-fir forest.” Glacier does have forests in which Douglas-fir is the climax species; these are chiefly dry areas, below 1,800 meters, with south and west exposures. But we usually associate the park with its Engelmann spruce-subalpine fir forests, found extensively between 1,200 and 2,100 meters, and with the western redcedar-western hemlock forests in the McDonald valley. Because forests mature slowly and change is usually imperceptible, we are tempted to think of them as static and eternal. But since a forest is a community of living things, it responds to changes in the environment. Subtle physical or climatic changes, such as a rising or falling water table or a slight increase or decrease in annual precipitation, will favor some species of trees and hinder others, eventually altering the composition of the forest. Other changes are more dramatic. Most notable of these is fire. From Fire to Forest