On Gunsight Pass, the rain lancing down, I found a sharpedged rock that
split the continent in two. On both sides the rain rivulets ran down, a
fraction of an inch determining the stream’s destination: Pacific or
Atlantic.
The Continental Divide is a mighty barrier, a line of consequence that
does more than determine watersheds. Its effect in Glacier is dramatic,
as a look at the forests will reveal.
Obstructing the eastward flow of the moisture-laden Pacific winds, the
Divide extracts a heavy annual tribute of precipitation from the air
mass, forcing it to rise up the mountain chain, where it cools and
condenses. Chief benefactors are the low western valleys, which respond
with a lush growth of Pacific coastal-type forests.
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.