In general, the western side of the park represents mixed conifer forests. In the southwest region you can find, the lush, sometimes ancient, Pacific cedar-hemlock forest types. Here, western hemlock and western red cedar reach their extreme eastern limits in the U. S. Their range fits perfectly within the eastern finger of the maritime climate of the Pacific northwest. Huge cedars and hemlocks have been growing for hundreds of years in the park and some trees of the lower Avalanche Creek area are over 500 years old. The cedars have stringy, spiraling bark and hemlock trunks sport frequent conks and horizontal lines of small woodpecker holes. From a distance, hemlocks can be recognized by their drooping tops. Most of Glacier's other tree species can also be found in these forests including the less common white spruce, paper birch, white pine and grand fir.
Low-growing Pacific yew is part of the understory in shady areas. It is anything but fire resistant, so it only occurs extensively in areas which have been free of fire for a hundred or more years, such as the cedar-hemlock forests. Junipers spread their low branches in open dry areas. More shrub-like mountain maples feed moose and elk in winter in the forest understory. Mountain ash are also fairly small and their clumps of orange berries add winter color to the Glacier landscape.
By contrast, aspen parklands are common on Glacier's east side where the landscape is frequently visited by chinook winds and summer downslope winds. The parklands are formed by open forests of quaking aspens in lowlands, sparse limber pine stands on the ridges and open grasslands between. Aspens fare better in the lowlands because their leaves can not survive the drying effect of the frequent winds of the ridgetops.
Uniform, even-aged stands of lodgepole pine reflect the recent fire history of the park, because their serotinous seed cones open and re-seed immediately following fire. Ponderosa pine, though common in Montana, makes up only a minor portion of stands in Glacier and is found only on the west side. It occupies the warmest and driest sites that will support forest. Douglas fir, also a dry forest species, occupies sites that are just slightly cooler and moister than the ponderosa forests. On the next most cool, moist slopes, another fire-dependent species, western larch, grows to heights of 150 feet. Larches provide beautiful autumn color in Glacier, being the only conifers in the park which yellow and lose all of their needles.
In the mid-elevations, subalpine fir and Engelmann spruce begin to become important forest components on cooler, moister sites. They become more and more dominant as elevation increases into the subalpine. White spruce is found in moist areas along streams and bottomlands as is black cottonwood (balsam poplar) at lower elevations.
Near persistant snowfields at treeline, alpine larch are uncommon, but found in relatively uniform stands. Whitebark pine once dominated the drier areas near treeline, but have been severely reduced by white pine blister rust infestation and to some extent, fire exclusion in the last hundred years. Clark's nutcrackers open the cones on whitebark pines and plant thousands of seeds each year by caching them in sunny treeline openings. Above 6000 feet in elevation, the trees become increasingly stunted and twisted in a form known as krummholz.