Glacier Park has 30 species that are "endemic" to the region, those with ranges limited exclusively to the northern Rocky Mountains. All but one of these occur in cold, open areas characteristic of harsh, post-glacial environments. Many are relics of the post-glacial age or occur here because the diverse combination of environmental conditions create unique micro-habitats. Three major North American watersheds arise from Glacier National Park (Arctic, Atlantic and Pacific). Two climate zones (Pacific Maritime and Prairie/Arctic) are separated by the Continental Divide. Biomes range from the lower elevation pacific cedar-hemlock forest to the high alpine tundra. These life zones, separated along an altitudinal gradient, contain a range of biodiversity unmatched in the Northern Rockies.
Plant species in Glacier Park have affinities with four major floristic provinces: (1) Cordilleran [49%], including the southern and central Rocky Mountains as well as the Cascade Mountains of the Pacific Northwest; (2) Boreal [39%], similar to what one would find across Canada; (3) Arctic-Alpine [10%]; and (4) a few representatives from the Great Plains [1%].
Moist, temperate conditions on the west side of Glacier Park have allowed the eastern-most extension of Pacific cedar-hemlock forest to develop in the Lake McDonald Valley. Moisture from the Pacific coast condenses during its rise to the Continental Divide. Rainfall ranges from an average of 23 inches in the park's driest locations along the northeast and northwest edges of the park to 30 inches at West Glacier. Precipitation in excess of 100 inches may fall in isolated cirques near the Continental Divide.
On the east side of the Park, dry chinook winds sculpt trees along the high ridges while calmer conditions prevail in the aspen groves below. The difference in rainfall is not extreme, but the desiccating winds have made the plant communities very different on the east side. The dark, ancient cedar/hemlock forests of the west side are a stark contrast to the more open forests, glades and grasslands of the east side. Plant varieties change somewhat north to south as well because the north half of the Park is in the rainshadow of the Whitefish Range. The cedar-hemlocks give way to drier Douglas fir and lodgepole pine forests in the North Fork, Flathead River drainage.
The Park's plant cover is roughly 33% moist coniferous forest, 29% barren or sparsely vegetated rock/snow/ice, 16% dry coniferous forest, 8% dry meadow and prairie, 6% deciduous forest (primarily aspen and black cottonwood), 5% wet meadow or fen, and 3% lake surface water (with aquatic plants occurring in the shallower zones).