The verdant forests of Glacier National Park appear robust and enduring, but they are constantly changing. Without periodic disturbances caused by fire, blow-down, avalanches and forest diseases, conditons favorable for new growth would not exist. Trees grow old, die and are replaced by other species through a process called "succession". On the Trail of the Cedars along Avalanche Creek, and on the Johns Lake Trail, visitors walk through some of the shadiest parts of the park. Huge western red cedars, hemlocks and cottonwoods absorb nearly all the direct sunlight in their expansive canopies. Some plants that thrive in such heavily shaded understory, like fungi and saprophytes, do not even bother with photosynthesis. The large trees that grow in this area date back to around 1517 and are now approaching the 500 year mark in age!
Because the cedars and hemlocks shade the area so completely, only their own kind can reproduce. That makes them the "climax" species. But they weren't always there. Once, a very long time ago, a large fire probably burned this area. Then grasses and shrubs invaded the new openings. Lodgepole pine, whose serotinous seedcones are specially adapted to open during a fire, started to grow in the new sunlit openings. Under the growing lodgepoles, shade-tolerant trees like spruce and firs could also grow. When the spruce and firs matured, extremely shade tolerant trees like western hemlock and in the moister areas, red cedar, could take root and grow. Eventually this pattern of succession culminated in the climax forest we see today.
Forest succession usually isn't this neat. Fires, wind, disease and avalanches can set the process back at any time. These interruptions are the rule rather than the exception. Add variables like soil type, moisture, elevation and climate, and the process becomes very complex. Prior to the large fire of 1929 the lodgepole pine forests along the park's west-side contained large stands of cedars and hemlocks. Old stumps of those trees remain today and a few small cedar and hemlock seedlings aspire to dominate the forest canopy a few centuries from now. We don't know whether another cedar-hemlock forest would develop in our existing climate.
Although the difference between a "tree" and a "shrub" is qualitative, botanists count 90 or so species in Glacier Park as shrubs and 20 as trees. Of the trees, none is more interesting than whitebark pine which grows near timberline in the subalpine zone. This gnarled-looking tree is visited by Clark's nutcrackers, grizzly and black bears, red squirrels, crossbills, blue grouse, and many other mountain animals flocking to it for the nut crop. In the early part of the 20th century white pine blister rust was accidentally introduced to North America. This disease has killed about half of the whitebark pines in the park and most of the others are infected. Further complicating the plight of whitebark, fire suppression during the past hundred years has allowed shade-tolerant trees to out-compete the whitebarks.
Whitebark pine is a slow-growing and long-lived timberline tree. Some of the oldest specimens in Glacier Park reached about 700 years in age before they recently succumbed to blister rust. A whitebark pine in Idaho's Sawtooth range has been dated at more than 1200 years old. Geologists and climatologists use their extreme age to advantage in aging prehistoric events. It is an important "keystone" species and the uphill struggle to restore whitebark pine communities through prescribed fire and replanting rust-resistant trees has begun in the park. On the wind-swept ridges of the east side, limber pine plays much the same role as the whitebarks. This species has been infected to a lesser degree by the same disease. Restoration of both species is a management priority in Glacier National Park.