North Cascades National Park - Time and Change

At first glance the dim, serene forests of the North Cascades seem permanent, unchanging. But the reality is constant flux, an ebb and flow of destruction and rebirth that affects every living thing. The cycle may begin when disease or insects kill a few trees. Years later lightning strikes, and their tinder-dry skeletons burst into flame. The fire may consume only a few acres or it may last for months, charring thousands of acres. Such was the Big Beaver Valley near Sourdough Mountain; burning until October snow doused the last sparks, it devastated 40,000 acres.

When the snow melted the next spring, the scene was one of utter desolation. Charred trees littered the ground; those still standing were dead and bare. But tiny green sprouts soon rose from the blackened soil. The process of succession -- the change from bare land to mature forest -- had begun.

Succession follows no single set of rules. Elevation, soil, weather -- all may cause variations in the sequence. But there is a pattern, too. The healing process begins with the so-called pioneers, sun-loving plants that are the first to colonize a devastated land. Often the earliest pioneer is the aptly named fireweed, which thrives nearly anywhere it can get full sunlight. Growing up to six feet tall and topped by spires of lovely purple-rose flowers, by midsummer it may brighten, the charred landscape with a riot of color. Soon the fireweed is joined by other pioneers -- delicate ferns, for example, and fragrant blue lupines.

Eventually the pioneers are crowded out by woodier plants. The newcomers are almost certain to include huckleberries, small shrubs that grow throughout the Cascades. Sometimes red, sometimes deep blue, their tangy pea-sized fruits are prized by a variety of creatures, including human trailside snackers. Briery salmonberry and thimbleberry are among the other plants making up this second stage in forest succession.

Deer soon move in to browse on all the succulent new growth. (In the North Cascades they are either the Columbia black-tailed deer or the closely related mule deer.) The brushy thickets may also attract black bears. They come to gobble up the plentiful supply of berries, but also dine on almost anything else that is edible.

Although the tangles of new growth seem almost impenetrable, young trees eventually poke through, starting the third stage of succession. Curiously, though the Cascades' forests are dominated by evergreens, those first saplings to appear are likely to be broadleaf trees -- bigleaf maple, for instance, or black cottonwood, or western red alder. The red alder, clad with smooth gray bark, rarely grows more than 100 feet tall, but it grows quickly, crowding out the smaller, sun-loving shrubs that preceded it.

The final phase in succession begins whenever a break occurs in the canopy of broadleaf trees. A few alders may die from disease or be toppled in a windstorm. Sunlight streams through the openings, and stunted young evergreens that had been struggling for existence in the dim light on the forest floor suddenly gain a new lease on life. Taking advantage of the opening to reach for the sun, they begin to grow rapidly and soon overtop their broadleaf neighbors.

The prime benefactor of such openings is usually the Douglas fir, an adaptable tree that thrives in most Cascade environments. Able to survive drought and insulated against minor fires by foot-thick bark, it may live for 1,000 years and reach heights of 300 feet. Thus it is that the skyscraping Douglas fir dominates most Cascade forests -- but not all of them. In many cases other trees win out in the battle for survival and bring a welcome note on variety. On wet sites, western red cedars may crowd out the Douglas firs and dominate the forest. The smaller, hardier Pacific silver firs often take over at higher elevations where the climate is more severe. Another successful competitor is the western hemlock. Tolerant of dense shade, its seedlings may grow slowly but tenaciously for decades in dark, damp glens beneath the Douglas firs. When a tree of the older forest topples, the hemlocks quickly fill the breach in the canopy. And when the hemlocks in turn die of old age, there is nothing on the forest floor to replace them except more of the shade-tolerant young hemlocks. And so the mature forest remains a stand of hemlocks, self-perpetuating until fire, flood, or storm destroys it, and the process of succession begins anew.