The Ecology of the North Cascades is heavily influenced by the high elevation and rain shadow effects of the mountain range. The North Cascades is a section of the Cascade Range from the South Fork of the Snoqualmie River in Washington, United States, to the confluence of the Thompson and Fraser Rivers in British Columbia, Canada, where the range is officially called the Cascade Mountains but is usually referred to as the Canadian Cascades. The North Cascades Ecoregion is a Level III ecoregion in the Commission for Environmental Cooperation's classification system.
The terrain of the North Cascades is composed of high, rugged mountains. It contains the greatest concentration of active alpine glaciers in the conterminous United States and has a variety of climatic zones. A dry continental climate occurs in the east and mild, maritime, rainforest conditions are found in the west. It is underlain by sedimentary and metamorphic rock in contrast to the adjoining Cascades which are composed of volcanics.
The North Cascades has a diversity of plant and animal species. It contains more than 1630 vascular plant species. The range has a number of top predators, including bald eagles, wolves, grizzly bears, mountain lions and black bears. The range is home to at least 75 species of mammals and 200 species of birds that either pass through or use the North Cascades for a breeding area. There are also 11 species of fish on the west side of the Cascades. Examples of amphibian species occurring in the North Cascades include the western toad (Bufo boreas) and the rough-skinned newt (Taricha granulosa).
The ecology of the area can be understood by following a west-to-east line at the southern end of the North Cascades, at approximately 47.5 degrees north. As the line passes through the Cascade range, it passes through a number of ecoregions, first getting higher and colder, then getting warmer, yet drier. Each of these component ecoregions can be described by either a tree indicator species, or by a lack of trees: western hemlock, Pacific silver fir, subalpine mountain hemlock, alpine, subalpine fir, and grand fir/Douglas fir.
Western Hemlock Ecoregion
The Western Hemlock Ecoregion huddles in the lower west-side elevations of the North Cascades. Western hemlock is found from sea level up to in elevation. In the Alpine Lakes Wilderness this ecoregion can be found in the lower elevations around The average annual temperature is and it gets between in precipitation per year. This ecoregion is evidenced by the dense stands of western hemlock, Douglas fir, western red cedar and red alder. The understory is primarily composed of salal, hazel, salmonberry, devil's club and Oregon grape. The western hemlock (Tsuga heterophylla) is an extremely shade tolerant tree and it is common to find its seedlings and saplings in the understories of the forest floors. It prefers moist temperate conditions. As conditions get drier and colder they don't fare as well. Western hemlocks can reach over in height with a diameter of . They can be identified by their drooping leader at the top of the tree. It is not uncommon to find western hemlocks growing in a row on a nurse log. The Western Hemlock Ecoregion offers an abundance of life. Black-tailed deer graze in their understories. Fox, coyotes, cougars, and an assortment of herbivore mammals and birds can also be found in these low elevation forests.
Silver Fir Ecoregion
The mid-elevation forests in the North Cascades with an elevation between is the Silver Fir Ecoregion. The Silver Fir Ecoregion makes up for much of the valleys in the Alpine Lakes area. The average annual temperature in this ecoregion is and the average precipitation in centimeters is between . Typical montane forests in this ecoregion is dominated by Pacific silver fir and also contains noble fir, Douglas fir, and Alaska yellow-cedar. Coarse woody debris is very characteristic of the Silver Fir Ecoregion, providing microsites for organisms. The Pacific silver fir zone is in some of the steepest parts of the topography and heavy snow often leads to avalanche gullies. In every major drainage basin along the western slopes of the Cascade Mountains there is evidence of avalanche tracks breaking up the forested vegetation with nonforested vegetation. These gullies provide sites of new successional growth as they move toward a coniferous forest again. Because of the dense forests of the Silver Fir Ecoregion, it was the preferred area for commercial logging prior to designation of areas as wilderness.