The Ecology of the North Cascades is affected by high elevations and rain shadow effects. The North Cascades is part of the Cascade Range, stretching from the South Fork of the Snoqualmie River in Washington, United States, to where the Thompson and Fraser Rivers meet in British Columbia, Canada. This area is officially called the Cascade Mountains but is often called the Canadian Cascades. The North Cascades Ecoregion is classified as a Level III ecoregion by the Commission for Environmental Cooperation.
The terrain of the North Cascades includes high, rugged mountains. This region has the most active alpine glaciers in the conterminous United States and features many different climate zones. A dry continental climate is found on the east side, while the west side has mild, maritime, rainforest conditions. The area is made of sedimentary and metamorphic rock, unlike nearby parts of the Cascades, which are made of volcanic rock.
The North Cascades supports a wide variety of plant and animal life. Over 1,600 species of vascular plants grow there. The region is home to top predators such as bald eagles, wolves, grizzly bears, mountain lions, and black bears. It also has at least 75 mammal species and 200 bird species that breed or pass through the area. Eleven fish species live on the west side of the Cascades. Amphibians found in the region include the western toad (Bufo boreas) and the rough-skinned newt (Taricha granulosa).
The ecology of the area can be studied by imagining a line drawn from west to east at the southern end of the North Cascades, near 47.5 degrees north. As this line crosses the Cascade Range, it moves through several ecoregions. The regions change in temperature and dryness, with each having distinct tree species or no trees at all. These include western hemlock, Pacific silver fir, subalpine mountain hemlock, alpine areas, subalpine fir, and grand fir/Douglas fir.
Western Hemlock Ecoregion
The Western Hemlock Ecoregion is located in the lower west-side elevations of the North Cascades. Western hemlock trees grow from sea level up to 2,500 feet (800 m) in elevation. In the Alpine Lakes Wilderness, this ecoregion is found in lower areas between 1,000 and 2,000 feet (300 to 600 m). The average yearly temperature is 47 °F (8 °C), and this region receives between 70 and 300 centimeters (30 and 120 inches) of rain each year. This ecoregion is identified by thick forests of western hemlock, Douglas fir, western red cedar, and red alder. The area beneath the trees, called the understory, is mostly covered with plants such as salal, hazel, salmonberry, devil's club, and Oregon grape. The western hemlock (Tsuga heterophylla) is a tree that grows well in shaded areas. It is common to find young western hemlock trees growing under the forest canopy. This tree prefers moist and temperate conditions. When the environment becomes drier and colder, the tree does not grow as well. Western hemlock trees can grow over 200 feet (60 m) tall and have trunks up to 3 to 4 feet (0.9 to 1.2 m) in diameter. These trees can be recognized by their curved top part, called a drooping leader. It is common to see western hemlock trees growing in a line on a nurse log. The Western Hemlock Ecoregion supports many forms of life. Black-tailed deer feed in the understory. Foxes, coyotes, cougars, and many other herbivores and birds also live in these low-elevation forests.
Silver Fir Ecoregion
The mid-elevation forests in the North Cascades, located between 2,000 and 4,300 feet (600 and 1,300 m), are part of the Silver Fir Ecoregion. This ecoregion covers much of the valleys in the Alpine Lakes area. The average yearly temperature in this region is 42 °F (6 °C), and the average yearly rainfall ranges from 220 to 280 centimeters (87 to 110 inches). Typical forests in this region are dominated by Pacific silver fir and also include noble fir, Douglas fir, and Alaska yellow-cedar. Coarse woody debris is a common feature in the Silver Fir Ecoregion, creating small areas where organisms can live. The Pacific silver fir grows in some of the steepest parts of the landscape, where heavy snow often causes avalanche gullies. Evidence of avalanche tracks can be seen in every major drainage basin along the western slopes of the Cascade Mountains, where nonforested plants replace forested vegetation. These gullies create spaces for new plant growth as they gradually become part of a coniferous forest again. Because of the dense forests in the Silver Fir Ecoregion, this area was often chosen for logging before it was designated as wilderness.
The Pacific silver fir (Abies amabilis) grows well in shaded areas but does not thrive in dry conditions or when temperatures are high. It can grow up to 180 feet (55 m) tall and reach a diameter of 3–5 feet (0.9–1.5 m). The plants growing beneath the Pacific silver fir vary based on the amount of moisture available. Common shrubs in the understory include vine maple, salal, Cascade Oregon grape, blueberry, mountain huckleberry, devil’s club, and fool’s huckleberry. Common herbs include bear grass, twin flower, pipsissewa, dwarf dogwood or bunchberry, bead lily, trailing blackberry, low false Solomon’s seal, foam flower, trillium, oak fern, and lady fern. The forest canopy helps create a cooler and moister environment in the summer and a warmer one in the winter. Pacific silver fir seedlings and young trees often grow under their own canopy or in mixed forests. Along streams, areas where the forest is broken are replaced by mountain alder, willow, and vine maple. Herbs such as saxifrage, yellow willow-herb, monkey flowers, and bluebells can also be found in these areas. Mountain alder and vine maple are often found near lakes, on flat or gently sloping land, and in bogs or marshes. Examples of Pacific silver fir forests can be found in the Commonwealth Basin and Snow Lake areas of the Alpine Lakes Wilderness.
Subalpine Mountain Hemlock Ecoregion
As elevation increases from the Silver Fir Ecoregion, between approximately 4,000 and 5,400 feet (1,200 and 1,600 meters) on the western side of the Cascade Range, the area transitions to the Subalpine Mountain Hemlock Ecoregion. This region has a colder average annual temperature of 39 °F (4 °C) and receives between 160 and 280 centimeters (60 and 110 inches) of precipitation each year. This ecoregion includes mountain hemlock forests, subalpine meadows, streams, lakes, wetlands, and avalanche gullies, which create unique patterns of new growth. At the lower parts of this region, forests have continuous tree canopies, while higher areas have mixtures of meadows and patches of mountain hemlock forests. The mountain hemlock forests are made up of mountain hemlock, subalpine fir, Alaska yellow-cedar, and Pacific silver fir. In Washington, alpine and subalpine areas cover about 4.4% of the state’s total land area.
As elevation increases from the Silver Fir and Mountain Hemlock Ecoregions, mountain hemlock (Tsuga mertensiana) becomes the most common conifer, though it may share dominance with Alaska cedar and Pacific silver fir. Mountain hemlock trees can live up to 1,000 years, longer than Pacific silver fir. Trees aged 500 to 700 years may grow to 100 to 125 feet (30 to 40 meters) tall. Hemlock cones are 2–3 inches (5–8 centimeters) long and grow at the ends of branches. These trees are easily identified by their dense, grayish-green needles. According to Franklin and Dyrness, when mountain hemlock and Pacific silver fir share dominance, the area below the tree canopy is dominated by tall mountain huckleberry. When Alaska cedar is dominant, the area below the canopy is covered with rhododendron, huckleberry, and mountain ash.
At the highest parts of the subalpine ecosystem, where conditions are harsher, trees grow in scattered patches. These trees often take on a krummholz form, which means they have a bent or twisted shape. Trees in this area may appear to have branches only on one side, showing the direction of the wind. The height of the tree’s lower branches indicates the depth of snow cover, as branches do not grow where snow accumulates. Trees take this shape because strong winds and ice particles damage the outer layers of the tree, preventing branch growth. Wind also causes the needles to dry out, leading to branch death on the affected side.
Between the scattered tree patches in the upper areas are open meadows. Events like fire, avalanches, snow movement, and climate change cause the boundaries between trees and meadows to change over time. The forest patches in these areas usually include mountain hemlock, subalpine fir, and Alaska cedar. Trees often spread into meadows, and this happened most during the 1930s due to warmer temperatures. Tree invasion into meadows can also occur after disturbances like fires or landslides.
The meadows in this region are popular with hikers because of their beautiful wildflowers. Flowers found here include tiger lily, glacier lily, bead lily, queen’s cup, columbine, aster, trillium, pearly everlasting, valerian, skyrocket, shooting star, penstemon, lousewort, mountain bog gentian, monkey flower, monkshood, bluebell, bellflower, bleeding heart, Tweedy’s lewisia, balsamroot, and wild orchids. These flowers bloom most brightly in meadows and along streams from mid-July to mid-August. The parklands of the Mountain Hemlock Ecoregion are shaped by the climate and the land’s physical features.
The two main types of vegetation in this region, forests and meadows, have very different microclimates. The amount of sunlight and ultraviolet exposure varies greatly based on the time of day, slope, season, cloud cover, and vegetation. Temperatures change depending on how much sunlight is received. Parklands in this area have a much larger temperature range than hemlock forests, up to 50 °F (28 °C), while forests rarely have a range larger than 20 °F (11 °C). This is because the tree canopy in forests creates a more protected environment compared to open meadows. Soil temperatures also affect biological activity, such as the life of soil organisms and plant roots. Daily and seasonal temperature changes influence how much heat the soil gains or loses. However, snow cover acts as a protective layer, reducing temperature changes in the soil. Mountain hemlock forests are the wettest and coldest of the Cascade forest zones.
Many animal species pass through this region for at least one season each year, including mountain goats, black-tailed deer, American black bears, elk, cougars, and numerous bird species. Only the whistling hoary marmot lives exclusively in alpine and subalpine areas. In addition to mammals, many insects play a key role in supporting the abundance of flowering plants in this region. Hummingbirds are also important pollinators. Scientists continue to study the relationships between species in both alpine and subalpine ecosystems to better understand how they depend on each other.
Alpine Ecoregion
The Alpine Ecoregion covers much of the North Cascades. Alpine areas are rough and rocky, with snowfields, areas that have some plants, and are above where trees naturally grow. In the Alpine Lakes Wilderness, the line where trees stop growing is at about 6,000 feet (1,800 meters). The average yearly temperature is 37.5 °F (3 °C), and the yearly rainfall is about 46 centimeters (18 inches). These conditions, along with strong winds and blowing ice, make it hard for trees to grow. Because of the cold temperatures and little rain, there are fewer plant types compared to areas at lower elevations, and these plants are simpler in structure. However, the lack of rain is a bigger problem for plants than the temperature. Both plants and animals have developed special ways to survive in this difficult environment.
This high-elevation habitat has strong winds, long snow cover, steep slopes, wide temperature changes, and strong sunlight. These conditions lead to special ways that species adapt. Alpine areas often have less oxygen in the air, which makes it harder for animals to use energy. At higher elevations, animals usually have shorter times to breed, as seen in the North Cascades. Also, many animals move to different elevations seasonally to find food and shelter. However, some animals, like the white-tailed ptarmigan, hoary marmots, and pikas, stay at high elevations all year, relying on scattered alpine plants. Most animals move to lower elevations at some point in their lives. Examples of adaptations include hummingbirds resting to save energy, mountain goats and coyotes blending into their surroundings, animals storing extra fat, and birds using strong winds to fly efficiently. The white-tailed ptarmigan changes its feathers to white in winter and brown in summer to stay hidden. Many high-elevation species have fewer young but spend more time caring for them.
Although alpine areas have tough conditions, they also offer benefits for animals. In winter, heavy snow cover exposes plant stems and seeds for animals to eat. In spring, insects carried by wind from lower areas land on snow-covered areas, providing food for birds and mammals that breed in the alpine. When snow melts, it creates a gradual change in plant growth, giving herbivores food over time to eat and follow. Spring foraging is important for species like mountain goats. In late summer, plants near the edges of snowfields begin to grow and produce fruit, offering food for animals. Black bears, songbirds, and marmots in the North Cascades and Alpine Lakes can find shelter in thick plants near subalpine forests. There is also evidence that animals in high alpine areas have fewer infections and parasites, which is another benefit. For example, the alpine grouse has few blood infections or intestinal parasites. In summer, some animals move to higher elevations to avoid insects and feed on meadows.
Subalpine Fir Ecoregion
The Subalpine Fir Ecoregion is located on the east side of the Cascade Range and covers elevations between 4,200 and 6,000 feet (1,300 and 1,800 m). This area has the same average yearly temperature as the Mountain Hemlock Ecoregion, which is 39 °F (4 °C). However, it receives much less average yearly rainfall, between 100 and 150 centimetres (40 and 60 inches), with a greater amount falling as snow than as rain. The ecosystems in this region include subalpine fir forests, subalpine meadows, avalanche gullies, and freshwater wetlands, streams, and lakes.
The subalpine fir forests in the North Cascades contain trees such as Douglas fir, Engelmann spruce, subalpine larch, and whitebark pine. Engelmann spruce and subalpine fir are often found growing together. At the highest elevations of this ecoregion, subalpine fir trees grow in a twisted, low form called krummholz. The Subalpine Fir Ecoregion is similar to the Mountain Hemlock Ecoregion in that both have areas of forest and meadows in their upper regions.
Grand Fir/Douglas Fir Ecoregion
Located on the east side of the Cascade Range is the Grand Fir / Coast Douglas-fir Ecoregion, which has a highly varied forest. This forest contains the greatest number of different tree species among the forested ecoregions in Washington state. The trees found here include grand fir, Douglas fir, Engelmann spruce, subalpine fir, ponderosa pine, lodgepole pine, western white pine, whitebark pine, western larch, and subalpine larch. The average temperature in this ecoregion each year is 46 °F (8 °C), and it receives between 60 and 110 centimetres (24 and 43 inches) of precipitation annually. The elevation of this ecoregion ranges from 2,000 to 5,000 feet (600 to 1,500 metres).
In the higher areas of this ecoregion, the main trees are mountain hemlock and subalpine fir, while in the lower areas, grand fir and Coast Douglas-fir are most common. This region also has a wide variety of plants growing beneath the trees, such as pinegrass, elk sedge, sedges, low shrubs, vine maple, white alder, and huckleberry. This area provides a home for many species, including animals that eat plants, such as deer, elk, black bear, herbivores, and many types of birds.
Fauna
The North Cascades are home to many types of animals, including reptiles, amphibians, mammals, birds, and arthropods. A few grizzly bears (Ursus arctos horribilis) live in the far northern Cascades, near the border between Canada and the United States. In 2008, a group of wolves was found breeding in Okanogan County, marking the first such group in Washington state since the 1930s. Other predator species in the area include mountain lions, black bears, fishers, and wolverines.
More than 75 mammal species live in the region, such as the mountain goat, which is found in high alpine tundra areas. Birds in the area include bald eagles, ospreys, and harlequin ducks. Amphibians found in the North Cascades include the western toad (Bufo boreas) and the rough-skinned newt (Taricha granulosa). A unique trait of rough-skinned newt populations is that about 90% of the adult population is perennibranchiate.