Plant and Animal Communities

• Life Zones
• Biodiversity

Life Zones1

Climate, elevation, soil development and other physical factors, which can be divided into distinct zones, influence the abundance and diversity of life. Each zone has its unique complement of continually evolving plant and animal communities. The northern Cascades are an excellent showcase of the North American life zone system.

Life Zones of the North Cascades

Humid Transition Zone

Douglas Fir (Pseudotsuga menziesii)

Dense Douglas fir and western hemlock forests characterize the west slope of the Cascades from sea level to about 1,500 feet. Streamside vegetation includes maple, alder, cottonwood, and dogwood. Ferns, mosses, mushrooms, and flowering plants like Oregon grape, salal, and salmonberry dominate the understory. This zone is home to western Washington's ancient old-growth forests.

Birds in this zone include spotted, barred, and great horned owls, ruffed grouse, band-tailed pigeon, Vaux's swift, pileated woodpecker, Stellar's jay, winter wren, chestnut-backed chickadee, golden crowned kinglet, Swainson's thrush, song sparrow, and purple finch. (See the NPSEC Spatial Data Inventory of Bird Species Distribution.)

Canadian Zone

Pacific Silver Fir (Abies amabilis)

You will notice a gradual change in vegetation types as you enter the Canadian zone from 1,500 to 4,500 foot elevation. Western hemlock, red cedar, and silver fir dominate the wet western slopes.

Birds of this zone include Barrow's goldeneye, red-breasted sapsucker, Stellar's and gray jays, mountain chickadee, red-breasted nuthatch, winter wren, dipper, varied thrush, MacGillivray's warbler, Lincoln's sparrow, and red crossbill. (See the NPSEC Spatial Data Inventory of Bird Species Distribution.)

Hudson Zone

Mountain Hemlock (Tsuga mertensiana)

From 4,500 feet to timberline you are in the Hudsonian zone. Similar to the northern climes of central Canada, this zone is characterized by mountain hemlock, subalpine fir, and white-barked pine.

Though no bird species are confined to this zone, typical inhabitants include blue grouse, rufous and calliope hummingbirds, three-toed woodpecker, Clark's nutcracker, Townsend's solitaire, hermit thrush, Townsend's warbler, fox sparrow, and white-winged crossbill. (See the NPSEC Spatial Data Inventory of Bird Species Distribution.)

Arctic-alpine Zone
The arctic-alpine zone is that area above timberline that epitomizes the mountainous terrain of North Cascades National Park. Here on the windswept ridges lie alpine meadows of heather interspersed with luetkea, huckleberry, Labrador tea, and a plethora of showy wildflowers. Snowfields, rock, and ice dominate the landscape.

Partridge Foot (Luetkea pectinata)	Red Huckleberry (Vaccinium parvifolium)	Labrador Tea (Ledum groenlandicum)

A few hardy bird species use this zone and include white-tailed Ptarmigan, black swift, common raven, horned lark, water pipit, and rosy finch. (See the NPSEC Spatial Data Inventory of Bird Species Distribution.)

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Biodiversity2

The North Cascades ecosystem contains many different habitats. Habitat is a combination of the physical environment – the rocks and land and water – as well as all the other organisms that live in the same place. Together these plants and animals make up an interacting, interdependent community. Over millions of years of geologic time, living things adapt to their environment. Every species finds a unique place in its habitat and community. Life is limited by the physical environment (temperature, wind, moisture, space, and length of growing season) and by other organisms (competition and predation). These habitats range from the microhabitat of the forest floor, a world of fir needles and decaying wood, home of the centipede and wolf spider, to the trout-filled waters hidden away in the backcountry and accessible only by days of rugged cross-country travel. The mountain forests support communities of plants and animals different from those of the river valleys. Plants that flourish in subalpine meadows are strangers to the more severe conditions of alpine ridges.

As one hikes up the river valley toward the montane forest, the plants and animals change. The giant ponderosa pines of the lowlands give way to Douglas fir and Pacific silver fir. The merganser and harlequin ducks of the lower river make way for dippers and spotted sandpipers. As we leave the river behind and climb higher, we enter the subalpine world of meadows and stunted trees.

Harlequin Duck (Histrionicus histrionicus)	Spotted Sandpiper (Actitis macularia)
Red-breasted Merganser (Mergus serrator) Photo © Mary Scott

Small cushion plants dominate the high meadows with their low growing, prostrate masses of green and brightly colored flowers. Climbing higher, toward the glacial snows, we enter a world of rock and ice. Along the rocky summit ridge of the peak, the only living things we find are lichens, a few insects, and rosy finches.

Montane Forests
Hiking trails wind through most of the major river valleys. Travel off-trail is difficult at best and nearly impossible in many places. River valleys are nearly impenetrable jungles of vegetation. Creeks and streambank habitats are choked with armies of inhospitable brush, including spiny devil's club, prickly currant, salmonberry, thimbleberry and willow. There are tales of mountaineers who spent days trying to bushwhack cross-country out of the Picket Range, crawling through brush that yielded less than a mile's passage for each day's effort.

Salmonberry (Rubusa spectabilis) Photo Credit: Markku Savela	Thimbleberry (Rubus parviflorus) Photo Credit: Markku Savela

Lowland river valleys contain magnificent forests dominated by western hemlock, western red cedar, and Douglas-fir trees. While some forests on public and private land are managed for timber harvest and some are managed for multiple use, others are designated wilderness areas. Old-growth stands on public and private lands in the North Cascades are those areas of virgin, uncut forest that contain large, old trees together with trees of different ages. The diversity of age classes in an old-growth forest provides a rich understory and canopy of vegetation. The different levels of canopy allow sunlight to touch the forest floor in places. Old-growth forests also contain snags – standing dead trees – and "nurse" logs that provide shelter for a variety of plant and animal life. Eventually, the snags and logs decompose and become new soil. Some of the last extensive stands of old-growth forest left in the United States are the lowland forests of the Cascade Mountains.

The greatest concentration of old-growth western red cedar in the North Cascades is found along the lower reaches of Big Beaver Creek below 2,000 feet. Western red cedars are a long-lived species; the oldest are estimated to be over 1,000 years old. Red cedars grow in moist habitats; therefore, they are ideally suited to the North Cascades, where summers are cloudy and winters are mild. Tapering from heavily buttressed bases often over 10 feet in diameter, they rise 200 feet above the forest floor, balanced on a shallow, wide-spreading root system that provides stability in the wet valley soils. They are distinguished by their stringy, fibrous bark, which peels away in thin strips. Lacy sprays of flat needles give western red cedars a delicate appearance despite their great size. Native peoples living on the west side of the Cascades and along the coast used all parts of the "tree of life," making clothing and blankets with the inner bark and totems, dugouts, cooking utensils, and lodging with the wood.

Another dominant tree of the river valleys is western hemlock. Both western red cedar and western hemlock are shade-tolerant; their seedlings thrive in the dense shade of lowland forests. Western hemlocks in an undisturbed area eventually out-compete less shade-tolerant trees like Douglas fir to form the climax community of the low-elevation forests of the North Cascades. Growing in scattered, open glades throughout the hemlock forest are red alder and bigleaf maple. These two deciduous trees must have some scattered sunlight for their seedlings to grow. They appear along trails and streams and in openings created when a forest giant falls during a windstorm. Red alder is an important pioneer species that adds nitrogen to the soil through nodules of symbiotic bacteria (mycorrhizae) on its roots. Bigleaf maple is easy to identify with its giant five-lobed leaves spanning 16 inches or more.

When walking along a forest trail, occasionally climbing over fallen trees and ducking under low-hanging branches, we see a spectrum of the many shades of green. Huckleberries tantalize the hiker along the trail. In drier locations, salal and Oregon grape, both evergreen shrubs, dominate the understory, while sword fern and its many relatives grow throughout the forest. In springtime the forest floor is thick with wildflowers. Trillium, bleedingheart, yellow violet, calypso orchid, twinflower, and arrow-leaf balsamroot display their delicate colors and brighten the forest shade. Mosses, lichens, and fungi grow thickly over and under the ground and on exposed rocks and fallen trees.

Wild Bleedingheart (Dicentra formosa) Photo Credit: Bjorn Thon	Twinflower (Linnaea borealis) Photo Credit: Steve Johnson

Only gradual changes in the physical environment and in the plant and animal communities distinguish river valleys from mountain forests. A slight chill in the air is felt that brings an awareness of snowy peaks rising from these mountain forests. As we climb, the land becomes drier and more open. The red cedars give way to a mixture of western hemlock and Pacific silver fir. Red alder, Douglas fir, and Bigleaf maple still grow in sunlit openings.

Every now and then, a trail crosses an open slope, a remnant of avalanches that thunder down from surrounding peaks every winter. Vine maple and many other shrubs compete vigorously for space and light on these avalanche paths. At about 4,500 feet elevation mountain hemlock replaces western hemlock in the forest canopy.

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References Cited

1. National Park Service. North Cascades: Life Zone of Birds. <http://www.nps.gov/noca/birdzones.htm>

2. National Park Service. The Diversity of Life and its Processes. <http://www.nps.gov/noca/biodiversity.htm>

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