Major Vegetation Types in Alaska

The word “tundra” derives from the Finnish word for barren or treeless land. The tundra is the simplest biome in terms of species composition and food chains. There are two types of tundra: arctic and alpine. Arctic tundra occurs at higher elevations in mountain ranges farther south. Tundra occurs in regions above and/or beyond treeline that have cold climates, with short growing seasons of 6-10 weeks. Vegetation is low and is dominated by perennial forbs, grasses, sedges, shrubs, mosses, and lichens. The permafrost that occurs below most tundra impedes drainage, root growth, and plant decomposition. The arctic tundra, which is partially represented in Alaska, covers 20% of the North American continent.

Common Characteristics of Tundra

Low diversity of plant species, only about 700 species, low productivity, and low diversity of animal species as well.

Most plants are relatively long-lived (200 – 100 years) herbaceous perennials.

Soils are poorly developed, thin irregular, and often waterlogged.

Nutrients are largely unavailable because of lack of decomposition, which is due to the cold and the presence of mostly acidic plants.

Two-thirds of flora is insect-pollinated, so flowers are large and showy.

Some of these flowers are dish-shaped to trap solar radiation and create more favorable thermal environments for pollinators.

Short growing season, only 50 – 90 days, and growth rates are slow.

Small flora; no trees except near rivers, treeline, and sheltered slopes.

Low annual precipitation, typically 4 – 20” (10 – 50cm) per year.

Taiga or Boreal Forest

Both names can be used as synonyms to describe this biome. Taiga, or boreal forest, stretches across most of Canada and Alaska and dips down into the Great Lakes states and New England to about 45 N l atitude. Taiga is a Russian word applied to Eurasian conifer forests that are described as damp, wild and, scarcely penetrable. Taiga is translated from the Russian language as “the land of little sticks,” which can be somewhat misleading. Ecologists have given the term taiga a broader meaning in order to describe this biome. The taiga is a forest with trees of moderate height and the biome has low species diversity. It is a low elevation forest that extends over large distances and is interrupted by lakes, ponds, and moss-covered bogs. As shown on the map of Alaska, taiga is the primary biome that is found in the Interior. Examples of taiga vegetation include the forest, marshes, and bogs surrounding Talkeetna, the vast forests along the Parks Highway between Talkeetna and Denali, and the forest surrounding and within Denali National Park. Remember that in Denali, there is a treeline Ecotone between taiga and tundra; the treeline Ecotone will be discussed at the end of this section.

Common Characteristics of the Taiga

Cold, west region in which the mean temperature is below freezing for more than 6 months of the year

Annual precipitation is generally 12 – 34” (30 – 85cm), and most precipitation falls in summer.

Taiga soils are acidic, pH 3 – 5 (neutral pH is 7), with upper layers leached of clay, organic matter, and many nutrients.

All taiga conifers possess mycorrhizae and there is evidence to suggest that tree growth and survival would be minimal without them.

Litter tends to accumulate, and as much as 60% of all ecosystem carbon is locked up in humus.

Nutrient cycling is slowed by layers of feather mosses, up to 12 inches (30cm) thick, which act as a nutrient sponge, preventing nutrients from reaching the root zone.

White spruce, black spruce, birch, and aspen dominate taiga forests in Alaska.

Conifers are closely spaced, with dense canopies, and are relatively small [60 – 70’ (20m) tall, 12” (30cm) DBH – Diameter at Breast Height] with short longevity (100 – 300 years).

Many species of evergreen and deciduous shrubs are present but cover is modest, and the ground layer is dominated by feather mosses, lichens, and ferns – the forest is essentially a two-layered community.

Near bogs, the forest canopy thins and only scattered trees of black spruce and tamarack remain, which are underlain by shrubs and/or Sphagnum Moss.

Taiga Is a disturbance (disclimax) forest: small areas are often disturbed by wind thrown trees from winter storms or by fire from summer lightning strikes.

Succession often proceeds through meadow, then broadleaf trees, before reaching an even-aged spruce-hemlock climax in a span of approximately 300 years.

Microenvironment Differences in Taiga

Since most of the taiga biome in Alaska is located I the zone of discontinuous permafrost, there are significant differences in vegetation composition depending on exposure (which means which way they are facing relative to the sun).

On south-facing hills, where permafrost is absent, deciduous trees grow preferably. Here, you will find vast stretches of beautiful birch, aspen, and cottonwood forests (aspen and cottonwood cannot grow on permafrost). The understory is mostly composed of grasses, forbes, and shrubs. If a forest on a south-facing hill is old enough, white spruce trees will move in and eventually take over the space.

On north-facing hills and low lying areas, where permafrost is present, there will be no aspen, cottonwood or white spruce trees. The vegetation here is comprised of black spruce and birch trees and a wide variety of typical wet and moist tundra plants (mainly willow and alder shrubs, mosses, sedges, and berries such as lowbush cranberries and blueberries.) A black spruce does not have a taproot, like white spruce. Instead, their roots spread out in a mat just below the surface. This makes it thick. All plants in this permafrost environment are adapted to fairly wet and cool soils. The black spruce usually look sickly. They are never more than a few inches in diameter but they can be 100 years old! If you were to cut one down and count tree rings, you’d need a magnifying glass.

Creek and riverbeds are usually lined by beautiful big spruce (maybe 2 feet across and 100 feet high). They can grow right next to the water because the body of water dissipates heat and keeps permafrost from forming. Just a few yards away the effect of the warmer water is lost and you find permafrost from forming. Just a few yards away the effect of the warmer water is lost and you find permafrost and black spruce growing.

Observing the vegetation, you can easily determine the presence or absence of permafrost in the ground. Looking at a beautiful aspen/birch forest? – No permafrost. Looking at a flat area or a slope with scrawny (or even dead) spruce trees? – Permafrost. Looking at a flat area with some tall spruce trees lining up? – Those are white spruce and they line a creek or a river; no permafrost here.

The Treeline Ecotone

An Ecotone is an intermediate habitat, or transitional area, that lies between two biomes. Species from both biomes can be found in the transition zones. The treeline ecotone in Alaska and Canada lies between the tundra to the north and the taiga or boreal forest to the south. In some areas, this ecotone can be as much as 185 miles (300 km) wide. Within this ecotone, the forest first thins, and fruticose lichens cover the ground, then the trees become restricted to patches and are dwarfed, twisted, and shrub like. The German word krummholz, meaning twisted wood, has been applied to the dwarfed and twisted trees as well as this ecotone; another term that is used is elfinwood. These patches of dwarfed and twisted trees are surrounded by shrubby tundra. Eventually, all trees disappear as one reaches the tundra. This ecotone can be found in Denali National Park. For example, when traveling along the park road from the east entrance, one can see the taiga forest thin out, eventually giving way to tundra. This transition area between taiga and tundra is characterized by small to moderately sized trees that are spaced farther apart than the trees of the taiga forest.

The reason that trees do not grow in the regions beyond treeline is not that it is too cold. Cold itself does not kill trees; the coldest temperatures in the northern hemisphere occur in forested country, not in the tundra. It is the lack of summer warmth that sets a northern limit to tree growth (treeline is usually associated with the 10 C July isotherm). Sunlight – the “energizer” for photosynthesis – is plentiful in the arctic and sub arctic summer, but the photochemical reactions of photosynthesis cannot happen unless plant tissues are warm enough. The growing season in these regions is too short for a plant to produce wood, it has to put all its efforts into producing leaves without which it would starve, and roots where the food produced in the summer is stored to tide the plant over to the next year. Producing wood is a luxury plant in this environment cannot afford.

Temperate Conifer Rainforest

This biome includes Southeastern Alaska, along the Gulf of Alaska, and up through the eastern edge of the Kenai Peninsula. This biome is usually identified as a part of the biome that stretches up from the forest of the Pacific Northwest. The temperate conifer rainforest includes some of the largest trees; and stands of forest with the greatest biomass, that are found anywhere on earth.

Common Characteristics of Temperate Conifer Rainforest

Overstory trees are commonly 165 – 250’ (50 – 75m) tall and more than 6.5’ (2m) DBH.

Lifespan of these large trees are often beyond 500 years, and is sometimes beyond 1000 years.

This is the most productive forest in North America (in terms of biomass).

Softwood trees (evergreen) dominate for three reasons: evergreens are favored because potential for positive growth outside of the usual growing season is high; water stress can be high, favoring evergreens; and evergreens have a higher nutrient-use efficiency than hardwood (deciduous) trees.

The most common overstory tree is the Sitka spruce. Its needles tolerate deep shade as a sapling; it then adapts to full sun as a full-grown, emergent tree.

Winters are generally mild and approximately 80% of the annual precipitation falls during winter.

Annual precipitation may vary greatly within this biome, depending on elevation, geography, and microenvironment differences, but the areas that receive the highest precipitation can receive 120” (300 cm) or more per year.

The Role of Fire in Communities

Fire is an agent of change in communities. It can destroy much of the existing vegetation and return the successional cycle back to an earlier successional or seral community. Both plants and animals are well adapted to fire, so while a fire may initially seem very destructive, it also benefits many members of a community. Here in Alaska, fire typically sets the spruce-hemlock climax forest back into an earlier and more productive sequence of plant succession. A more productive sequence of plant succession refers to the fact that the communities that occur after a fire can support more wildlife (i.e. biomass) than the climax spruce-hemlock community. Generally speaking, fires occur every 80-200 years in an Alaskan forest community, although in some areas, wildfires may occur more often. It generally takes 150 to 300 years after a fire before the spruce-hemlock forest will return to the area. 

When a fire sweeps through area, it typically produces a mosaic pattern of burned and unburned areas. Winds, the amount of fuel, local geography, and the types of plants in the area will all play a part in how the mosaic pattern of burned areas come about. This mosaic pattern helps to provide a variety of habitats for the plants and animals which live in Alaska. With a wider range of habitats, there will generally be more biodiversity as well. Look for examples of this mosaic pattern along the Kenai River in the Pothole Lake burn area that occurred in 1991. 

Plant Adaptations to Fire

Black Spruce

It burns readily in wildfires, but its cones only open in the heat of a fire, thereby releasing seeds for a new generation of trees. This tree also grows in wet areas, which reduces the chances of being lost in fires. 

White Spruce

It is very susceptible to destruction by wildfire but depends upon heat to open up cones and regenerate. 

Birch and Aspen

These species produce thousands of wind-borne seeds which will re-establish them in burned areas.

Balsam Popular and Black Cottonwood

These trees rely upon their thick bark (up to 4 inches thick) and their location near rivers and streams for protection against fire.

Many types of plants, shrubs, and even some trees will re-sprout from their roots after a wildfire. Aspen, willows, and fireweed usually re-grow in this manner after a fire. One week after the Pothole Lake Fire in 1991, fireweed and cow parsnip were already
re-sprouting in the burned areas at Bear Beach, on the Kenai River?

Sources and Suggested Readings

Michael Barbour, Jack Burns, and Wanna Pitts. Terrestrial Plant Ecology. 1987. Benjamin/Cummins Publishing Co. Inc.

B. F. Chabot, and H. A. Mooney, editors. Physiological Ecology of North American Plant Communities. 1985. 

John C. Kricher, and Gordon Morrison. Ecology of Western Forests (Peterson Field Guides). 1993. Houghton Mifflin Co.

E. C. Pielou. A Naturalist’s Guide to the Arctic. 1994. The University of Chicago Press.

Peter H. Raven and George B. Johnson. Understanding Biology. 1988. Times Mirror Mosby.

United States Forest Service. Fire In Alaska.

Robert A. Wallace. Biology, the World of Life, 4th Edition.

Contributing Authors

Scott Thomas and Claudi Hoefle