- Also called:
- boreal forest
News •
Climate
Coldness is the dominant climatic factor in taiga ecosystems, although a surprising diversity of climates exists. Several factors—namely, the solar elevation angle, day length, and snow cover—conspire to produce this cold climate. In the taiga biome the Sun is never directly overhead (90°) as it can be in the tropics. The maximum solar angle decreases with increasing latitude. At latitude 50° N in the southern part of the taiga biome the maximum solar angle is 63.5°, and at the Arctic Circle it is only 47°. As a result, solar energy is less intense in the taiga biome because it is spread out over a greater area of Earth’s surface than it is in equatorial regions. Day length also affects temperature. Long winter nights at high latitudes allow radiation emitted by the surface of Earth to escape into the atmosphere, especially in continental interiors where cloud cover is less abundant than it is near the coast. Snow cover too affects the climate, because it reflects incoming solar radiation and amplifies cooling. During winter a snowpack persists for at least five months in the southern portion of the taiga biome and for seven or eight months in the northern reaches. The taiga actually mitigates this cooling because it roughens and darkens what would otherwise be a smooth, snow-covered, energy-reflecting surface for much of the year. It has been estimated that Earth would be significantly colder without the taiga.
The northern limit of the North American taiga coincides with the mean position of the Arctic front—the boundary between Arctic and mid-continental air masses—in the summer; its southern limit coincides with the mean frontal position in the winter. Mean annual temperatures in the taiga range from a few degrees Celsius above freezing to −10 °C (14 °F) or more. Areas with a mean annual temperature below freezing are susceptible to the formation of permafrost soils (frozen ground; see below Soils).
The mean temperature of January, the coldest month, is generally less than −10 °C (14 °F) across the taiga. The taiga includes areas that experience some of the lowest temperatures on Earth, excluding Antarctica. At the height of winter an intensely cold pocket of air develops over inland areas of far eastern Siberia; mean temperatures of −50 °C (−58 °F) have been recorded in this region. As this Siberian cold air flows over the unfrozen northern Pacific Ocean, a great temperature contrast develops that results in strong, westward-moving storm systems. The movement, position, and strength of these storms control much of the weather in the Northern Hemisphere.
Boreal forests do not grow on areas surrounding the Bering Strait. A rigorous cold climate with a very short snow-free season precludes the growth of trees on the Russian side of the Bering Strait in the Chukotka region of the Russian Far East. On the North American side, in western Alaska, summers are too cool for trees to grow, because of cold air masses moving off the Bering Sea.
The growing season in the taiga is generally cool. The mean temperature of the warmest month, July, is generally between 15 and 20 °C (59 and 68 °F). Summer daytime high temperatures are typically cool to warm—20 to 25 °C (68 to 77 °F)—for much of the growing season in the taiga. Cool summer temperatures can actually produce higher photosynthetic efficiency in plants than can warmer conditions. Species adapted to cold respire less (use up less of their food stores) when photosynthesizing at cool temperatures in intense summer light than they do at higher temperatures, allowing a greater net gain in biomass (dry mass of organic matter).
Areas of the taiga located in the centre of continents generally receive 30 to 50 cm (12 to 20 inches) of annual precipitation. Precipitation totals are relatively modest in these locations because they are a significant distance from unfrozen oceans that supply moisture. Some taiga regions are semiarid and may even include grasslands interspersed with the forest. These forests are found in regions of Yukon and Alaska that occur on the leeward side of mountains which are sheltered from moisture-bearing winds, as well as in some portions of the interior of the Far East region of Russia. Annual precipitation in low elevations of these regions is 30 cm or less. The highest annual precipitation total in the taiga, which can exceed 100 cm, is in eastern North America and northern Europe. During ancient eras of colder climate, these regions also received relatively abundant precipitation, which resulted in the buildup of glacial ice sheets. Today these once heavily glaciated regions support extensive lakes, streams, and wetlands.
Extended periods of clear, dry weather in the boreal region are caused by persistent strong polar high pressure systems. If strong high pressure persists during the long days near the summer solstice, temperatures can warm to 30 °C (86 °F) or higher. Intense heating at the ground surface often produces convective storms with lightning but little rain, causing forest fires.
Soils
Taiga conifer litter is highly acidic. Soils of the more humid and southern taiga are highly leached spodosols, which are characterized by the leaching of iron, aluminum, and organic matter from the chemically and biologically distinct surface layer—horizon A—to the next layer—horizon B. Much of the soil of central and eastern Canada—granitic Canadian Shield—has been repeatedly scraped clean by glacial advances. Thus, productive forests often are restricted to portions of the landscape where soil material has been deposited by glaciers. Peaty wetlands occur where surface drainage is impeded by permafrost, youthful glacial topography, or aggraded rivers; their soils are characteristically organic soils, or histosols. Soils in much of boreal western North America and Asia are inceptisols, which have little horizon development. Very thin surface salt deposits are found in the most arid portions of the taiga.
Cold soils are characteristic of taiga regions, which overlaps the zone of permafrost. Permafrost is soil or earth material that remains below 0 °C (32 °F) for at least two years. The surface, or active, layer of permafrost thaws in the warm season and freezes in the winter, but the soil below the active layer remains continuously frozen. Because the plant rooting zone is restricted to the active layer, nutrient supply is limited and secure anchoring for roots is lacking. Some trees and other plants of the taiga (especially black spruce [Picea mariana] and tamarack [Larix laricina] in North America and larches in Siberia) can grow on permafrost if the active layer is sufficiently deep, but several species are eliminated from permafrost.
The taiga itself is an important contributing factor to the development of permafrost. The latter stages of forest growth—characterized by development of an intact forest canopy, growth of an insulating moss cover in summer, and accumulation of forest litter—may cool the soil to such an extent that permafrost develops. Warming of the soil is promoted by forest fires, which remove the canopy, moss, and forest litter layers. In the absence of an intact canopy, a deeper and more effective insulating layer of snow accumulates in the winter. The presence of dark ash following a fire increases solar energy absorption on the site for several years.
The taiga of Europe generally lacks permafrost, but east of the Ural Mountains and from central Canada northward permafrost is common. In southern and central parts of the taiga, permafrost occurs sporadically and occupies only a small percentage of the landscape that experiences the coldest temperatures. The northern portion of closed-canopy forest and the lichen woodland zone are in a region of discontinuous permafrost, where permafrost is found on north-facing slopes and in cold air drainage basins but is absent from south-facing slopes and newly deposited alluvial sites. Most of the forest-tundra is within the continuous permafrost zone.
Forest productivity in the middle and northern taiga zones is directly related to soil temperature. Warmer soils decompose organic matter more quickly, releasing nutrients for new plant growth and creating a more productive site. Productive forest types occupy warmer, south-facing slopes and river terraces, and less productive dwarf or sparse forest occupies the north-facing and basin permafrost sites.
Floodplains throughout the taiga biome are free of permafrost, high in soil fertility, and repeatedly disturbed in ways that renew the early, rapid growth stages of forest succession. Floodplains are a mosaic of productive shrubland and forest that serve as a major habitat for moose (Alces alces), which influence ecosystem structure and function.
South-central Alaska and adjacent Yukon and British Columbia support the most extensive ice sheets and glaciers in the world outside the polar desert regions of Antarctica and Greenland. Glacial meltwater is a large part of the flow of larger rivers such as the Yukon and Tanana in Alaska and the Yukon territory. Glacial meltwater carries a heavy load of suspended sediment that deposits in riverbeds and causes frequent channel shifts. Glacial river floodplains are extensive, very dynamic, and constantly renewed with fertile soil material. In the ancient past exposed deposits of glacial silt were picked up by strong winds and deposited on surrounding hillsides. Fertile soils, known as loess, resulted, on which highly productive upland forests are found today. Because the beds of glacially fed rivers are rising, the landscape through which they flow is partially drowned from the impeded drainage, often preventing forest growth and favouring the development of marshes and mires.
The biota and its adaptations
Nearly all major taxonomic groups have fewer species of animals and plants in the taiga than they have in other terrestrial ecosystems at lower latitudes. This accords with the species diversity gradient that is observed from lower to higher latitudes, with numbers of species decreasing in a poleward direction.
Trees
Scotch pine is the most widely distributed pine species in the world, growing from northern Scotland to the Russian Pacific shore. The relatively humid and productive taiga of northern Europe and south-central Siberia is dominated by this species. Forest management has greatly favoured this species in Scandinavia and Finland. It is a thick-barked species and easily survives light ground fires, often reaching ages of 350 to 400 years and some individuals being older than 700 years. European aspen and Siberian spruce are essentially transcontinental in distribution as well.
The species composition of Eurasian taiga is different east of central Siberia from that which prevails westward into Europe. Distinctive European species include Norway spruce (Picea abies), a large dominant species of the productive humid parts of the taiga, and Sukaczev larch (Larix sukaczewii), an early successional species (one of the first species to colonize an area after a disturbance) of European Russia. Gray (Betula populifolia) and white birch (B. pendula) occur across northern Europe and well into central Siberia. The birches often form dense stands of light- or white-barked trees that are considered a characteristic feature of the taiga. Siberian larch (Larix sibirica) and Siberian fir (Abies sibirica) are restricted to north-central Asia. Species restricted to northeastern Asia include chosenia (Chosenia arbutifolia), an early successional broad-leaved tree of floodplains; Siberian stone pine (Pinus sibirica), a short shrub or tree; and Asian spruce (Picea obovata).
All North American tree species are distributed across the continent except jack pine (Pinus banksiana), lodgepole pine (Pinus contorta), and balsam fir (Abies balsamea). Jack pine is a relatively small, short-lived, early successional tree occurring in the eastern and central parts of taiga east of the Rocky Mountains. Lodgepole pine is a longer-lived, early successional species growing in western Canada and along the Rocky Mountain axis from central Yukon southward to well south of the taiga limit. Balsam fir is a shade-tolerant, late successional, but relatively short-lived tree that occurs only in the eastern and central parts of the North American taiga.
Major taiga tree species are well adapted to extreme winter cold. The northernmost trees in North America are white spruce that grow along the Mackenzie River delta in Canada, near the shore of the Arctic Ocean. The northernmost trees in the world are Gmelin larch (Larix gmelinii) found at latitude 72°40′ N on the Taymyr Peninsula in the central Arctic region of Russia.
A representative profile of the vegetation is shown in the .
Other plants
A distinctive feature of the flora of taiga is the abundance and diversity of mosses. About one-third of the ground cover under taiga is dominated by moss. Much of the ground cover in older conifer stands is moss, which grows on rocks, on tree trunks, and in the pits formed by upturned trees. Extensive peaty wetlands in the boreal region are often thick accumulations of dead sphagnum and other mosses, sedges, and other plants; a living moss layer continually grows at the surface.
Lichens (a symbiotic association of a fungus and algae) constitute a significant part of the ground cover in the lichen woodland or sparse taiga. Lichens are also generally well distributed on tree trunks and especially in the canopy of older conifers throughout the taiga. Because lichens and mosses are dispersed by airborne spores that can travel long distances, many species of both groups are found across the entire circumpolar taiga.
Many vascular plants are also widespread across the circumpolar north. Some forest understory species dominate their habitats; they include twinflower (Linnaea borealis), lingonberry (Vaccinium vitis-idaea), baneberry (Actaea rubra), and Swedish and Canadian dwarf cornel (Cornus suecica and C. canadensis). Several taiga plants are adapted to rapid colonization and growth in recently burned areas, such as fireweed (Epilobium angustifolium). The extensive peatlands of the boreal north support a typical flora that usually includes species such as Labrador tea (Ledum palustre), cloudberry (Rubus chamaemorus), cotton grass (Eriophorum species), and crowberry (Empetrum nigrum or E. hermaphroditum). In northern Europe crowberry also grows as shrub mats under Scotch pine forests or woodlands. Crowberry has been shown to produce secondary chemical compounds that inhibit or kill Scotch pine seedlings. Periodic light ground fires reduce the abundance and vigour of crowberry and allow tree regeneration.
Specialized orchids in the forest understory include calypso (Calypso bulbosa), coralroot (Corallorrhiza trifida), and lady’s slipper (Cypripedum species). The roots of these plants form particular associations with fungi (mycorrhizae). Willow shrubs (Salix species) are one of the first plants to emerge following disturbances on floodplains and occasionally on uplands as well. Important grasses across the boreal region include species of bromegrass (Bromus species), bluegrass (Poa species), reed bent grass (Calamagrostis species), and vanilla grass (Hierochloe odorata). Many freshwater aquatic plants such as sedges (Carex species) and pondweeds (Potamogeton species) are distributed widely across the boreal zone of both continents because migratory waterfowl and shorebirds are effective in dispersing their seeds. Several species of ferns are common to the taiga regions of the two continents, especially in regions of higher precipitation.
