Biome

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.

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.

Insects

The taiga is the home of relatively few species of insects, but extensive and usually uniform areas of habitat periodically support high populations of species that do live there. The taiga lacks the elaborate complexes of invertebrate predators and parasites that serve as stabilizers of the insect populations in warmer regions. As a result, boreal insect populations occasionally increase rapidly and cause outbreaks. Some outbreaks can injure or kill trees across widespread areas of the taiga. Once an outbreak reaches a certain size, it can become self-sustaining, much like a forest fire; the effects of the spruce budworm and spruce beetle in North America are well-documented examples. Outbreaks can be triggered by unusual weather or physical injuries that stress trees and make them vulnerable to the insects; they can end for a variety of reasons, including production of defensive chemicals by the host plants or depletion of susceptible host plants.

Perhaps the insects most noticeable to humans in the taiga are mosquitoes, which belong to several species. Mosquitoes feed on and are fed upon by many of the birds of the taiga. Wetland areas of the boreal region, such as sites having poor drainage because of permafrost, provide extensive mosquito breeding sites. Where well-oxygenated flowing water is found, biting flies are abundant. Almost all food webs that support fish in the streams of the taiga are dependent on insects.

Conifers serve as hosts for a variety of wood-boring beetles, spruce beetles, bark beetles, and ips beetles (Ips species). These insects aid in wood decomposition and nutrient release. Some beetles have outer shells with specialized indentations specifically matched to the shape and size of the spores of wood-decomposing fungi. Fungal spores become securely lodged in these cuplike structures. As the beetles burrow into wood, they inoculate it with fungi.

A variety of lepidopterans (moths and butterflies) are adapted to feeding on the leaves of boreal trees. These include defoliators and leaf rollers.

Soil organisms

The species richness and total biomass of soil organisms are significantly lower in the taiga than they are at lower latitudes. Dominant soil organisms are protozoans, nematodes, rotifers, and tardigrades. These organisms live primarily in soil water film and soil pore water. The soil fauna of the taiga is distinctive because it generally lacks large invertebrates such as millipedes, isopods (springtails), and earthworms, especially in the middle and northern taiga. Larger soil invertebrate animals perform the function of biting off (shredding) pieces of leaf litter in forest soils and passing them through their guts. As a result of this activity, a thick layer of several years' accumulation of only partially decomposed plant material is characteristic of soils in the taiga biome.

Fungi are the dominant organisms in the task of decomposition of litter in the taiga, but flushes of bacterial growth occur in response to triggering factors. The soil animals generally do not attack the forest litter directly but instead exert their influence by grazing on the fungi and bacteria. The rate of decomposition in taiga soils does not keep pace with the rate of production, causing the progressive accumulation of organic matter. At middle depths of the forest floor, small invertebrates, especially dipteran larvae, partially consume or skeletonize leaf litter before emerging as adults.