I.

Introduction

Taiga or Boreal Forest (also Northern Coniferous Forest), forest biome covering about 11 per cent of the Earth's land surface, or one third of its total forested area (about 1.5 billion hectares/3.7 billion acres). It occurs in the Northern hemisphere, in a circumpolar band, running though Canada, Scandinavia, Russia, China, Mongolia, the Korea Peninsula, and northern Japan. Although taiga is the Russian name, it can be used interchangeably with “boreal forest” for the biome as a whole. In addition to these forests in high latitudes, pockets of the biome can also be found at high altitude in more southerly mountainous regions. These are known as oroboreal forests, from the Greek word oro meaning “mountain”.

The region is of great importance because of its size and biodiversity. These areas constitute some of the largest extent of wilderness on the planet. The impact of human activity and habitat loss in such areas is, however, becoming a matter of concern.

II.

Climate

Climate controls the distribution of the high latitude boreal forests. They occur in regions with subarctic and cold continental climate characterized by long severe winters (up to six months with mean temperatures below freezing) and short warm summers (50 to 100 frost-free days). Average winter temperatures are as low as -30^o C(-22° F), while summers are cool averaging 6 to 14^o C (43 to 57° F). Due to the size of the land areas, there is often little moderating influence from the sea, and temperatures can be extreme. Precipitation is also low, ranging from 350 mm to 500 mm (14 in to 20 in). The strong winds cause high evaporation rates and the wind chill factor is high. Due to the high latitude, the daylight period is short in winter and long in summer. To the north, the forest grades into the tundra biome, where temperatures are too low to support full-sized tree growth. In the south the taiga gives way to grasslands in the drier continental interiors or deciduous forest in the more moist, maritime climates.

III.

The Taiga Ecosystem

A.

Plants

The forests are dominated by gymnosperms. These are cone-bearing tree species, and those found in the taiga have needle-shaped leaves. The most common species are members of the spruce (Picea), fir (Abies), and pine (Pinus) families, along with deciduous trees such as larch (Larix). In North America, black and white spruce are found as well as jack pine and balsam fir. In Scandinavia and western Russia the Scots pine (Pinus sylvestris) is common.

Conifers are adapted to the cold and the drought conditions of winter when water is frozen. Their conical shape allows snow to fall off easily, while the small surface area of their needles reduces water loss through transpiration, and a thick, waxy cuticle protects the leaf stomata from the drying winds and prevents water loss. By retaining their dark-coloured leaves throughout the year (see Evergreen), the trees are ready to photosynthesize as soon as temperatures rise above 6° C (43° F) in spring. The evergreen strategy enables them to exploit fully the short growing season.

The coniferous forest is the climatic climax vegetation. However, variations in topography, soils, and local climate result in a mosaic of different types of forest. Patches of broadleaf temperate forest occur in sheltered areas and to the south of the region. Species from the birch (Betula), aspen (Populus), and alder (Alnus) families are common. Where drainage is poor and the terrain low-lying, extensive bog areas with lakes and swamps are found with associated peat soils. Forests dominated by larch cover large tracts in Eurasia where they are underlain by permafrost. The winter drought here occurs for such long periods that conditions are too harsh to support the other boreal species. These different types of mosaic are very important because they increase the biodiversity of the taiga.

B.

Animals

The taiga supports a variety of animals. The major herbivores are deer, with more species found here than in any other biome. Small herbivores include rodents such as beavers and voles. Predators include members of the weasel family, as well as lynxes and wolves.

Many animals show specific adaptations to the conditions found in the biome. For example, many have thick coats of fur or feathers for insulation. Large body size, relative to similar species found in more southern biomes, is another strategy that prevents heat loss. For example, the elk is the largest member of the deer family, and the wolverine is the largest of the weasels. Several species, such as the northern red-backed vole, shrews, and lemmings, have developed the ability to live beneath the snow-covered ground in winter. Others (such as the brown bear, Siberian chipmunk, and squirrel species) over-winter by hibernation. Some animals do not hibernate, but hoard or store food to enable them to survive the winter period.

Migratory behaviour is also a common adaptation to the climate of the taiga. In North America, for example, Barren Ground caribou migrate south into the biome from the tundra in winter. By contrast other birds and mammals only spend the summer months in the taiga, moving further south when temperatures fall.

C.

Soils

The interaction of the vegetation and the climate produces a soil type known as a podzol. Slow decomposition, due to the low temperatures, results in a thick litter layer. The low rates of microbial activity produce a type of acidic humus known as “mor”. When precipitation exceeds evaporation, usually following snowmelt, soils can drain freely and become leached. The distinctive mineral layers of the podzol soil can develop. This involves the removal (or eluviation) of materials from the upper bleached A-horizon (see Soil: Soil Horizons), and their partial redeposition (or illuviation) in the B-horizon below. Iron deposition often forms a hard pan, which may impede drainage, causing waterlogging and leading to the development of a gleyed podzol.

Where waterlogging is extreme, often over permafrost, the anaerobic conditions prevent the decay of organic matter altogether, and acidic peat soils form. The vegetation here consists of mosses, lichens, and shrubs rather than trees.

D.

Nutrient Cycling

In the acidic environment of a podzol, the populations of soil biota are low and little mixing of the soil occurs. The vegetation is shallow-rooted, penetrating only the leached A-horizon. Any nutrients lost to the lower B-horizon are effectively unavailable to the plants and so strategies have evolved to minimize losses. The evergreen habit helps trees to retain nutrients as well as being efficient given the short growing season and slow decomposition rates. Microbial activity and the action of symbiotic (mutualistic) fungi associated with the tree roots, release the nutrients stored in the litter layer. Because decomposition occurs most rapidly in the growing season, the trees can capture nutrients as they are released.

E.

Natural Disturbance

When the tree cover is disturbed, plant succession can take place. This promotes biodiversity by producing a shifting mosaic of vegetation types. Pioneer species, such as grasses and herbs, move into the gaps, followed by shrubs. Broadleaved trees, such as aspen or birch and, later, spruce and fir, may eventually replace the early colonizers.

Fire, caused by lightning, is probably the most important natural disturbance. Fire severity depends on such factors as quantity and dryness of litter on the forest floor and related weather conditions. Fires tend to occur in cycles of between 50 to 200 years, depending on the type of forest, but they usually affect only small areas. Damage by insects may also create opportunities for succession. Spruce bark beetles or spruce budworm are common. Although damaging for commercial operations, outbreaks can be beneficial because insects consume the combustible materials such as plant litter so that fire risk is reduced.

IV.

Problems Facing the Taiga

Many indigenous people live in boreal forests, including the Dene, Inuit, Cree, and Athabaskans of North America; the Saami of Scandinavia; the Ainu of northern Japan; and the Nenets, Yakut, Udege, and Altaisk of Siberia. In recent times, settlement of peoples from outside boreal regions has taken place and the exploitation of the resources is now posing a threat to the fragile ecosystem and its native peoples. The main impacts come from timber industry (logging), mining, and hydroelectric power development.

A.

Logging

The taiga is an important resource of softwoods and has been exploited throughout human history. In recent years, however, logging has reached a scale that has a major impact on the ecosystem.

Mechanization has increased the rate of deforestation. Clear-cut felling is normally employed. The heavy machinery used can disrupt the land surface, leaving the newly exposed soil vulnerable to erosion. The soil can dry out in summer, limiting seedling establishment and growth. Some trees (for example, spruce and fir) require shaded conditions to regenerate, and so their frequency declines after logging. Trees are often floated down rivers for processing in saw mills and pulp and paper plants, so damaging and blocking river channels and sometimes causing flooding.

Following harvesting, afforestation may occur. However, the resulting plantations often contain fewer tree species, and so are more susceptible to disease. Single-aged stands are also more prone to storm damage. Plantations also exclude plants that might compete with the tree seedlings, so the forest floor vegetation is poorer than in a natural forest. Thus habitat diversity is reduced and wildlife is affected adversely. Some animals, especially top level carnivores such as Siberian tigers, may not be able to move or find forested areas large enough to support viable populations. If slow-release fertilizers are used to promote seedling growth, nutrient material can be washed into rivers, causing eutrophication.

B.

Mining

The inhospitable conditions of the boreal zone have meant that until recently mineral resources have not been fully developed. Mining usually involves clear-felling of large areas of forest; however, the impacts can go beyond the site of mining activity. Once operations commence, environmental impacts arise from the disposal of acidic effluent from tailings, the silting of rivers, and pollution from processing plants.

C.

Hydroelectric Power

Recently some very large schemes have been created, flooding vast areas of land. They have disrupted natural drainage and rivers and dramatically altered the landscape. The traditional lands of the indigenous peoples, which are valuable for hunting and fishing, have also been damaged. Much opposition to the schemes is based on the fact that they disturb the natural environment for local people but mainly benefit those from outside the region.

V.

Sustaining the Taiga Biome

The boreal forests are a major global store of carbon and so play an important role in regulating the Earth’s climate (see Carbon Cycle (ecology)). Recent research suggests that currently the forests are not taking up as much carbon as they did prior to the mid-1970s. If this is the case, then the net transfer of carbon into the atmosphere will have contributed to the trend of increasing atmospheric carbon dioxide (CO₂) levels and the greenhouse effect. Several factors seem to be responsible for the net loss of carbon from the biome.

The impact of logging and clearance of these forests for other uses could be a factor, together with more widespread disturbance of these forests by fires, insect outbreaks, storms, and damage due to acid precipitation. However, it is also thought that forest decline may have been brought about through global warming itself. Higher temperatures may, for example, reduce the natural rates of recovery of these forests following disturbance, so that forest die-back can occur. Even if the clear-cut forest areas are converted to plantations, a net transfer of carbon to the atmosphere results, because old growth forests tend to support more biomass than commercial ones.

If global warming does disrupt the boreal forests, and results in significant net transfers of carbon into the atmosphere, then a dangerous positive feedback loop could be established. The carbon released would increase the rates of global warming and further increase the rate of decline of the forests themselves. Although it is possible that the biome could expand into the tundra, such northward migration would be slow and unlikely to compensate for the decline in the forests in the south. The further loss of boreal forest could therefore have a major impact on global climate.

Given the problems now facing the taiga, the need to develop strategies for managing these areas in a more sustainable way has become a focus of current concern. At the global scale, the fate of the forests depends on attempts by the international community to control CO₂ emissions and slow the rate of climate change. At national, regional, and local scales, more sustainable ways of using the resources in the biome must be found. These include afforestation programmes, which produce more natural forms of plantation, integrated forms of ecosystem management, and the development of forestry based on selective felling, rather than clear-cut felling. The conservation of old growth forest and the biodiversity resources of the taiga are also high priorities.