Taiga

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.