- Also called:
- boreal forest
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.
Community structure
Natural disturbances
The taiga is well adapted to development following natural disturbances, which include fire, floods, snow breakage, and insect outbreaks. Characteristic of the taiga is the general lack of late successional species that develop under an intact forest canopy. (For further information on succession, see community ecology: Ecological succession.)
Fire is the primary agent responsible for natural disturbances in the taiga. It can result from natural causes, such as lightning, or it can be set by humans. Large-scale insect outbreaks can weaken or kill trees over vast areas, thus creating an environment less resistant to fire. In the period between 1981 and 1989 an estimated 3 million hectares (7.4 million acres) burned annually in the Soviet Union, almost all of which occurred within the taiga region of Russia. The so-called Black Dragon Fire of 1987 in China and Russia may have been the largest single fire in the world in the past several hundred years. During the 20th century about 1 million hectares of taiga in Canada burned annually; a great majority of the burning occurred in the less-accessible boreal forests of the northern and western parts of the country. In Alaska in years that have prolonged hot and dry periods of summer weather, millions of hectares burn, primarily in a few very large fires. Intervals of about 200 years occur between fires in the uplands of northwestern Canada and in the interior of Alaska. In much of the central and western taiga of North America, replacement of vegetation on upland sites, presumably by fire, appears to be necessary for forest regeneration. Floodplain islands usually do not burn and contain white spruce trees as old as 400 years. In the northern taiga of Europe, a pattern of periodic light ground fires in Scotch pine forests was typical before the era of fire control. The thick bark of these mature trees allowed them to survive these fires. In much of the taiga only wildland fires that threaten high-value resources are actively suppressed. Complete fire suppression would cause soil temperature to decline gradually, promoting permafrost development that would cause a significant decrease in site productivity.
Jack pine and lodgepole pine have cones that remain closed on the tree (serotinous), and black spruce has semiserotinous cones; these cones do not open to release their seeds until a wax layer is melted by the heat of fire. White spruce seedlings require the bare mineral soil produced by burning of thick organic layers of the forest floor for proper establishment; they may time their periodic production of seed to dry periods when fire is more likely.
Effects of human use and management of the taiga
Different degrees of forest development have had various effects on biodiversity around the circumpolar taiga biome.
A highly developed forest industry based on intensive forest utilization is maintained in boreal Scandinavian countries and Finland. About 95 percent of the productive forest types of Finland and the Scandinavian countries have been harvested at least once. Finland is located almost entirely within the boreal region and is one of the most-forested countries in the world. About 9 percent of Finnish land, which includes large areas of marginal forest, woodland, and tundra, is protected from human modification. In contrast, only about 5.5 percent of Sweden’s total land area is protected, and about 300 species in the country have been given protected status.
The Canadian taiga represents nearly 7.5 percent of Earth’s forested area. Much of the harvesting of Canadian forest has been carried out in primary (previously unlogged) forest, and some 18 percent of Canada’s primary forest remained by the early 21st century. Considerable effort has been devoted to forest regeneration and tending of new stands, although a certain amount of land does not meet reforestation goals.
In Alaska the amount of land with at least 10 percent forest cover in the boreal region is estimated at about 46 million hectares, or 12 percent of the state, only 5.5 million hectares of which is considered productive timberland. Of all areas in the world, Alaska probably has the largest percentage of its surface area, about 40 percent, devoted to strict protection of natural habitats and species. Local-scale logging traditionally was carried out for much of the 20th century.
The taiga of Siberia covers 680 million hectares and represents nearly 19 percent of the world’s forested area and possibly 25 percent of the world’s forest volume. About 400,000 hectares of the Russian taiga are logged annually, and nearly an equal area is burned, with perhaps half of the burned area resulting from destructive fires of human origin. Social and economic problems in the early postcommunist era slowed the amount of logging by one-third to one-half. However, illegal felling accounted for 30 percent of the harvest by the early 21st century, and forestry officials feared that the practice was increasing. The fate of the Siberian taiga has become a matter of international concern.
Large areas, perhaps exceeding two million hectares, of the Russian taiga near Norilsk and the Kola Peninsula have been destroyed by air pollution. Many oil pipelines are leaking in Siberia, and repairs and maintenance are minimal. In July through September 1994 more than 150,000 metric tons of crude oil were spilled in the Kolva, Usa, and Pechora river basins of the republic of Komi in Russia. Other, smaller spills since the 1994 spill have resulted from leaks as well as illegal pipeline tapping.
