- Distribution of boreal forests ( boreal )
The boreal forest, also called a boreal forest or taiga, is the northernmost forest type on earth. It grows in the cold temperate zone, but without exception, on the northern hemisphere. As GLOBAL NETWORK landscape type, it is the northernmost vegetation zone in the growth of forests is possible. If the forest is dominated by evergreen conifers such as pine, spruce or fir, also referred to as the " Dark Taiga" in contrast to " light taiga ", which is formed by the deciduous larch. However, the larch taiga is found only in central and eastern Siberia.
Just north of the boreal forest zone is the tundra, in the south, close cool temperate deciduous broad-leaved forests or forest-steppe on. The ecosystem of the boreal forest zone forms the largest contiguous forests in the world.
Boreal coniferous forests are in Eurasia ( northern Europe, Siberia, Mongolia ) and North America (Canada, Alaska). As the tropical rain forest they form make worldwide belt, which lies approximately between 50 ° north latitude and the Arctic Circle. The boreal zone begins in the south where the climate for hardwood deciduous trees is too unfavorable, ie where the summer is too short and the winters are too long. Here the number of days decreases with daily average temperatures above 10 ° C under 120, and the cold season lasts longer than six months. The northern limit of the boreal zone is located where the number of days with daily mean temperatures decreases above 10 ° C under 30. The boreal forest is approximately 1.4 billion acres of the largest contiguous forest complex of the earth and the most economically important forest region. Of this area, however, about 150 million hectares are caused temporarily not planted by fire, storm, large insects, or human activities.
The deciduous hardwood forests graduate into a coniferous forest zone in which there is a pronounced Thermoperiodik during the year. In the transition region both penetrate tessellated or forming mixed stands of their tree species and their other plant and animal world. In the Taiga, which extends as a broad belt of Fennoscandia through Siberia and continues in North America, there are long, snowy winters and short, cool summers pretty. On average, less than four months above 10 ° C. The cold season lasts six months. The vegetation endures a cold period of eight months. The foliage of xeromorphe with the exception of larch (Larix spp.) Evergreen conifers is adapted to both cold and desiccation on. Photosynthetic activity ceases at -4 ° C, when the needles freeze, but continue at higher temperatures again. The importance of temperature increases, the more the rich forests to the north of oceanic or continental regions in. The transition to winter dormancy a hardening process occurs. Spruce needles, which die off without him already at -7 ° C, can tolerate -40 ° C after the Resilience. In the spring there is a " softening ". This is due to an increase in the sugar concentration in the cell sap. There are other protective substances in the protoplasm enriched, the cells are lower in water content and the Zentralvakuole fissured in a variety of Kleinvakuolen. The heat adjustment is rapid, since the rise in temperature in one day, and sometimes can be carried out even faster. On hot days, the heat resistance is higher in the afternoon than in the morning. The softening in cold weather is happening within a few days. The Abhärtungstemperaturen must be so high that they act on the protoplasm as stress. In most land plants, this is the case, usually from 35 ° C. In the euro area the Siberian air to the north and east is toward more extreme, which is reflected by particularly low winter temperatures. In the East Siberian taiga with permafrost larch dominate (Larix spp.), Which shed their needles and thus have the greatest resistance to cold.
In the continental part of the boreal climate zone the annual precipitation is 150-250 mm per year. At 50-100 days, the daily mean temperatures rise above 10 ° C. In oceanic embossed areas, such as Scandinavia, the rainfall is about twice as high, while the monthly mean temperature in the coldest months may be at about -10 ° C.
The flora is characterized by coniferous forests, which are interspersed in southern and oceanic affected areas with birch and aspen.
The boreal forest is often determined in its core areas through only one or two tree species and therefore is one of the less species-rich forests. This is primarily due to the short growing season of only 2 to 4.5 months (or the low energy input into the ecosystem ). The main reason for the dominance of evergreen conifers is the fact that they have a fully developed photosynthetic apparatus (ie needles) already at the beginning of the growing season. With decreasing length of growing period of the metabolism of deciduous tree species is therefore always less economical. A threshold is the number of at least 120 days in the year on which the average daily temperature exceeds 10 ° C. Most conifers also withstand temperatures down to -40 ° C. In the extreme zones, however, it is cold even for most conifers. In Yakutia temperature averages are from -50 ° C or even less possible. Then the nadelabwerfende larch takes the place of spruce and pine.
To the north, close to the boreal forest to the tundra, in which only occur dwarf plants. The forest thins out, so can be found depending on the location, for example, reindeer lichen on the ground, steppe grasses or Daurian rhododendron bushes. In the south of the boreal forest turns into temperate mixed forest with deciduous broad-leaved trees.
In boreal forests, plant communities find a class Vaccino - Piceetae. In the tree layer one finds the Siberian larch (Larix sibirica ) and the Dahurian larch (Larix gmelinii ), the Siberian fir ( Abies sibirica ), the ( Picea abies ) Birch (Betula platyphylla ), and the common spruce or further to the east the Picea obovata (sometimes considered a subspecies of Picea abies ) and Scots pine (Pinus sylvestris). In the less extreme climates growing next to the Siberian stone pine (Pinus sibirica ).
In North America, the appropriate tree species Tamarack balsam fir (Abies balsamea ), white spruce (Picea glauca ) and black spruce (Picea mariana ) come (Larix laricina ), above. The pine species jack pine (Pinus banksiana ) is also part of the North American boreal forest, but they are found only in the southern parts of the same.
Among the characteristic species include not only trees, but also shrubs and herbs. These are blueberry and cranberry, forest Cow-wheat, Seven Star, Blue Honeysuckle, Moosglöckchen, Sprossender club moss and fir club moss, as well as numerous mosses, which are all acid pointer. Imagine either because of the starting substrate of the bottom one, or conifers create through their influence on soil properties favorable for this crop conditions: The degradation of the already difficult decomposable needle litter is further slowed under the climatic conditions of this vegetation zone. The powerful humus layer has acidic properties.
The boreal forest is home to over 300 bird species. Besides, this is the habitat of many mammals such as moose, wolf, wolverine, bison, reindeer and caribou, bears, deer, lynx, fox, hare, marten, otter, beaver, skunk, flying squirrels, chipmunks, squirrels, lemmings and coyote. In parts of eastern Siberia, there are Tiger and Snow Leopard, come into Alaska and Canada as predators pumas before. Rivers and lakes of the boreal forests are habitat for many fish species, including many species of salmon. Despite the cold, there are relatively many species of amphibians and reptiles.
The conifers with their generally difficult decomposable litter ( the process takes about 350 years, which runs a hundred times slower than in central European deciduous forests ) require this, in combination with the very low energy input into the ecosystem and the decreased metabolic activity of microorganisms, the emergence of powerful humus of up to 50 cm. This also leaves a large part of the important for the growth of the forest nutrient elements such as potassium, calcium and mainly organically bound nitrogen in the humus layer, and is the plant not be available. The lack of return of bases due to the formation of an acidic podzol. This is promoted by strong acids, which are formed by fungal organisms. So podzols have a low pH.
Two thirds of the boreal forests are on permafrost, also known as permafrost. Permafrost free the intense precipitation zones are on the western edge of Eurasia, where mighty snow cover prevent cooling of the soil to great depths. The permafrost thawed at the surface only in the early summer ( up to depths of 0.5 to 1 meter ) and a tendency by the resulting water to waterlogging. The root mass of trees is therefore hardly deeper rooted than 20-30 cm in the soil. The silting caused also an incomplete decomposition of organic matter and insufficient mineralization of nutrients bound in her due to lack of oxygen. So many places also form Moore. The thick humus layer protects the permafrost ground against summer sunlight. If this litter layers destroyed by forest fires, as formed under circumstances swampy lakes of 10-50 ha of land where the forest can not initially take more distance. Therefore, other types of vegetation found here first one (succession ). In the mires it comes thanks to the large amounts of organic material to the formation of peat. The world's largest peat supplies ( about 2/3 of the deposit ) are located on the territory of the former Soviet Union.
The further one goes north, the omission of the forest cover of the boreal zone, until it finally passes into the tundra. The phytomass - the entire plant matter ( organic and Nekromasse ) - this is lower. In the southern part of the boreal zone is about 400 t / ha dry matter in the " Middle Taiga" halved this already to 166 t / ha. In the same measure, net primary production is reduced to four to eight tons per hectare per year ( an average of five tonnes). Overall, the growth, including that of the conifers, due to the short growing season, but also because of the mitverursachten by themselves podzol, comparatively slow. The trees are on average 15-20 cm lower than in the Central European forests. The diameters are also smaller in correlation to the amount. However, benefits from the cut wood and paper industry, which can process large trees uniformly better. However, increases with slower growth, the quality of wood. Larch, for example, very long-lived ( 300-500 years) and therefore highly valued as timber. The trees of boreal coniferous forests produce only a third of the dry mass of the tropical rainforest.
Fire and forest fires play an important role in the ecosystem and the development dynamics of boreal forests. The reason lies in the powerful humus layer, which impedes regeneration of forests. The seeds of the trees find no contact with the ground, where there are plant-available nutrients and they could take root. By fire the mineral soil is exposed again. At the same time stored in the organic mass elements are released ( a lot of it is due to leaching and lost again ). Fires are regular, natural occurrences in this ecosystem. The period between two fires events on a surface is described by the concept of fire rotation. In summer dry areas of Alaska and Canada, these are 50-100 years, in wetter areas 300-500 years. Under natural circumstances, the fires are caused by lightning.