Boreal Zone
The boreal zone is one of the nine global eco-zones according to J. Schultz. Today it occupies about 13.1% of the earth's land surface. At the beginning of the 21st century, around 70% of them are largely in a near-natural state .
Its expansion roughly coincides with the cold-temperate climate zone . According to the predominant vegetation, it can be further subdivided into the landscape types forest tundra and boreal coniferous forest .
In reality, the boundaries of the boreal zone are fluid, so that an exact extent - as shown on the map - cannot actually be determined. This fact becomes understandable if one uses comparable geozonal models , e.g. T. have significant deviations. For example, consider the comparable boreal zonobiom on the map of the zonobiome according to Walter and Breckle or the FAO Ecozones .
In the north, the boreal zone ends at the polar tree line with the forest tundra before it merges into the polar / subpolar ecozone . In the south, the moderate limits deciduous and mixed forests of the humid middle latitudes , or the steppes of dry mid-latitudes on. The boreal zone's ecosystem includes the largest contiguous forests on earth and extends over approximately 2 billion hectares .
The name boreal zone is derived from the Greek βορέας boreas , which means "north" and also refers to Boreas , the Greek god of the winter north wind.
distribution
The boreal zone spans the northern hemisphere quite regularly with a band around 700 to 2,000 kilometers wide. In Eurasia it extends from Norway across Siberia to Kamchatka , in North America from Alaska across Canada to Newfoundland . On the western sides of the continents, i.e. in Norway and on the North American Pacific coast, the occurrence is pushed further north by warm ocean currents than on the east coasts.
To the north, the polar tree line delimits the boreal zone, which is not, however, a sharp dividing line, but an ecotone several dozen kilometers wide . The forests there become lighter and lighter, forest and tundra alternate like a mosaic and form the so-called forest tundra until the treeless tundra joins. Climatically, the northern edge of the boreal zone coincides roughly with the 10 ° C July isotherm . On the Taimyr Peninsula in Russia, the boreal zone extends to 70 ° N, on Hudson Bay it changes into the tundra at 55 ° N.
To the south are the middle widths, which, depending on the amount of rain, can be defined as forest or steppe. Here, too, the transition is not abrupt, but coniferous forest and deciduous forest, or coniferous trees and forest-free steppe merge. The vegetation period has to last about six months in order to speak of the warm-temperate zone: to the north of it in the boreal zone, to put it bluntly, winter is longer than summer. According to Treter, the southern boundary of the boreal zones roughly coincides with the 18 ° C July isotherm.
In the southern hemisphere, areas that would be climatically comparable are almost exclusively in the sea. There are some similarities in the extreme south of South America and New Zealand, as well as on the sub-Antarctic islands, but the differences in climate and ecology are so great that one does not speak of a boreal zone there.
Boreomontane
The boreal zone and the mountainous regions of the warm temperate zone are collectively referred to as boreomontane. Since, like the boreal zone, the higher mountain regions of the warm-temperate zone were originally almost completely covered by coniferous forest, with numerous climatic and ecological similarities as well as common flora and fauna elements, the term "boreomontane" was coined to reflect the common features of the two spatially to name separate coniferous forest regions.
climate
The cold temperate climate is characterized by long, cold winters and short, moderately warm summers. Temperatures reach minimum values of down to −40 ° C in winter in the interior of the continents (down to −70 ° C in continental Eastern Siberia) and rise to mean temperatures above 10 ° C for two to three months in summer. The climate is largely determined by the arctic cold air - in winter the arctic front is often on the southern border of the boreal zone, while in summer warmer air currents from the Pacific or Atlantic are decisive. The growing season is usually four to five months. The summers in the continental climate are even shorter, but also warmer. Under the influence of the ocean , the summer is longer, but also remains cooler. If the average temperature stays below 0 ° C, permafrost soil forms, but the limit of the permafrost is not identical to the limit of the boreal zone.
The average rainfall in most areas is between 250 and 500 millimeters, which initially seems very little. Due to the low level of evaporation, however, there is almost never a lack of water ( humid climate ). In addition, thawing snow continues to supply water for a long time and water cannot seep away on permafrost soils. The rain usually outweighs the snow parts slightly, the snow cover remains for up to seven months.
During the summer there are long-day to all-day conditions. The length of the day at the time of the summer solstice is 16 hours on the southern border and 24 hours on the northern border. In this way, the low intensity of solar radiation can be compensated for at least for some time. The air temperature remains very low, however, as the energy is used up to melt the snow and ice.
The temperature differences between individual regions within the boreal zone can be very high due to different continental or oceanic conditions. The cold continental type of climate, e.g. B. occurs in the Siberian taiga, stands for extremely cold winters, in which the temperature can drop to -70 ° C. On the other hand, there are relatively warm, short summers with maximum temperatures of +30 ° C. The annual mean temperature there is very low at −10 ° C. These continental parts of the boreal zone have the highest temperature amplitudes. The cold ocean climate type, e.g. B. from southern Alaska or Norway, however, has slightly cooler summers, but also milder winters. The temperature amplitudes are not so high here, the annual mean temperatures are higher. The depth of snow and general rainfall are higher here.
Small-scale microclimates can also have a major impact on the ecosystem. Even slight slopes change the radiation balance with the prevailing low sun positions and lead to different conditions on north and south slopes.
ground
Large landscapes of the boreal zone are formed by old continental shields that have not seen any changes due to plate tectonics or volcanism for a long time. Due to the long-term erosion, the relief energy is low, plains and flat hilly areas dominate. When weathered, the rocks provide acidic, nutrient-poor soils. The Rocky Mountains in North America and the East Siberian Mountains in Asia are more recent . During the Ice Ages , large parts of the glaciers were covered and formed superficially by the ice movements, so the soil formation only started relatively recently.
In the drier continental areas, for example in central and eastern Siberia, mineral permafrost soils ( cryosols ) dominate. The subsoil is permanently frozen, while the topsoil thaws to different depths in summer. The expansion during freezing causes elevations to form; if the ground thaws, depressions form. The regular ground movements ( cryoturbation ) form ice wedges and frost pattern soils , as they are more pronounced in the tundra. Whether a permafrost soil forms or how deep the soil thaws can be influenced by minor changes in the environmental conditions, so that the soil conditions change frequently. In the forest, the sun mainly warms the crown area, little heat arrives on the ground, it remains in the only superficially thawing permafrost soil. On unforested, cleared or burned-down areas, the ground thaws deeper and sinks; the subsequent flow of water increases radiation absorption and heat storage - a lake forms (Thermokarst, Alass). The reverse process also occurs: if a lake silts up, the vegetation isolates the ground. Then the summer warming turns out to be less, and the advancing permafrost transforms the lake into a bulging hill of ground ice (Pingo, Bulgunnjacha).
The low seepage and evaporation or long-lasting frost at greater depths lead to waterlogging.
Podsols are common in the more humid oceanic areas . Under these cold, wet and acidic conditions, dead plant remains of animals are only moderately worked into the mineral soil and only slowly decomposed by bacteria and fungi. A raw humus layer therefore forms on the surface of the mineral soil . Released organic acids penetrate the upper mineral soil, which often has pH values below 4. Now iron and aluminum become mobile, which are enriched again in deeper horizons. In addition, organic matter moves downwards. This podsolisation called process leads to the formation of a nutrient-poor, very acidic, pale topsoil (Ae horizon ), where the dark, humus-rich BH and the intensely colored iron oxide-rich B-horizon follow, which may be ausghärtet and waterproof. Nutrients are only released slowly through mineralization and are poorly available to plants. At the low pH values of the mineral soil and the litter, all H + and Al 3+ ions are bound to the ion exchangers. The soil type is sandy , the water capacity is low, and the root space can be limited by hardened soil horizons.
Podsolation does not take place on very base-rich original rocks, but an only moderately decomposed litter is formed, which only slightly mixes with the mineral soil. The resulting soil is called Cambisol .
In places where water cannot flow away easily or where the groundwater is shallow below the surface, peat forms from dead plant parts . From a thickness of the peat layer of 30 centimeters one speaks of a bog . In terms of soil science, soils with at least 40 to 60 cm of peat belong to the histosols .
plants and animals
The biodiversity in the boreal zone is low compared to more southern ecosystems. For long stretches, individual plant species dominate the picture, conifers from the pine family - essentially only twenty different tree species in the entire boreal zone. For shrubs or perennials in the undergrowth there is usually little left of light and nutrients. The ecological relationships between organisms are not very specialized; herbivorous insects are only represented by a few species.
Flora
Coniferous forests and moors determine the flora. Conifers from the genera spruce , fir , larch and pine form partly mixed, but often monodominant stands. The trees are pollinated by the wind and mostly spread their seeds by the wind. The stands reach a height of about twenty meters, the trees do not compete for light as much as they do for nutrients in the soil. The high nutritional needs of the deciduous trees in order to produce new leaves every spring is also considered to be crucial for the dominance of the conifers. In addition, the evergreens can make better use of the short growing season. (In Asia, however, the boreal tree line is formed by the deciduous Dahurian larch ( Larix gmelinii ), which dominates large areas in eastern Siberia.) In the shrub layer there are deciduous trees, mainly birch , poplar , willow and alder . Pollination and seed dispersal by animals is also more common here, for example in the case of whitebeams , snowballs , cloudberries , Swedish dogwoods and the various dwarf shrubs from the heather family , such as blueberries . Mosses and lichens still grow close to the ground ; they are more common in the herbaceous layer than vascular plants . Fungi are abundant and all trees form a symbiosis with them ( mycorrhiza ).
Bogs are widespread in the boreal zone because the soils are strongly influenced by stagnant water and organic matter decomposes only slowly. Their area share is on average 20 to 30%, but can also increase to over 50%. Forests with boggy soil and forest-free bog areas merge smoothly. The moors are interspersed with many open bodies of water, moors and bodies of water are mostly poor in nutrients (oligotrophic). The growth rates of the peat layer are less than one millimeter per year, in the northern area only values of 0.2 millimeters per year are achieved. Nevertheless, due to the huge area, a significant amount of carbon is stored in the boreal bogs.
Deciduous forests are only found in a few places in the boreal zone. Individual deciduous trees such as birch and poplar can be found everywhere in the coniferous forests, but real deciduous forests are only formed in Scandinavia and Kamchatka, which are strongly oceanic, where birches form the boreal tree line. In floodplains and in favored locations, they can occur anywhere in the boreal zone and then form small forests interspersed with tall herbaceous vegetation.
The rejuvenation or change of the ecosystems often takes place on larger areas at the same time: Events such as forest fires, storm damage on the shallow thawing soils, floods or siltation, damage caused by animals affect the forests so often that the trees usually do not reach their maximum age, but rather die over large areas and be replaced by a new generation. The herb and shrub-rich succession areas are an important habitat for many animal and plant species. The accumulated litter is mineralized, especially through forest fires, and shrubs and deciduous trees grow on the nutrient-rich, sunny ash.
Wildlife
The animal population is small both in terms of the number of species and individuals. The prevailing coniferous forests and moors offer little food, the sporadic deciduous forests and succession areas are somewhat cheaper. Well-known animal species that have their distribution in the boreal zone are elk , brown bear , mountain hare , wolverine , lynx and various martens . Due to the pronounced winters, all animals have developed corresponding adaptations: Many birds migrate south, mammals and insects hibernate or are active under the protective snow cover. Reptiles and amphibians are largely absent, as is larger soil organisms - dead organic matter is usually decomposed by fungi.
Land use
Despite the wealth of natural resources, the use of this zone is limited due to the inhospitable climatic conditions. The human impact is comparatively small here. The most important type of land use is logging, along with the production of resin and honey as well as peat mining , traditional fur hunting and the gathering of wild berries . The extraction of mineral resources is made more difficult by the difficult transport conditions and the permafrost soil. Agriculture (such as rye , barley , oats and potatoes ) and grazing are of little importance because of the climate and the poorly fertile soil, only to ensure the self-sufficiency of the local population.
The entry of nutrients by winds affects the actually nutrient-poor waters and moors, the previously clear lakes are clouded by stronger algae growth, siltation and peat formation run off faster. Since the very sensitive lichens make up an important part of the undergrowth, industrial immissions and substance inputs from mining areas have a noticeable effect on the boreal zone.
The logging in the boreal forests covers around 90% of the world's demand for paper and sawn timber. The forestry yield is low, however, as the growth rates are low in the cold climate. There are many problems with the commercial use of wood: The remoteness and the size of the areas lead to long transport routes. Due to the sparse tree population, there is only a small amount of usable wood per area. Due to low temperatures and high snow cover in winter, the wood quality is low, as the height is very low. Most of the time, the wood can only be used to make paper or as fuel, and afforestation or natural regeneration takes a relatively long time. Therefore, damage has occurred in many places through clear cutting and peat removal.
Individual evidence
- ↑ Schultz, J. (2008): The Earth's Eco Zones . Stuttgart: Ulmer. ISBN 978-3-8252-1514-9
- ↑ Values from J. Schultz (Schultz, J. (2008): Die Ökozonen der Erde. Stuttgart) without Antarctica and Greenland converted to the entire land surface. See the article Eco zone # The individual eco zones . (See also tabular overview of various landscape zone models and their proportions ; PDF; 114 kB)
- ↑ Map data for the studies "Last of the wild", "Intact forest landscapes" and "Review of status and conservation of wild land in Europe", summarized in the source description for the "Wilderness World Map" on Wikimedia Commons
literature
A detailed list of literature can be found in Schultz 2000, pp. 220–225.
- G. Grabherr: Color Atlas of Earth's Ecosystems . Ulmer-Verlag, Stuttgart 1997. ISBN 3-8001-3489-6
- H. Remmert: Special ecology: Terrestrial systems. Springer-Verlag, Berlin 1998. ISBN 3-540-58264-9
- J. Schultz: Handbook of the eco-zones. Ulmer-Verlag, Stuttgart 2000. ISBN 3-8252-8200-7
- J. Schultz: The Earth's Eco Zones. Ulmer, Stuttgart 2016 (5th edition) ISBN 978-3-8252-4628-0
- M. Richter: Vegetation Zones of the Earth. Klett-Perthes, Gotha 2001. ISBN 3-623-00859-1
- U. Treter: The Boreal Woodlands . Westermann, Braunschweig 1993. ISBN 3-14-160312-X
- J. Pfadenhauer, F. Kötzli: Vegetation of the earth. Springer Spectrum, Heidelberg 2014. ISBN 978-3-642-41949-2 .
- W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the world. 2nd Edition. Springer Spectrum, Heidelberg 2014. ISBN 978-3-642-36574-4 .
- Further information
- JA Larsen: The boreal ecosystem . Academic Press, New York 1980. ISBN 0124368808
- HH Shugart, R. Leemans, GB Bonan (Eds.): A systems analysis of the global boreal forest . Cambridge University Press, Cambridge 1992. ISBN 0521405467
- H. Walter, S.-W. Breckle: Ecology of the Earth. Spectrum Academic Publishing House, Heidelberg 1999.
- Vol. 3. Special ecology of the temperate and arctic zones of Euro-North Asia. ISBN 3825280225
- Vol. 4. Temperate and Arctic Zones outside Euro-North Asia. ISBN 3-437-20371-1
