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Arctic area of the tundra and Antarctic dominion.

Sitting at the end of the hollow was the old dust devil. His body was of ice, ice-cream cones for hair and a cloak made all of snow. Three black wolves lay at his feet rising with their mouths open as the sun rose. From the maw of the first one arose a penetrating cold, from the second, a mountain wind which froze the marrow and the third vomited impenetrable darkness. The light tumbled through a ditch of walls covered with frost.

Selma Lagelof

Nils Holgerssons underbara resa genom Sverige (1906-1907)

1 Winter all year round

1. Permanent cold and half a year of darkness

1.1 The Arctic tundra, an extreme environment

The word tundra comes from the (Lapp) Samer language and literally means a high and dry place. In the north of Scandinavia and in the Kola Peninsula, the areas where the Samer * live, the term tundra is used for the treeless hilltops and plateaux that rise above the typical northern landscape of conifers or, the taiga. The scientific literature has extended this term to include the plains covered in low vegetation that run across the northernmost regions of Eurasia and North America around the Arctic Ocean.

The extreme climate is one of the factors determining the tundra's characteristic features, such as the lack of trees and the abundance of bryophytes (mosses, liverworts, hornworts) and the large number of lakes. Although the area it covers is relatively restricted in terms of latitude, the tundra is subject to a very steep gradient of climatic conditions.

Conditions related to latitude

The harshness of the tundra's climate is explained, in the first place, by its location at a high latitude. In fact, the sun's rays hit earth's surface at a more oblique angle at the poles than in the tropics. Furthermore, the angle of Earth's inclination with respect to the Sun gives rise to the alternation of the seasons, so that for six months of the year the North Pole is exposed to the sun, while for the other six months the South Pole is exposed to the sun. The annual solar radiation reaching the earth's surface at the latitude of the tundra is 70-80 kcal/[cm.sup.2], higher than that received by the Arctic islands at higher latitudes (57-67 kcal/[cm.sup.2]) and slightly less than in temperate regions of Central Europe (80-100 kcal/[cm.sup.2]).

To assess the real quantity of heat retained, which is what allows the growth of some plants and the survival of some organisms, knowing the solar radiation that reaches Earth's surface in a given place is insufficient. Earth's surface reflects most of the solar radiation, emitting it once more into space. Surfaces covered by ice and snow are highly reflective, and their albedo value, the fraction of incident light that is reflected, reaches an average value of 95% at the highest latitudes of the Arctic regions. In the tundra, the albedo value is 50%; in the taiga, it is only 35-38%.

It is especially interesting to know the radiation budget, which is the difference between the total incoming radiation and that reflected by the earth's surface. The annual energy budget of the tundra varies between 10 and 20 kcal/[cm.sup.2]. In comparison, it is worth pointing out that the Arctic islands at high latitudes show values of 6-10 kcal/[cm.sup.2], temperate forests values of 40-50 kcal/[cm.sup.2], and African savanna values of 70-80 kcal/[cm.sup.2] per year. Thus, from mid-October to March, the radiation reflected is greater than the radiation absorbed, with a negative balance that may reach 60%. The soil surface can accumulate heat only from April to July.

The low temperatures and the wind

The tundra is a cold environment, although in summer it may be hot, and some days very hot. In the northern hemisphere, the coldest winter temperatures do not occur in the tundra, but much further south in the taiga. The "cold pole" of the northern hemisphere, where the minimum winter temperatures may frequently drop below--58[degrees]F (-50[degrees]C) (the absolute minimum ever measured is -90.4[degrees]F [-68[degrees]C]), is in the area of taiga in eastern Siberia, near Verkhoyansk, not far north of the Arctic Circle. In the taiga, however, trees can grow because the harsh winter cold is compensated by the heat of a very long summer.

Because of the strong winds that sweep across it, the tundra is dominated by woody plants that hug the soil. Although the trees growing on the border between the tundra and the taiga may belong to species with normal growth forms in forest formations, they are low and gnarled with asymmetric crowns leaning to one side in the direction of the prevailing winds.

Limited precipitation and surface water

The tundra contains more lakes and wetlands than any other region, except perhaps for some forested regions of taiga. It is paradoxical that there is so much water, as annual rainfall is very low (8-12 in [200-300 mm]), more or less the same as the dry subboreal steppes of Eurasia. The very low temperatures, the insignificantly low evaporation, and the dominance of frozen soils combine to prevent the water filtering away.

Summer temperatures scarcely exceed 32[degrees]F (0[degrees]C) and promote the condensation of the scarce humidity contained in the air masses of polar origin and its precipitation in the form of rainfall. Approximately half the tundra's annual rainfall occurs in the four months when average temperatures exceeds 32[degrees]F (0[degrees]C). In the Arctic deserts further to the north, average annual precipitation is about 4 in (100 mm), so low it can be compared with a true desert. The tundra's high humidity in summer is also largely due to the continuous unfreezing of the surface horizons of the soil. It has been calculated that the melting of a thickness of 1 ft (1 m) of soil releases a quantity of 30 ml of water/[cm.sup.2], about twice as much water as that which falls in the form of rain during the period of the year when temperatures are above freezing.

The mosses and lichens that cover the floor of the tundra absorb and retain this water. This unusual feature, together with permafrost, (soil that is permanently frozen and prevents water draining to the subsoil), promotes the formation of wetlands and small lakes in the smallest depressions. The rivers are shallow, with a low volume of flow (1.5-2 m3/s) and wind slowly, forming many meanders and ox bows, or cut-offs. The main source of water for the streams (50-70% of the total) is the melting of snow; thus after the spring, when the rivers carry a great deal of water, the volume of flow is much less. In winter, some streams freeze solid, completely halting the flow of water.

Although there is a lot of water in the tundra (up to 30% of its area, in regions covered by shallow pools and peat bogs), it cannot be said to consist entirely of wetlands. In the most sheltered sites, the sunnier sites, the snow melts early and the soil dries out. These regions with more continental climates may have very hot summers, and so in the middle of the tundra there may be areas similar to the steppes. These northern steppes, like those in some areas of eastern Siberia, were undoubtedly the last refuge of the mammoths.

1.2 The seasonal cycle and the hyperboreal climate

One characteristic feature of the functioning of the tundra is its highly seasonal nature, with alternating periods varying greatly in terms of incoming sunlight, and more importantly, of temperature. Both the tropics and the latitudes close to the pole show major oscillations in the solar radiation over the daily (circadian) and annual cycles, but they are very different in periodicity: in the tropics, the most important variations are circadian (there is no solar radiation at night), whereas in the tundra, the most important oscillations are between different seasons.

The luminous but tepid summer

At high latitudes, after the spring equinox, the length of day increases rapidly. At the summer solstice, to the north of the Arctic Circle, the sun is above the horizon 24 hours a day. Although there are changes in the intensity of the light over the course of the day (at midnight the sun is not very high above the horizon), there are almost no circadian variations in temperature.

The total quantity of solar radiation received by the tundra in summer is considerable, almost the same as in subtropical latitudes, with the unusual characteristic that it is relatively rich in ultraviolet radiation with respect to visible light (up to three times greater at the height of summer). Yet air temperatures stay low, despite the intense solar radiation. In the central areas of tundra, the hottest mean monthly temperature, normally in the month of July, is 45-50[degrees]F (7-10[degrees]C); further norther it decreases to 37-43[degrees]F (3-6[degrees]C), and in the southern areas it may rise to 50-53.6[degrees]F (10-12[degrees]C). It is precisely at this period of the year, when the latitudinal thermal gradient is at its greatest. For example, to the north, there are only 10 days a year with a mean daily temperature greater than 50[degrees]F (10[degrees]C), while the southern regions may reach 86[degrees]F (30[degrees]C) for a period of 30- 40 days. The thermal gradient is less clear in winter.

There are two main reasons for the low summer temperatures: the enormous amount of heat required to melt the snow and to warm the surface layer of soil, and the proximity of the cold Arctic ocean. The masses of cold air that form over the Arctic Ocean enter the areas of tundra and cool them, not only because of the low temperature of the air but also because this air contains little water vapor and thus cannot retain the long wave radiation, a phenomenon that is to some extent the opposite of the greenhouse effect. In August, the air temperature is almost the same as in July, but in September it drops sharply, although it is still above 32[degrees]F (0[degrees]C).

The cold dark winter

The solar radiation received by the tundra declines greatly in winter, although this does not mean that the hyperboreal latitudes are completely dark for 24 hours a day. The sun, even though it is always below the horizon, provides a feeble light during the day.

In most tundra regions, the cold period lasts from October to May. The average monthly temperature in these months remains below above 32[degrees]F (0[degrees]C). Although latitudinal differences in temperature are not very large in winter, there are differences between regions situated at the same latitude. In the tundra of Eurasia, the mildest winters occur near the Atlantic coastline, towards the east, the temperatures are lower and only become more moderate again on the Pacific coastline. In North America, the Pacific coastline is more temperate. Thus, the average value of the lowest monthly temperature (January or February, depending on the site) may vary greatly, ranging from -28[degrees]F to -34[degrees]F (-2[degrees]C to -37[degrees]C).

In the tundra, there is continuous snow cover for 220-280 days a year, more than in any other biome (in the boreal forests of eastern Europe, for example, the snow cover lasts 120-160 days, and in the steppes, from 60-80 days). In this period in the tundra, the wind most often blows from the south, as in winter the continent cools down more quickly than the ocean. In winter the thickness of the snow layer increases, reaching 28 in (70 cm) in the southern regions and 16-20 in (40-50 cm) in the central regions; in much of the northern tundra, this layer may be much thinner, only 4-8 in (10-20 cm), but it must be borne in mind that there is no direct relationship between the thickness of the snow and the time it lasts. There is more snow in the southern tundra than in the northern tundra, yet it melts earlier. Throughout the winter, the winds redistribute the snow, forming drifts, depending on the site's relief. This is of great importance to many tundra organisms, because compacted snow has a thermal conductivity somewhere between tens and hundreds of times greater than looser snow. In addition, compacted snow makes it more difficult for animals, such as reindeer, to find food beneath the snow.

In spring, when the snow begins to melt, some depressions in the relief show what are called "snow greenhouses." These are cavities that form between the soil surface and the snow layer, and within the cavities there is a very special microclimate. The solar radiation that penetrates the snow heats the soil, and the heat reflected by the soil surface is retained by the upper snow layer, which melts slowly and forms a transparent layer of ice that acts like the glass in a greenhouse. The temperature in these "greenhouses" is usually 35.6- 41[degrees]F (2-5[degrees]C) (and up to 50[degrees]F [10[degrees]C]) warmer than the exterior. This means that the plants in these sites start growing very early, when the general temperature of the tundra has still not reached 32[degrees]F (0[degrees]C).

2. Permanently frozen soils

2.1 The beginnings of knowledge of the tundra soil

Soil formation processes in the tundra are essentially no different from those of other regions, except for the tundra's specific climatic conditions, such as low temperatures, the action of ice and soil desiccation caused by the action of ice.

Permafrost: the impermeability of frozen water

All the soils of the tundra have ice at depth, seasonally or permanently. If it is permanent, the soil or rock horizon is more or less saturated with water that stays frozen throughout the year and is known as permafrost, sometimes called pergelisol, or permagel. Permafrost is very thick in continental regions, where its southern limits generally lie in the southern taiga forest, as happens for example in eastern Siberia, where it reaches furthest south. Yet in more oceanic regions, such as the Labrador Peninsula in northern Quebec, its limit coincides approximately with the boundary between the tundra and the taiga.

There are several types of permafrost: continuous permafrost, where the average yearly temperature is below 23[degrees]F (-5[degrees]C), discontinuous permafrost, where it is between 23[degrees]F and 28[degrees]F (-5[degrees]C and -2[degrees]C), and sporadic permafrost, if it is always above 28[degrees]F (-2[degrees]C).

Sporadic permafrost mainly occurs in waterlogged peat bogs or marshy depressions, on the boundary between the tundra and the taiga, whereas discontinuous permafrost occurs at the base of northeast- or east-facing slopes, in alluvial terraces or shallow wetlands. Even in the area of continuous permafrost, the sediment may remain unfrozen under thick layers of water, such as lakes or rivers; this formation is known as talik.

It is possible to distinguish between two different layers in the permafrost: an active layer that melts every summer, in which most of the soil processes take place; and true permafrost, which is permanently frozen and of variable thickness. The permafrost may vary greatly in thickness, from a few centimeters in waterlogged soils, to tens or even hundreds of meters on rocky substrates. For example, in the Bolshezemel'skaya tundra on the eastern banks of the River Pechora, the permafrost is about 66 ft (20 m) thick, whereas only 186 mi (300 km) northeast, on Vaigach island, to the southeast of Novaya Zemlya, it is 984 ft (500 m) thick. There is no permafrost under the ocean, and under the beds of large lakes and rivers, its upper limit is at a great depth.

The active layer that melts also varies in thickness from year to year, depending on the summer weather, although in general it is never very thick, between 12 in (30 cm) and 79 in (2 m). The deepest melting is typical of sandy, well-drained soils, whereas soils that are under a layer of peat show least melting, due to peat's low thermal conductivity. The thickness of the snow layer also conditions the depth and the thickness of the layer that freezes and melts every year. In the southernmost tundra, the soil under thick snowdrifts does not freeze. The quantity of water in the permafrost may also vary greatly, from almost nil in some rocks in dry permafrost to the equivalent of 400% of liquid water.

During the summer, on flat or depressed relief, a water table may form over the permafrost layer. Only when the surface snow has melted does the soil begin to melt, with a fall in the level of the permafrost table as a consequence. In soils where ice occurs only at depth seasonally (i.e., there is no permafrost), melting begins in summer at the surface and at the base of the frozen soil, so that the frozen soil layer becomes thinner on both sides until it disappears. On the other hand, the high ice content of tundra soil during the winter means that it shows high thermal conductivity in the winter. Yet in summer, when tundra soil totally or partially melts, the thermal conductivity is more directly affected by the water content of the soil surface. The most important consequence of these changes in the physical state of the soil water is sharp variation in the soil's temperature regime over the course of the year.

The plant cover and the lichen and cyanobacteria crust also affect the heat level in the summer: Bare soil has a higher reflectance than soil covered by mosses and lichens. Irregularities in the microrelief, sediments and vegetation also give rise to changes in the depth of the summer melt and in biological activity. As the vegetation is low, the wind is a major factor influencing the soil, because it affects both the thickness of the snow cover in winter and the soil temperature in the summer, and even biological adaptation, through the creation of special habitats.

Freezing and melting: the molding of a changing microrelief

Permafrost, together with the freezing and later seasonal melting of the soil, has given rise to the development of a special microrelief that is highly characteristic of the tundra.

Soil ice is generally concentrated in specific points in the form of lenses, with their longer plane lying more or less parallel to the isotherms; the deeper they occur, the further apart they are. This form of segregation occurs in all sorts of relatively unconsolidated porous materials containing fine particles, such as clays and mud: The water migrates through them along the capillary gaps towards the ice front, pulled by a suction gradient (cryosuction) caused by the freezing and crystallization of the water. This migration of water induced by the heat gradient allows the water content of the horizon to increase, in the form of segregated ice. But not all materials are geliexpansible, or prone to expand on freezing owing to the growth of the many sheets of ice that may form within them. For example, many sandy and gravelly soils are not geliexpansible; the spaces that form are larger and do not retain the water so strongly, ice forms only in the interstices or between the grains, and capillary movement of water is impossible. A typical feature of soils with geliexpansible materials is that they are either bare, because the soil heave caused by the ice breaks the roots of vascular plants, or they have a cover of a thin crust of lichens and cyanobacteria and a little moss.

One of the characteristic models of the tundra's microrelief is what is known as polygonal micro-relief, consisting of rectangles or polygons 16-65 ft (5-20 m) in diameter, separated by deep, vertical, ice-filled cracks (see fig. 7). When the melt begins, the thermal gradient of the permafrost is reversed, so the higher temperatures now occur at the surface, not the reverse. This means that cryosuction is towards the soil surface, the opposite of what happens in winter. This results in the enrichment in segregated ice of the surface horizons of the permafrost, which causes the soil to heave. This sometimes results, for example, in polygons with a raised center instead of a flattened one or causes the stones present in a geliexpansible material to be moved and arranged so that they are transported upwards until they emerge at the surface. When the ice expands, it pushes the stones towards the surface; but when the ice melts, the space it occupied is partly filled with soil particles. The stone cannot sink back down to its former level, so it moves towards the surface. This process is very rapid and is not restricted to tundra soils. Although it only occurs in soils subject to the action of ice, it is also very active in winter in temperate latitudes, where it is often possible to observe the emergence of stones in cultivated fields (which was known in the past and turned to advantage by collecting the stones and getting rid of them in order to make it easier to work the soil). To sum up, starting from mixed materials of varying particle size, such as moraine deposits, the action of ice sorts the materials by particle size forming stony layers on the surface and finer ones at depth.

In addition to the microrelief, some authorities on the tundra distinguish forms that can be classified as nanorelief, mainly as a consequence of the processes of freezing and melting occurring on the surface soil horizon. One of the most notable results of these processes is called soils with structured surfaces. These soils may occur in several different configurations, but they generally show regular repetition of slightly or highly structured forms, mainly regularly spaced circles or polygons formed by stones sorted by size and bands in the form of geometric figures. The development of these regular geometric shapes is due to the formation of convection cells in the circulation of the meltwater in the topmost soil horizons. Water is densest at 39.2[degrees]F (4[degrees]C), and begins to sink while the soil is melting from the surface, then cools when it meets the melting front (thus lowering its density), and rises again. Thus, in the warmest period of the year, the soil is continually melting, and permanent convection cells may also form.

2.2 Soil processes

All these forms of microrelief and nanorelief do not presuppose any special type of soil morphology, except that freezing causes the precipitation of any carbonates or gypsum the water might initially have contained. Even so, soil formation processes and their functioning in the tundra are closely related to the development of the microrelief. The underlying permafrost, the very low temperatures, the regular freezing and melting, the extremely high humidity, and the characteristic layer of mosses and lichens are the main factors responsible for soil development. The soil's saturation with water prevents oxygenation and creates conditions favorable for anaerobic organisms that normally play a major role in the breakdown of organic material; as a consequence, this breakdown is very slow, and much of the dead plant materials turns into peat.

Physical weathering

The mechanical weathering of rocks provides the parent material for soil development, which is very important in the tundra, because the rate of chemical weathering is very slow because of the low temperatures and the shortness of the summer. The high cryosuction occurring at very low temperatures means that there may be a marked local dissolution at the surface of the materials through which the capillary water circulates, since these materials show a high amount of hydric activity.

Frost wedging caused by the ice, also known as gelivation or congelifraction, occurs along the planes of weakness of rocks (the result of their tectonic past) and is at the same time combined with the fracture planes resulting from the stresses caused by sudden changes in temperature. This leads to fissuring parallel to the isotherms, which may be promoted by reopening of exfoliation junctions or of fissures from a previous fracturing, especially in the ones that lie parallel to the penetration of the freezing front (the 32[degrees]F [0[degrees]C] isotherm). In the case of volcanic rocks, the crystalline junctions are separated by differential expansion and hydration. All these junctions, after melting, form the routes the meltwater circulates through, so this is where the levels of weathering or solution are highest. The efficiency of weathering is greatest where the winter temperatures are lowest (greater cryosuction), the summer temperatures are highest (drying out, expansion) and moisture is abundant (which may even be derived from mist).

Weathering is also more effective if translocation of organic material takes place under the plant cover. During the frost period, the formation of ice crystals produces mechanical pressures that break the rock into fragments, flakes, or granules as well as hydraulic pressures within small fissures that cause them to expand. This network of cracks readily fills up with fine detritus materials, residual clays or mud (dolomite crystals), or translocated organic material (flocculation [causing to aggregate in a loose, fluffy mass] by calcium ions), and these materials locally increase the geliexpansibility of the horizon and allow ice lenses to form.

If these broken-down detritic materials are buried by a fine sedimentary or residual matrix (sands, muds, or clays), water retention increases and the breakdown is more effective. The presence of salts, such as chlorides and sulfates, also affects the production of detritus and leads to the breakdown of the rocks into tafoni, as happens in Mediterranean regions and in deserts. Weathering by salts is also more common in arid environments closer to the polar desert, such as Ellesmere Island, at the northern tip of the Canadian Arctic Archipelago, than in the true tundra.

Soil nutrients and biological activity

In the tundra soil, the decomposition of organic materials is also very slow. In practice, the favorable period for humus formation and weathering of the substrate is even shorter than the brief period of plant growth, because of the low summer temperatures, the waterlogging typical of the melt season (especially over permafrost), and the physiological drought due to the wind and low temperatures, which means that the soil's mineral nutrient content is low.

The living organisms are concentrated mainly in the surface layer, among the plant remains. Fungi, including forms that can grow at temperatures below 32[degrees]F (0[degrees]C), play an important role in the breakdown of organic material, at least in the well-aerated surface layer, the only layer where they can be active. In deeper layers, breakdown is undertaken by bacteria.

Tundra soils are poor in nutrients: Almost all the nitrogen is in organic form. In wet sites, mineral nitrogen is present as ammonium ions, but in dry soils it is present as nitrates. In tundra soils, the main input of nitrogen is by bacterial fixation of atmospheric nitrogen. For example, Nostoc commune is very common in the tundra and can live independently or form filaments associated with mosses. It may even form a symbiosis with ascomycetes to form a lichen, such as one of several species of the genus Peltigera. The biomass of nitrogen-fixing organisms is high in wet areas and low in dry ones. Inorganic nitrogen and phosphorus mainly reach the soil in the precipitation. Although general nutrient cycles in the tundra have not been thoroughly studied, it can be stated that phosphorus, not nitrogen, is the nutrient that limits primary production. Herbivores, such as lemmings (Lemmus), in years of abundance, may play an important role in the phosphorus cycle.

Humus formation

Humus formation is not very important in tundra soils when compared with its formation temperate or tropical soils, owing to the shortness of the growing season. The activity of the fauna may be very important in wet material that is well drained and protected from the wind, since in the tundra environment desiccation and cold are the factors limiting humus formation. Sites with excess water (hydromorphic soils) are dominated by thecamoebas and nematodes, while limnicolous prosothecan oligochaetes of the Enchytraeid family and collembollas (springtails) prefer wet but well-drained sites. Organic residues are mainly colonized by dipteran larvae and by caterpillars. If these residues are well drained, only typical lumbricid earthworms live in them, such as Dendrobaena octaedra in acid soils, and Eisenia nordenskjoldii. In drained sub-Arctic soils, acarid mites are also common.

The types of humus, often coprogenic, vary from immature ranker or moder on hydromorphic or dry soils, and mull/moder in continental acidic grasslands (grasses, sedges) on loamy soils. The C/N ratio varies between 10 and 30. Bacterial activity is not very important due to the bactericidal activity of the lichen acids, although some bacteria play an important role in the precipitation of iron hydroxides. The processes of degradation of organic material are mainly due to the activity of actinomycetes, yeasts and other types of fungi, followed by the insects of the soil fauna, even in cracks in rocks. The segregation of ice also breaks down plant tissues by tearing the roots.

The least developed form of humus is the so-called cryptogam crust, dominated by cyanobacteria and fungi, whether lichenized or not, so that the only true cryptogams are a few bryophytes. The C/N ratio is very low, less than 10. This cover is very resistant and can tolerate drastic environmental conditions such as drought, soil-heaving, seasonal waterlogging and geliturbation. Its elasticity is also adapted to cryogenic deformation, giving the soil surface the appearance of elephant skin. It is also very resistant to shearing, thus limiting the splashing effects of water drops or deflation by wind. However, the cryptogam crust is very sensitive to the action of piprakes (needle ice) and to abrasion by the load of snow borne by the wind or human disturbances. The characteristic fauna of this inhospitable environment consists of springtails, thecamoebas, and nematodes.

Cryogenic consolidation, illuviation, and solifluction

Ice formation can be considered as a type of thermal desiccation. The sheets of ice cut the sediments into aggregates in the form of blocks or sheets, which are initially formed by contraction, and later undergo a reconsolidation caused by severe ice-caused desiccation called cryogenic consolidation. The lower the winter temperature, the greater the cryosuction and the consolidation of the aggregates.

The stability of the aggregates depends on the degree of cryogenic consolidation, which is affected by several factors, such as the temperature or composition of the material. The interstitial water freezes at 19[degrees]F (- 7[degrees]C) in muds and at 25[degrees]F (-40[degrees]C) in clays. Other factors that affect consolidation are the presence of salts as flocculating agents, or other possible consolidating agents, such as organic polymers or precipitated amorphous minerals, such as immogolite, iron hydroxides, or silica.

The process of freezing takes place more quickly at the surface. Therefore there is not so much time for the aggregates to consolidate, and so consolidation is weaker than at depth. When the melt starts, it also takes place more quickly at the surface than at depth, as the thermal gradient is greater at the surface, leading to the collapse of relatively unconsolidated aggregates at the beginning of the winter. The melt water's high viscosity and low relative permittivity also promote the process. As a result, the particles in the surface horizon are dispersed and illuviated deeper into the soil, a process favored in spring by the reversal of the thermal gradient. Clays and muds accumulate at depth as a covering of pores and coarse elements during the summer, and may be incorporated into the soil matrix by the pressure of freezing in the winter. Successive annual cycles of translocation of fine materials from the surface and their later incorporation into the soil matrix at depth lead to the progressive formation of a Bt subsurface horizon, enriched in clays and thus showing greater geliexpansibility than the surface horizon. This diagenesis (change that takes place at a low temperature and pressure after deposition) is continuous and may even be increased in summer by crust formation due to the impact of water droplets on the soil surface. In the case of detritic materials broken up by the ice, the fine residues, mainly muds, are also translocated downwards and a loam-enriched Bt horizon forms.

If the soil is on a slope, during the melt it undergoes soil creep, layer by layer, between the lenses of melting ice, as if it were a laminar flow. This process is known as gelireptation. When there is abundant water during the advance of the melt front, lubrication increases because of the greater hydrostatic pressure, and the sliding is faster. If the cryogenic structure is stable, the water drains between the aggregates and the movements soon slows down; this is called gelifluction. If, to the contrary, the aggregates collapse and the water does not drain easily, the supersaturation causes liquefaction to occur, and mudslides (surface flows) occur. Most forms of solifluction are due to ice creep. Gelifluction and mudslides are located under patches of snow or occur only during hot summers, when the ice-enriched surface permafrost horizon melts and provides enough water.

Differential raising, cryoturbation, and the geliexpansibility gradient

In polar and tundra environments, the small differences in particle size, composition, water retention and water content of the materials, affected by the surface microrelief, lead to differences in geliexpansibility. The accumulation of ice lenses on the soil causes differential raising of the overlying or adjacent materials. If drainage is insufficient, as in waterlogged depressions, when the frosts start in the autumn, the water content of the surface horizon is very high. The surface freezes as hard as concrete and rises like a rigid mass. All the differences in the raising of the ice are thus expressed below, giving rise to geliturbations. The microrelief is caused by the expression on the surface of the differential raising. In well-drained areas, forms develop that rise above the landscape, called hummocks, but where the drainage is limited or poor, the relief is flat. Sands are the first to freeze, as they have a low water retention capacity and a low reserve of latent heat of crystallization; the organic horizons with humus or those with a high mud content are the next to freeze, as their greater water-retention capacity allows the water to continue migrating through the frozen soil to the growing ice lenses. When the subsoil is warm, the thermal gradient is greater, cryosuction is more effective, and deformation is faster, as happens on the warm edges of the tundra.

Geliexpansible materials, when they are not frozen, are squashed and compressed between materials that have already frozen, such as sands and gravels. These internal pressures within the soil are responsible for the plastic deformations that occur under the freezing front. Under natural conditions, a sequence of stratified material or superimposed horizons will form a gradient of geliexpansibility. The gradient will be positive when the surface horizon is more susceptible than the subsurface horizon, and negative when it is not. The resulting morphology is characterized by injections or intrusions of gravel or geliexpansible materials along thermal contraction or desiccation fissures. This process is faster when the difference of geliexpansibility of the horizons is greater, as happens in the circles made of materials sorted by grain size, where gravels and muds derived from the cracking of dolomites by ice are juxtaposed. Podzols also show similar contrasts; the Bs horizon is enriched in organic material and fine particles, and is more geliexpansible than the eluvial E horizon, so that the Bs; horizon extrudes through the E and Al, forming a bubble of mud that rises above the surface of the herbaceous tundra. As these horizons are susceptible to changes in their structure through the simple incorporation or elimination of water, many tundra soils are spongy and ductile. The people who work on these soils know that stamping regularly in a given point in the tundra can make the surface of the tundra quake in an area of several square meters.

2.3 The types of soils

The tundra's soils are the result of the interaction between soil formation processes and the combined action of freezing and melting. The soils are difficult to classify within the systems used in other areas (FAO Soil Taxonomy).

Organic soils or histosols

In cryptogam tundra, the peat is the result of the growth and accumulation of bryophytes together with cyanobacteria, algae, diatoms, and, occasionally, lichens. The C/N ratio of these bogs is very low, less than 10, because of the excrement from bird colonies and the accumulation of nitrogen by the cyanobacteria. The deepest layers of peat may reach a thickness of 3-7 ft (1-2 m), and the organic material is kept cool, and normally preserved from breakdown by the parallel growth of the permafrost and the bog. As the nutrient reserve in the sediments is low, bog development often starts on the carcases of mammals or under colonies of marine birds. This process leads to the formation of histosols.

In shrubby tundra, the peat may reach greater thicknesses. It consists of an accumulation of bryophytes and some sedges, such as cotton grass Eriophorum vaginatum, or the sedge Carex bigelowii, often colonized by clumps of dwarf willows (Salix lapponum, S. herbacea) dwarf birches (Betula nana), and marsh rosemary (Ledum palustre). The C/N ratio is higher, but less than 25, and the pH is generally below 5. The average depth affected by the frosts is at least 20 in (50 cm), as occurs for example in Lapland, but the summer melt may affect the entire thickness of the soil. The changes in micro-relief and the drainage conditions of the bog may affect the mineralisation and the darkening of organic material. The growth of the peat on the slopes of the subarctic tundra leads to the formation of reticulate bogs.

Hydromorphic soils or gleysols

Despite the slowness of the chemical action of atmospheric agents, the capacity of the tundra soil to retain water sometimes creates conditions that favor the accumulation of oxides of iron and aluminium. In the absence of oxygen, the ferric compounds (Fe3+) are easily converted into ferrous (Fe2+) ones, and together with other compounds form greenish blue layers (gleying), mainly in depressions and flat surfaces with inadequate drainage.

The gleys found in the tundra range from the purely mineral ones found in the Barren Grounds of northern Canada, covered by a crust of cyanobacteria, fungi and lichens, to the peaty gleys of the wetlands in the shrubby tundra. The iron hydroxides released precipitate as an orstein (indurated soil horizon) in the form of a thin ferruginous layer near the surface. Due to the low temperatures, pyrite formation is very effective, although superficial, in poorly drained organic soils.

Podzolization and the accumulation of carbonates and salts

In sites with good drainage there are dark soils, known as podbur in Russia and Siberia, that recall the dark soils, rich in ferric (Fe3+) oxides, of more southerly regions. The process of soil darkening is similar to podzolization, since in the melt season there is intense formation of organic complexes and washing, which favors the development of soils with shallow washing horizons.

The slowness of the processes lengthens the residence time of organic compounds (between 40 years in well drained sites and 400 years in wetlands). Marked melanization of organic compounds occurs and is due to the desiccation caused by winter ice, summer drought and the frequent saturation with calcium. Mica is often transformed into vermiculite and the surfaces of quartz are dissolved by the action of lichen acids. Cyanobacteria and endolithic algae in the surface horizons of lithic soils also promote their weathering.

In crytopodzolic soils, seasonal washing allows the accumulation of breakdown products at depth in the horizons with higher pH values, and leads to the formation of a spodic Bs horizon, enriched in immogolite and in fulvic acids (uncolored organic acids), while in hydromorphic conditions, it is enriched in amorphous iron hydroxides. This accumulation follows the same thermal law that governs the translocation of particles and may even occur in the surface horizon of the permafrost. Furthermore, the accumulation of amorphous compounds (mineral or organic) results in an increase in the geliexpansibility of the material, and thus soil formation processes may mean that sands that were initially untouched are later affected by geliturbation.

The level of carbonic acid is very high because of the low temperature and the metabolism of the fungi and the cyanobacteria, causing a rapid loss of carbonates from the rocks, favoring their breakdown by ice and salts. In dry organic soils, carbonates, which may also be derived from the oxalates produced by lichens, precipitate during the summer, mainly as microstalactites in cracks within rocks or in the larger pores in the soil. The cryogenic precipitation of salts is restricted to free water only (frost, ice on the coast or on rivers), and most solutes are drained away in the first days of the melt. In polar deserts, the salts are concentrated in depressions and precipitate as evaporites on the surface, or, as in solonchak soils, similar in appearance to those that occur in some cold deserts at lower latitudes.

3. The polar space of the tundra and the Arctic deserts

3.1 The hyperboreal lands

The area covered by the tundra is about 3% of the total area of dry land. The presence of the Arctic Ocean prevents it from spreading any further north, whereas the latitudinal bands are shorter in the regions near the poles than in the lower latitudes. Thus, the equator is 24,854 mi (40,000 km) long, but the 70th parallel (N or S) is only 18,544 mi (3,750 km) long, so there is almost no space for the tundra.

The tundra's latitudinal limits

The tundra occupies a relatively narrow strip along the northern coastline. The width of this strip may vary greatly from one area to another. In some regions of the center of the northern coast of Eurasia, for example the Taymyr Peninsula or in the east of Northwest Territories (Canada), it may be 373-435 mi (600-700 km) wide.

The southern limit of the tundra, in both Eurasia and North America, does not always occur at exactly the same latitude, but tends to run parallel to the continents' northern shorelines. Thus, in some areas (for example, the Taymyr Peninsula) it reaches 72[degrees]N, while in other areas (such as the shores of Hudson Bay) it reaches a latitude as low as 52[degrees]N (more or less the same as Berlin or Irkutsk). In the case of the Arctic islands, only the most southerly ones are occupied by the landscapes typical of the tundra; those located to the north of 75[degrees]N (Spitzbergen, Franz Josef Land, the northern island of Novaya Zemlya, and much of the Canadian Arctic Archipelago and of Greenland) are covered by ice or occupied by Arctic desert, that is to say, dry rocky lands with almost no life--practically only some small scattered patches of lichens and mosses, not forming a continuous cover.

The tundra and Arctic deserts

Tundra landscapes and its plant communities tend to vary greatly with latitude. Normally, the tundra is divided into three subzones: (1) the southern, or shrub, tundra, (2) the typical, or central tundra, and (3) the Arctic, or northern, tundra.

The southern subzone of the tundra has a diverse vegetation, including not only mosses but also many grasses, rushes, cotton grasses, dwarf trees, and shrubs (Salix lapponum, S. herbacea, S. lanata, Vaccinium vitis-idaea, V. uliginosum), and even by true shrubs. The southern tundra borders the tree tundra and is similar to it.

The central subzone, the typical tundra, covers the largest area. The limits of this subzone can be defined with reference to the isotherm of the warmest month: its northern limit corresponds to the average temperature of between 39 and 41[degrees]F (4 and 5[degrees]C) in the warmest month, and the southern limit to a temperature of between 46 and 52[degrees]F (8 and 11[degrees]C). In the typical tundra, there are mainly mosses, above which there is normally a herbaceous layer of rushes. There are also dwarf shrubs. The few normal shrubs present are restricted to protected depressions, where the winter layer of accumulated snow protects them from the violent winds and the harsh frost.

The northern subzone, the Arctic tundra, is intermediate between the typical tundra and the Arctic desert. It is found mainly on the islands in the Arctic Ocean, whereas on the mainland it is found only in a few small enclaves that are very local, for example in some areas in the north of the Yamal and Taymyr Peninsulas. The vegetation of the Arctic tundra is very poor and is often found as dispersed patches in the middle of bare ground.

3.2 The geography of the tundra

The northern limit of the tundra is formed by the Arctic Ocean. There are, however, also tundra landscapes in many of the Arctic islands, from Iceland in the west to Wrangel Island in the east. The islands at more northerly latitudes, that is to say, Spitzbergen, Franz Josef Land, the northern island of Novaya Zemlya, and Severnaya Zemlya (North Land), are covered in glaciers or Arctic desert. The climate of these areas is drier and colder than that of the true tundra, and there is very little vegetation, only a few scattered patches of lichens covering small areas of ground.

The Scandinavian and Siberian tundra

The Eurasian tundra runs as a coastal strip of variable width along the coastline of the Arctic Ocean and that of the northern Pacific. The tundra's southern limit lies at different latitudes depending on the region. In the Taymyr Peninsula it reaches 70[degrees]N, while on the eastern edge of Eurasia it runs down the Pacific coastline to 59[degrees]N on the mainland and as far south as 55[degrees]N (approximately the latitude of Copenhagen) in the Komandorskiye Ostrova islands, off the southeastern coastline of Kamchatka, the most southerly area of tundra in Eurasia.

The Atlantic and Pacific Oceans at the westernmost and easternmost parts of Eurasia have a major influence on their heat pattern. The Atlantic attenuates the thermal pattern of the Scandinavian coastline, and the Pacific accentuates the cool summers on both sides of the Bering Strait, on the Kamchatka coastline and on the neighboring islands. For this reason, at the northwestern tip of Eurasia, northern Scandinavia, the tundra is reduced to a very narrow strip. In this region, the winter is relatively mild, with frequent thaws. The coldest average monthly temperature is 23[degrees]F (-5[degrees]C). The summer is cool, like that of many regions of continental tundra (more than 20 in [500 mm] per year). Precipitation is very high in comparison with other areas of tundra. In autumn and winter, during the period of the cyclones, rain and snowfall are relatively abundant. The climate is thus in effect cool maritime.

In eastern Scandinavia, the tundra stretches in the form of a narrow strip and occupies the flat coasts of the northern Kola Peninsula. The influence of the heat of the ocean (due to the warm currents of the North Atlantic) creates a warm winter, with thaws, even though the mean temperature of the coldest month does not exceed 14[degrees]F (-10[degrees]C). The snow cover lasts for seven months. To the east of the White Sea, the tundra zone widens and covers the entire Kanin Peninsula and large plains on both sides of the lower stretches of the River Pechora (the Malozemelskaya and Bolshezemelskaya tundras). The climate is even more continental: The coldest mean monthly temperature is 0.4[degrees]F (-18[degrees]C) throughout almost the entire region, while the warmest mean monthly temperature (July) changes considerably with latitude (2[degrees]F [1[degrees]C] for each 15-31 mi (25-50 km) northwards or southwards). To observe thermal changes of this size in the boreal latitudes, it is necessary to move hundreds of kilometers along the meridian. But in the tundra of the lower Pechora, in the north, on the shores of the ocean, the average temperature of the month of July is 46.4[degrees]F (8[degrees]C), while in the south, near the northern limit of the taiga, it reaches 54[degrees]F (12[degrees]C). The rainfall is less than in the tundra in the Kola Peninsula. The snow layer forms in October and may last until the end of May, or till the beginning of June in the north, and it increases in thickness from north to south. Further east, to the east of the Urals, in the region of the Yamal Peninsula and in the north of westernmost Siberia, the tundra is widest, reaching 373-435 mi (600-700 km) if measured along the meridian. The climate is more continental and the winter temperatures are lower. Thus, in the south of Yamal, the average temperature of the coldest month is -13[degrees]F (-25[degrees]C). The thickness of the snow cover, which decreases towards the east, is a limiting factor for the distribution of shrubs.

The tundra areas with the most continental climate are in the eastern part of the Taymyr Peninsula and spread east to the River Kolyma across the plains of the rivers Lena, Yana, and Indigirka. This region is very far from the Arctic front, so cyclonic activity is very weak here, especially in winter. Rainfall is scarce and the snow layer is too thin to provide resistance to the intense frosts that may reach temperatures as low as -58[degrees]F (50[degrees]C). The coldest average monthly temperature is normally below -22[degrees]F (-30[degrees]C), and in some regions is as low as--40[degrees]F (-40[degrees]C). Winter begins very early: soil freezing normally occurs in the second half of September, before the formation of the snow cover. Further east in the lower Kolyma, the mountains are closer to the coastline. Therefore, the tundra lowland is represented by a very thin strip of coastal plains bordering the Chukchi Peninsula and running south along the coastline of the Bering Sea down to 59[degrees]N. These regions are strongly influenced by air masses that form over the area of the Bering Sea, the Chukchi Sea and the Sea of Okhotsk. The cyclonic activity related to the Arctic front leads to strong winds and sudden changes in the weather, in both summer and winter. Yet the coldest monthly temperature is higher than in the tundras of neighboring eastern Siberia. Precipitation is abundant in winter and the snow cover is relatively thick.

Apart from the lowland tundra, represented by a continuous strip, typical tundra landscapes are also found in the area of the taiga forest, at altitudes at which the severe climate prevents tree growth. These montane tundras are found in Scandinavia, in the Kola Peninsula, in the northern Urals and especially in the eastern part of Siberia. The vegetation and fauna of the montane tundra is very similar to that of the lowland tundra.

The North American tundra

East of the Bering Strait, the situation in the southwestern coastline of Alaska is similar to that described in the easternmost coastline of Siberia and Kamchatka. Along the chain of Aleutian Islands, the tundra reaches as low as 52[degrees]N in climatic conditions where the winter is not so harsh (owing to the maritime influence and the latitude), similar to the conditions of a typical boreal forest but with a summer that is too cool (also due to the maritime influence) to allow the growth of trees. The tundra that grows is a shrubby tundra, including many ericaceous species, and with increasing altitude becomes a more typical tundra of mosses and lichens.

In northern Alaska, the Brooks Range, marks the division between the taiga, which occupies the sunny sites, and the tundra, which populates the highest areas and extends down to the shores of the Arctic Ocean in shaded sites. In this region the tundra is reduced to a very narrow strip, little more than 62.2 mi (100 km) wide, running along the Arctic coastline, but when it enters Canada at the lower Mackenzie River, the tundra becomes considerably wider. Its southern limit moves further south, and is roughly at the mouth of the Churchill River in northeastern Manitoba when it reaches the shores of Hudson Bay; the tundra's northern limit, running along the coastlines of the islands of the Canadian Arctic archipelago, reaches latitudes higher than 80[degrees]N in favorable sites on Ellesmere Island and the northern coasts of Greenland, where small populations of the flora typical of the tundra have been recorded, including lichens, mosses, ericaceous species (plants in the heath family), and even some dwarf willows, all growing as far north as 83[degrees]N.

The corresponding continental area consists of the Barren Grounds, which occupy part of the Northwest Territories and is a low shrubby tundra where none of the shrubs exceeds a height of more than 16-24 in (40-60 cm). Towards the north-eastern tip, and in some of the more southerly islands of the Canadian Arctic archipelago, the thinner snow layer and the strong winds even further limit the height of the shrub layer. A strip of tundra running along the northern coasts of Manitoba and Ontario on the shoreline of Hudson, interrupted in some areas by intrusions of taiga reaching as far as the sea, extends the Barren Grounds to the southeast. At the mouth of James Bay, the tundra reaches one of its lowest latitudes in North America (above 54[degrees]N). The only tundra occurring further south is on the Atlantic side of the Labrador Peninsula.

To the east of Hudson Bay, the Ungava Peninsula, the northern tip of Quebec, and the Atlantic side of the Labrador Peninsula complete the tundra distribution area in North America that reaches its southernmost point (a little over 52[degrees]N) precisely in the Atlantic area of Labrador. Off the coasts of Newfoundland, the tiny Saint Pierre et Miquelon Islands, at a latitude of 47[degrees]N, but located in the middle of the cold Labrador current, still show traces of tundra in the form of peat bogs, with cotton grasses and ericaceous species, that are frozen from November to April.

The Greenland tundra

Greenland, like some northern islands of the Canadian Arctic Archipelago, is a special case. Although most of the island is covered in ice (the largest block of ice on earth after Antarctica), patches of tundra contain spaces without ice. Many studies of the diversity of the flora of northeastern Greenland have been unrewarding owing to the difficulty of finding sufficiently large areas that are not covered by ice and also to the difficulty of moving through the ice and snow. Since 1983, however, numerous visits to Bronlund Fjord in Peary Land, have managed to produce a reasonably complete inventory of the most northerly vegetation, with a total of 106 species inventoried, including endemic species such as Saxifraga nathorstii.

After exhaustive studies, it has been demonstrated the type of vegetation that develops in the Greenland tundra is more closely related to the temperature than to the type of soil the vegetation is growing on. When the summer temperatures do not exceed 36.5[degrees]F (2.5[degrees]C), the vegetation is dominated by mosses and lichens, while the threshold that allows woody plants to grow is between 37 and 38[degrees]F (3 and 3.5[degrees]C). Dwarf shrubs dominate when temperature exceeds 38.3-39[degrees]F (3.5-4[degrees]C).

The plants are so adapted to an extremely harsh climate, short growing seasons, and the presence of permafrost that many plants considered typical of the Arctic tundra do not live south of 70[degrees]N, even in the most mountainous regions. These plants include some, such as the crucifer, Braya wulffii, that multiply vegetatively by stolons. To sum up, along the western coastline, on the slopes and on the shores that are warmest in summer, from Disko Island (Qeqertarssuaq), at 69[degrees]N, and the Sodre Stromfjord (Kangerdslugssuaq) at 67[degrees]N, to the south, the tundra is mainly low and shrubby with Salix glauca callicarpaea, Betula nana, and B. glandulosa, while on the eastern coastline, and also further to the north, the vegetation is more often small bushes members of the Ericaceae and Empetraceae.

1 The intense cold, the weak sunlight, and the low precipitation of the Arctic tundra make this biome inhospitable for life. Yet on the almost permanently frozen ice layer there are the tracks of a passing animal, showing that this environment is not uninhabited and that life has colonized even the regions of the planet with the most extreme climates.

[Photo: Jim Brandenburg / Planet Earth Pictures]

2 The incoming solar radiation at different latitudes and annual average temperatures. At the equator, the Sun's rays fall perpendicularly on the earth's surface and warm it. Owing to the curvature of the planet, however, the Sun's rays reach the poles at an angle of less than 90o, so the same amount of radiation has to cross a greater thickness of atmosphere, and also falls on a larger area, thus heating it less. Furthermore, the polar ice caps reflect much of this radiation back into space. These are the reasons (among others) why temperatures decrease with increasing latitude. The graph of temperatures shows that the southern hemisphere is considerably colder than the northern hemisphere, and that the northern hemisphere demonstrates smaller seasonal changes in temperature.

[Drawing: Jordi Corbera, based on Davenport, 1992]

3 The tundra, the biome of the Arctic region, extends from the limit of tree growth (see also figure 15) to the southern limit of the permanent ice (in white) and has a cold, dry climate. The average annual temperature is below 32oF (0oC). and precipitation, normally as snow, rarely exceeds 14 in (350 mm) per year. The climagrams, corresponding to four sites at similar latitudes and altitudes (except for Thule which is far to the north), show that the winters are long and extremely cold while the summers are short and cool. The relatively warm summer days when the sun shines and the wind does not blow are very pleasant.

[Drawing: Editronica, from several sources]

4 Longitudinal section of Alaska showing the profile of the permafrost. In most Arctic regions there is a belt of more or less continuous permafrost that coincides almost exactly with the distribution of the tundra. The presence of the permafrost layer means that the temperature gradient declines sharply with increasing depth. Beneath the permafrost, the temperature rises again owing to geothermal heat.

[Drawing: Jordi Corbera, from several sources]

5 Lens-shaped block of ice in the permafrost, uncovered at the edge of a road in Fairbanks, Alaska, at the beginning of summer (June). Ice accounts for 80-100% of the volume of the permafrost. The depth of the permafrost depends on the average annual temperature, the soil's composition (which determines its thermal conductivity), the nearness of the sea, and the topography. In some regions it reaches a thickness of 0.4 mi (600 m) or greater. The thickness of the surface layer that melts every year is determined by a large number of factors, including the nature of the soil, the amount of water it contains, the summer temperature, the thickness of the snow layer, the type of microrelief and the vegetation.

[Photo: Charlie Ott / Bruce Coleman Limited]

6 Thickness of the active layer of soil (in centimeters). The active layer varies in thickness depending on the type of substrate or tundra in question. Information has only been collected on the typical tundra and the southern tundra, which have more plant species. The northern tundra, which is more arid, has been omitted.

[Source: data provided by the authors]

7 Polygonal microrelief, in this case in the Siberian tundra, can form in well-drained sites but is more common in waterlogged sites. Swampy, waterlogged, soils take a long time to freeze, and when they freeze at the beginning of winter, they undergo sudden contraction, meaning that cracks form easily. A network of thermal contraction fissures thus forms that penetrates deep into the soil, dividing the area into polygons whose edges may be up to half a meter higher than the center. In soil lying over permafrost, when the melt begins, the meltwater percolates through these fissures and freezes again, increasing the impermeability of this horizon. Ice accumulates within the cracks and in time forms wedges that eventually deform both the active layer and the permafrost. The depressions within each polygon fill with water in the spring but dry out in summer, especially if the cracks and channels allow good drainage. Thus, the vegetation in the center of the polygon is normally no different from that found on the edges (see figures 37 and 118).

[Photo: Colin Monteath / Auscape International]

8 Palsa or hydrolaccoliths (water mounds), are small mounds of peat with a frozen center. They are very frequent in the Talkeetna Mountains in Alaska. They are the result of the soil rising when the water circulating in its active layer freezes on contact with the permafrost, and they appear especially in regions where freezing is greater at depth than on the surface. They are normally between 1.6 and 33 ft (0.5 and 10 m) high and between 6 and 33 ft (2 and 10 m) in diameter, but may be much larger. Some are almost 230 ft (70 m) high and 492-656 ft (150-200 m) in diameter. In Yakutiya they are called bulgunyakhi, in North America, especially Canada, pingo and in Finland palsa or palse.

[Photo: Steve C. Kaufman / Bruce Coleman Limited]

9 Congelifraction of a sedimentary rock in Greenland. The liquid water penetrates the fractures in the rocks, and when it freezes it increases in volume and cracks them. This phenomenon, called gelivation or cingelifraction, occurs in all types of rock and is the first step towards the formation of a soil that plant communities can grow on. The basic processes of soil formation are the same in the tundra in the temperate regions, but in this biome they are much slower, because the low temperatures and the short summer slow down chemical weathering and restrict the plant cover. The presence of permafrost, which slows down vertical circulation of water, even further limits soil formation and plant growth.

[Photo: B. & C. Alexander / NHPA]

10 Cyclic freezing and melting creates a series of surface microreliefs. Some of the most spectacular are the circles and materials arranged by particle size, shown here in the Norway's Svalbard Archipelago. Some may reach 328 ft (100 m) in diameter, but most are between 16 and 33 ft (5 and 10 m), and some are only a few centimeters across. Like all surface microreliefs, they play an important role in soil development and the distribution of the vegetation.

[Photo: Brigitte van Vliet-Lanoe]

11 If the water content of the surface layers is very high, when it freezes, the rocks and materials below are differentially compressed. They deform in a characteristic way, as shown in this photo taken in the Svalbard Archipelago in Norway.

[Photo: Brigitte van Vliet-Lanoe]

12 Blocks of peat cut and piled for use in building, or as fuel when dried, in the Andenes-Vesteralen peat bog in Norway. When they die, the mosses and other marsh plants are buried underneath a layer of living vegetation, where they gradually decompose and are compacted into peat. However, for peat to accumulate, the material lost by decomposition must be less than the input of dead plant materials. In the tundra, this is possible because the low temperatures slow down decomposition. As the soil rises because of peat formation, rushes and small shrubs and trees may take root (see figure 23).

[Photo: Ramon Torres]

13 Arctic podzols or spodosols are formed by the washing of organic materials from the surface horizons by the meltwater. Unlike what happens in other latitudes, Arctic podzols are shallow and have very thin horizons.

[Photo: Brigitte van Vliet-Lanoe]

14 Sea ice and the glaciers of the North Pole are not static accumulations of ice. The constant input of snow, together with ice formation, means that during the winter months the polar ice cap spreads southwards. When summer arrives, however, the ice once more retreats. The snow that accumulates on the land masses pushes the glaciers towards the sea, where enormous blocks break off and float in the sea as icebergs. Wind, and especially sea currents, push the icebergs towards the south; they usually follow the route marked on the map.

[Drawing: Editronica, from several sources]

15 The world's most northerly trees grow in the forest tundra in Taymyr (Siberia) at 72o, possibly the remains of denser forests that formerly occupied the region and that have retreated as climatic conditions have become colder. The trees of the taiga mark the southern limit of the tundra, which is thus highly variable. There are often, however, also zones of transition between the two biomes, consisting of tree tundra.

[Photo: Hellio & Van Ingen / NHPA]

16 The arrival of spring melts the snow layer covering the tundra during the winter, and the meltwater forms countless streams, such as this one meandering down from Creek Glacier, in Denali National Park, Alaska. After this, the stronger sunshine allows a wide range of plants to grow. Plants flower, millions of insects hatch, flocks of birds and herds of herbivores arrive from more southern regions to make use of the rich seasonal resources, and the tundra fills with life until the first snows return.

[Photo: Antoni Agelet]

17 This photo of the North American tundra on the banks of the Killik River in Alaska shows the biome's relatively pleasant appearance in summer. Rain is not abundant and most water is retained in the form of ice, but the less rigorous spring conditions cause the tundra's appearance to change as it acquires the colors of the many different leaves and flowers, which are sometimes surprisingly warm and attractive.

[Photo: Chlaus Lotscher / Jacana]

* Although they are traditionally known as Lapps by the Scandinavians, and Lappar by Russians, the term should be avoided. Of unknown origin, it appears to mean "people of the frontier" and is an insulting or even hostile term for the Samer and their language.
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Publication:Encyclopedia of the Biosphere
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Date:Jan 1, 2000
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