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The Arctic is an Ecosystem
by Bill Heal
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The Terrestrial Ecosystem - Or Ecosystems
  From space you see the Polar Deserts in the 'High Arctic' - the Earth's cold deserts - where the cover of shattered rocks bear only occasional cushions of saxifrage, small poppies and dwarf willows, with a few mosses, and lichens crust the south faces of larger rocks. But it is not all barren. For example, there is a small lowland cove, sheltered by south facing cliffs carved from the limestone plateau of Devon Island in northern Canada. The Truelove Lowland has a series of beach ridges, formed as the land rose from the sea at various times. The ridges hold back melt water and plants thrive, warmed by the sun's rays rather than the air temperature. Similarly, where melt water seeps from snow-beds, relatively lush swards of mosses and lichens grow through the month or so of 'summer'. Local topography can override the devastating regional cold. But availability of water is critical. The widespread stony soils have not yet accumulated organic matter, drain freely and dry in summer.
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Moss in early spring, Greenland. Photo: Jonas Allansson

Further South or near the coast, plant cover gradually diversifies and extends to cover half or more of the soil surface - the Semi-Desert. Again more lush vegetation grows in sheltered and moist areas with grasses, sedges, shrubs and dwarf willow, birch, and larch. Here, germinating seedlings survive better than on the open ground which drains and dries in summer.

The distinction between desert and semi-desert is blurred; it is more of a gradient, with small patches and tongues of one system mixed into the other. Three features, two obvious, one hidden, play key roles in the dynamics of these landscapes:

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Road in Greenland, one can easly spot several layers of snow on the left side of the picture. Photo: Jonas Allansson
Snow. Depth, quality and timing are critical. Fresh snow is an excellent insulator but compaction and ice crusts caused by thawing and freezing increases heat conduction up to a hundredfold. With its insulating properties, early deep falls buffer the soil and the upper layers (the 'active layer') remain unfrozen well into the winter despite the extreme cold. Lemmings and voles shelter and breed beneath the snow; ptarmigan burrow to escape predation by foxes; but reindeer and muskox have difficulty in exposing plants to eat. In spring, late snow lie prevents nesting of birds and emergence of insects. Shallow snow exposes plants to low temperatures and to grazing, but ice crusts can resist the hooves of reindeer. Early melt, before the ground thaws, floods the nest of small mammals, drowning them or exposing them to predation. Water in frozen and liquid form is critical.
  Cryoturbation. Perturbation by daily and periodic freezing and thawing gradually moves soil particles, stones and blocks, sorting them into different patterns; circles, boils, medallions and on slopes stripes and flows. This patterned ground disturbs plants but also creates finer, moister soil for recolonisation, however briefly. Cracks open with the freeze-thaw cycles providing further colonisation sites. The power of water, changing between solid and liquid, moulds the land surface.
  Permafrost. The surface layers may thaw to 20 centimetres in fine-grained moist or wet soils, deeper in coarser dry soils. But lower layers remain frozen, even if only a few degrees below zero. Surface temperatures may rise to and fall by 50o C or more over 24 hours in summer through the sun's radiation but slow diffusion of heat and cooling by the underlying bed of frozen soil damps these daily and seasonal oscillations. The permafrost remains at -1 to -3o C. This impermeable bed also prevents drainage, holding water in the active layer and causing surface or near-surface flow on slopes. The hidden element of the landscape.
  The deserts and semi-deserts of the High Arctic are sparsely covered and extend over vast areas, especially in Russia and Canada. The patterns are on a small scale within the vast landscapes - at the meso-scale of small landscapes covering a few hundred square metres or kilometres; at the micro-scale of centimetres or metres. At each scale you can see a basic structure, usually defined by the plant cover or physical form. At each scale there are the process of primary production, decomposition and circulation which define an 'ecosystem', including inputs and outputs. The ecosystem is never completely closed. In these deserts and semi-deserts the connections between the small scale ecosystems are often through surface water movement and through animals, often moving across large distances and using the small patches of richer vegetation in sheltered river valleys.
  There is no clear-cut end to these deserts and beginning to the Tundra that spreads across the Low Arctic. Small patches of tundra vegetation with dwarf shrub heaths or cotton grass tussocks or wet mires occur further North but they become the dominant and extensive systems in the Low Arctic, varying with the climate, underlying geology, soil conditions and slope.
  Shrub Tundra, with dwarf birch, willows and alders, bilberry, blaeberry, heaths, rhododendrum, some sedges and saxifrages often form a canopy extending 50-8- centimetres over a continuous mat of mosses and lichens. In sheltered areas the canopy may form a thicket up to 2 metres. Shrub Tundra occurs mainly in drier soils and its distribution reflects the climate. Thus it extends to 74oN in west Greenland but only to 62o on the east coast which lacks the warming influence of the Labrador Current. The plants respond to subtle changes in the physical environment and Sedge - Dwarf Shrub Tundras are identified over large areas of Russia, grading into Tussock - Dwarf Shrub Tundra with tussocks of cotton grass or sedge often on rather wetter and poorer slightly acid soils.
  Where drainage is poor because of permafrost, clay soils and / or flat land, Mires (muskeg, bog) tend to dominate large areas. These mires often have sedges as dominant plants but mixed with many other species and a strong moss and Sphagnum cover. Hummocks and pools cover much of the surface and, as with all the other types of Tundra, there is patterning resulting from freeze-thaw action. Frost boils, polygons, medallions, cracks and many other patterns occur and disrupt the vegetation. As the vegetation becomes more dense and continuous and increasingly insulate the ground surface, these phenomena become less frequent. But the power of the ice and permafrost can still alter the landscape as large mounds or even small hills (Pingos) force the surface to heights of 100 metres or so.
  As the climate eases towards the South, birch trees become more frequent, then pine, spruce and larch forming the Forest-Tundra. The ground vegetation remains similar to the more northerly Tundra but gradually gives way as the tree cover expands into the classic Taiga or Boreal Forest. Here the domination of spruce and pine shades out much of the ground cover and tends to dry the soil. It also keeps the soil cool and although the climate may be warmer, permafrost may remain in the forest whilst it disappears from open areas and is thus 'discontinuous permafrost'.
  This gradient from Polar Desert to Taiga, with its many variations and diverse patterns, is the landscape of the North. Much is written and the debates are vigorous over definitions of different plant communities, the beautiful adaptations of the animals and plants and their natural histories. But what about the System? What are the dynamics of this landscape? How does it function as an integrated Ecosystem or as a series of Ecosystems? These questions are becoming more and more important as the environment changes through climate, land use, industrial development and pollution. So let us explore some of so-called Ecosystem dynamics and function - Change over time; Food webs; Carbon and nutrient circulation; Biodiversity
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The Arctic is an Ecosystem, by Bill Heal.
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