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The Terrestrial
Ecosystem - Or Ecosystems
Definitions |
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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. |
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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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'. |
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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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