| Several peculiar features of the north
polar
environment combine to create a distinctive eco-region-the arctic tundra
of Eurasia
and North
America. One feature is the frozen ground, or permafrost,
which may extend to depths of hundreds of metres. Another is the weak
energy supplied by the sun, which remains below the horizon for the winter
and never rises very high in the sky, even during the summer when it does
not set. A third is the paradoxical status of much of the far north as a
cold, humid desert-a
desert because total annual precipitation
is low, but humid because so little of that moisture evaporates.
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| The tundra that forms in
response to these conditions is a low, flat to rolling, treeless plain
that is waterlogged over much of its extent during the growing season.
Once temperatures
warm sufficiently, the snow that melts tends to gather in shallow
depressions in the ground. Only the uppermost layer of soil
thaws, and the permafrost below impedes drainage. Much of the vegetation
in and around these pools and puddles consists of mosses and grasslike
sedges and rushes. On slightly higher and drier ground grow lichens, dwarf
shrubs-both evergreen
members of the heath
family and deciduous
willows and birches-and an assortment of wild flowers. Many of the flowers
have a compact "cushion" habit that resists the damaging effects
of the strong tundra winds.
In the long days of the growing season there are
enormous hatches of insects from the tundra pools, and these attract great
flocks of birds, which migrate to the Arctic
to feed and breed. The exuberant vitality of the tundra contrasts starkly
with the almost lifeless polar deserts that form in certain valleys
blown dry by summer winds coming from the continental
interior. Even more barren are the sheets of permanent ice that cover much
of Greenland.
In the south polar region, ice covers most of Antarctica
and polar deserts occupy the ice-free valleys. Only the very margin of the
continent supports plant life, and this consists almost entirely of
lichens, mosses, and algae. The vegetation of the drizzly, windswept islands
that stud the Southern
Ocean resembles that of the semi-deserts of Patagonia,
with tussock grassland
and cushion plants. |
