Ecoregions of the Continents: The Polar
Ecoregions
5
5.1
100 Polar Domain
Polar and Arctic air masses chiefly control
climates of the polar domain, located at high
latitudes. With the exception of the ice cap
climates, they lie entirely in the Northern Hemisphere. In general, climates in the polar domain
have low temperatures, severe winters, and small
amounts of precipitation, most of which falls in
summer. Polar systems are dominated by a periodic fluctuation of solar energy and temperature,
in which the annual range is far greater than the
diurnal range (see Fig. 4.2, p. 29). This contrasts
with the tropics where the major periodic fluctuation is the diurnal one, and the mid-latitude
systems, as we will see, are subject to
fluctuations in both annual and diurnal energy
patterns.
The intensity of the solar radiation is never
very high compared to ecosystems of the middle
latitudes and tropics. On the poleward margins,
although summer insolation persists for many
hours, temperatures do not get very high because
the intensity is low and because much of the
energy goes to evaporate water and melt snow
or ice. More energy is given off by terrestrial
radiation than is received from solar radiation.
In order for this situation to persist, a supplementary heat source must exist to provide the difference. This supplementary energy source is heat
carried poleward by wind and water currents. It
maintains the Arctic temperatures at a level
much higher than would solar radiation alone.
The high-latitude climates have low annual
total evaporation, always less than 50 cm,
reflecting the prevailing low air and soil
temperatures. The frozen condition of the soil
in several consecutive winter months causes
plant growth to cease. Snow that falls in this
period is retained in surface storage until spring
thaw releases it for infiltration and runoff. The
growing season for crops is short in the subarctic
zone, but low air and soil temperatures are partly
compensated for by great increase in day length.
In areas where summers are short and
temperatures are generally low throughout the
year, thermal efficiency, rather than effectiveness
of precipitation, is the critical factor in plant
distribution and soil development. Three major
regional divisions have been recognized and
delimited in terms of thermal efficiency—the
icecap, tundra, and the subarctic (tayga). The
world map of the polar ecoregions of the
continents (Fig. 5.1) shows the locations of
these ecoregions. Climate diagrams in Fig. 5.2
provide general information on the character of
the climate in two of these divisions.
These climate diagrams express differences in
the climatic regime of the divisions within the
domains by comparing annual temperature and
moisture cycles. A relatively dry season is
depicted by the precipitation curve falling
below the temperature curve. The location of
the weather station and its altitude, as well as
the average annual temperature and precipitation, are shown above the graphs.
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_5, # Springer Science+Media, LLC 2014
47
Ecoregions
5
5.1
100 Polar Domain
Polar and Arctic air masses chiefly control
climates of the polar domain, located at high
latitudes. With the exception of the ice cap
climates, they lie entirely in the Northern Hemisphere. In general, climates in the polar domain
have low temperatures, severe winters, and small
amounts of precipitation, most of which falls in
summer. Polar systems are dominated by a periodic fluctuation of solar energy and temperature,
in which the annual range is far greater than the
diurnal range (see Fig. 4.2, p. 29). This contrasts
with the tropics where the major periodic fluctuation is the diurnal one, and the mid-latitude
systems, as we will see, are subject to
fluctuations in both annual and diurnal energy
patterns.
The intensity of the solar radiation is never
very high compared to ecosystems of the middle
latitudes and tropics. On the poleward margins,
although summer insolation persists for many
hours, temperatures do not get very high because
the intensity is low and because much of the
energy goes to evaporate water and melt snow
or ice. More energy is given off by terrestrial
radiation than is received from solar radiation.
In order for this situation to persist, a supplementary heat source must exist to provide the difference. This supplementary energy source is heat
carried poleward by wind and water currents. It
maintains the Arctic temperatures at a level
much higher than would solar radiation alone.
The high-latitude climates have low annual
total evaporation, always less than 50 cm,
reflecting the prevailing low air and soil
temperatures. The frozen condition of the soil
in several consecutive winter months causes
plant growth to cease. Snow that falls in this
period is retained in surface storage until spring
thaw releases it for infiltration and runoff. The
growing season for crops is short in the subarctic
zone, but low air and soil temperatures are partly
compensated for by great increase in day length.
In areas where summers are short and
temperatures are generally low throughout the
year, thermal efficiency, rather than effectiveness
of precipitation, is the critical factor in plant
distribution and soil development. Three major
regional divisions have been recognized and
delimited in terms of thermal efficiency—the
icecap, tundra, and the subarctic (tayga). The
world map of the polar ecoregions of the
continents (Fig. 5.1) shows the locations of
these ecoregions. Climate diagrams in Fig. 5.2
provide general information on the character of
the climate in two of these divisions.
These climate diagrams express differences in
the climatic regime of the divisions within the
domains by comparing annual temperature and
moisture cycles. A relatively dry season is
depicted by the precipitation curve falling
below the temperature curve. The location of
the weather station and its altitude, as well as
the average annual temperature and precipitation, are shown above the graphs.
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_5, # Springer Science+Media, LLC 2014
47