Continental Types and Their Controls
4
The ecoregions on the Earth’s land masses are
arranged in predictable patterns and are causally
related to macroclimate, i.e., the climate that lies
just above the local modifying irregularities of
landform and vegetation. These macroclimates
are regularly arranged with reference to several
controlling factors.
4.1
The Controls of Macroclimate
The controls of macroclimate may be grouped
under three headings: latitude, continental position, and elevation.
4.1.1 Latitude
As mentioned previously, if the Earth had a
homogeneous surface circumferential zones of
macroclimate would result from the variation in
solar radiation and the resultant atmospheric circulation. These zones would be divided along
lines of latitude (see Fig. 1.8, p. 6).
Thermally Defined Zones The actual distribution of land and sea forms a more complicated
thermal distribution (Fig. 4.1). The thermal limits
for plant growth determine boundaries of these
zones. For example, trees in Eurasia and America
cannot grow beyond about 70
latitude.
We can delineate three major thermally
defined zones: (1) a winterless climate of low
latitude, (2) a temperate climate of mid latitudes
with both summer and winter, and (3) a
summerless climate of high latitude. In
winterless climate, no month of the year has a
mean monthly temperature lower than 18
C.
The 18
C isotherm approximates the position
of the boundary of the poleward limit of plants
characteristic of the humid tropics, such as
palms. In summerless climate, no month has a
mean monthly temperature higher than 10
C.
The 10
C isotherm closely coincides with the
northernmost limit of tree growth, separating the
regions of boreal forest (tayga
1 ) from the treeless
tundra.
If we also consider the annual and diurnal
energy cycles, we can differentiate these thermal
zones. The relative amplitudes of annual and
diurnal energy cycles vary in each region
(Fig. 4.2). Within the tropics, the diurnal range
is greater than the annual range. Within temperate zones, the annual range exceeds the diurnal
range, although the diurnal can be very large.
Within the polar zones, the annual range is far
greater than the diurnal range.
Moisture-Defined Zones Life-giving precipitation is generally higher in areas of higher temperature and, therefore, higher evaporation. It
ranges from about 200 cm in the equatorial
region to about 30 cm at the poles (Fig. 4.3).
1 In this book tayga, often spelled “taiga,” will be written
with a “y” following the Russian.
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_4, # Springer Science+Media, LLC 2014
27
4
The ecoregions on the Earth’s land masses are
arranged in predictable patterns and are causally
related to macroclimate, i.e., the climate that lies
just above the local modifying irregularities of
landform and vegetation. These macroclimates
are regularly arranged with reference to several
controlling factors.
4.1
The Controls of Macroclimate
The controls of macroclimate may be grouped
under three headings: latitude, continental position, and elevation.
4.1.1 Latitude
As mentioned previously, if the Earth had a
homogeneous surface circumferential zones of
macroclimate would result from the variation in
solar radiation and the resultant atmospheric circulation. These zones would be divided along
lines of latitude (see Fig. 1.8, p. 6).
Thermally Defined Zones The actual distribution of land and sea forms a more complicated
thermal distribution (Fig. 4.1). The thermal limits
for plant growth determine boundaries of these
zones. For example, trees in Eurasia and America
cannot grow beyond about 70
latitude.
We can delineate three major thermally
defined zones: (1) a winterless climate of low
latitude, (2) a temperate climate of mid latitudes
with both summer and winter, and (3) a
summerless climate of high latitude. In
winterless climate, no month of the year has a
mean monthly temperature lower than 18
C.
The 18
C isotherm approximates the position
of the boundary of the poleward limit of plants
characteristic of the humid tropics, such as
palms. In summerless climate, no month has a
mean monthly temperature higher than 10
C.
The 10
C isotherm closely coincides with the
northernmost limit of tree growth, separating the
regions of boreal forest (tayga
1 ) from the treeless
tundra.
If we also consider the annual and diurnal
energy cycles, we can differentiate these thermal
zones. The relative amplitudes of annual and
diurnal energy cycles vary in each region
(Fig. 4.2). Within the tropics, the diurnal range
is greater than the annual range. Within temperate zones, the annual range exceeds the diurnal
range, although the diurnal can be very large.
Within the polar zones, the annual range is far
greater than the diurnal range.
Moisture-Defined Zones Life-giving precipitation is generally higher in areas of higher temperature and, therefore, higher evaporation. It
ranges from about 200 cm in the equatorial
region to about 30 cm at the poles (Fig. 4.3).
1 In this book tayga, often spelled “taiga,” will be written
with a “y” following the Russian.
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_4, # Springer Science+Media, LLC 2014
27