Continental Patterns and Boundaries
11
11.1 Pattern Within Zones
The zones give only a broad-brush picture.
Variations within a zone break up and differentiate the major, subcontinental zones. For
example, the vegetation of the savanna is highly
differentiated related to variation in length of
the dry season (Fig. 11.1). The geographic
patterns of ecosystems within zones caused by
these variations are reviewed here; see the
author’s Ecosystem Geography (Bailey 1996 et
seq.) for details.
Within the same macroclimate, broad-scale
landforms (geology and topography) breaks up
the zonal pattern and provide a basis for further
delineation of mesoscale ecosystems, known as
landscape mosaics. The same geologic structure
in different climates results in different
landscapes. For example, limestone in a subarctic
climate occurs in depressions and shows intense
karstification, while in hot and arid climates it
occurs in marked relief with a few cave tunnels
and canyons inherited from colder Pleistocene
time (Fig. 11.2).
A landscape mosaic may be further subdivided
into microscale ecosystems called sites. Within a
landscape, the sites are arranged in a specific
pattern. For example, the Idaho Mountains, a
temperate-steppe regime highland in the western
United States, are made of a complex mosaic of
riparian, forest, and grassland sites (Fig. 11.3).
Even in areas of uniform macroclimate,
topography leads to differences in local climates
and soil conditions. Topography causes
variations in the amount of solar radiation
received, creating topoclimates (Thornthwaite
1954), and affects the soil moisture (Fig. 11.4).
Variations in drainage, and in steepness of
slopes, further affect the soil moisture and
biota, in turn creating ecosystem sites. A
sequence of moisture regimes, ranging from
drier to wetter from the top to the bottom of a
slope (Fig. 11.5), may be referred to as a soil
catena, or a toposequence (Major 1951).
Figure 11.6, in a simplified way, illustrates
how topography, even in areas of uniform
macroclimate, leads to differences in local climates
and soil conditions. The climatic climax theoretically would occur over the entire region but for
topography leading to different local climates.
Other topographic, hydrologic, geologic and/
or geochemical deviations may also occur. We
can place ecosystem sites into three basic
categories: (1) zonal, which are typical for the
climatic conditions, such as on well-drained
sagebrush terraces in a semiarid climate
(Fig. 11.7); (2) azonal such as riparian forests
and (3) intrazonal, which may occur on extreme
types of soil that override the climatic effect,
such as very dry sand dunes or black soil over
certain limestone (Fig. 11.8).
In summary, the pattern of ecosystems in a
region is the product of all these factors, some
climatic (resulting from the average state of the
atmosphere), and some edaphic (resulting from
the character of the soil and surface). In general,
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_11, # Springer Science+Media, LLC 2014
105
11
11.1 Pattern Within Zones
The zones give only a broad-brush picture.
Variations within a zone break up and differentiate the major, subcontinental zones. For
example, the vegetation of the savanna is highly
differentiated related to variation in length of
the dry season (Fig. 11.1). The geographic
patterns of ecosystems within zones caused by
these variations are reviewed here; see the
author’s Ecosystem Geography (Bailey 1996 et
seq.) for details.
Within the same macroclimate, broad-scale
landforms (geology and topography) breaks up
the zonal pattern and provide a basis for further
delineation of mesoscale ecosystems, known as
landscape mosaics. The same geologic structure
in different climates results in different
landscapes. For example, limestone in a subarctic
climate occurs in depressions and shows intense
karstification, while in hot and arid climates it
occurs in marked relief with a few cave tunnels
and canyons inherited from colder Pleistocene
time (Fig. 11.2).
A landscape mosaic may be further subdivided
into microscale ecosystems called sites. Within a
landscape, the sites are arranged in a specific
pattern. For example, the Idaho Mountains, a
temperate-steppe regime highland in the western
United States, are made of a complex mosaic of
riparian, forest, and grassland sites (Fig. 11.3).
Even in areas of uniform macroclimate,
topography leads to differences in local climates
and soil conditions. Topography causes
variations in the amount of solar radiation
received, creating topoclimates (Thornthwaite
1954), and affects the soil moisture (Fig. 11.4).
Variations in drainage, and in steepness of
slopes, further affect the soil moisture and
biota, in turn creating ecosystem sites. A
sequence of moisture regimes, ranging from
drier to wetter from the top to the bottom of a
slope (Fig. 11.5), may be referred to as a soil
catena, or a toposequence (Major 1951).
Figure 11.6, in a simplified way, illustrates
how topography, even in areas of uniform
macroclimate, leads to differences in local climates
and soil conditions. The climatic climax theoretically would occur over the entire region but for
topography leading to different local climates.
Other topographic, hydrologic, geologic and/
or geochemical deviations may also occur. We
can place ecosystem sites into three basic
categories: (1) zonal, which are typical for the
climatic conditions, such as on well-drained
sagebrush terraces in a semiarid climate
(Fig. 11.7); (2) azonal such as riparian forests
and (3) intrazonal, which may occur on extreme
types of soil that override the climatic effect,
such as very dry sand dunes or black soil over
certain limestone (Fig. 11.8).
In summary, the pattern of ecosystems in a
region is the product of all these factors, some
climatic (resulting from the average state of the
atmosphere), and some edaphic (resulting from
the character of the soil and surface). In general,
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_11, # Springer Science+Media, LLC 2014
105