with the introduction of trees. Forests were
planted around the farmsteads and villages, so
that today the buildings are all but hidden in
foliage during the summer.
In Canada and northern United States the
transition from grassland to boreal forest consists
of a narrow belt of poplar and aspen deciduous
forest. This belt is from 60 to 200 km wide.
The boundary between the prairie and the
forest is not so clearly related to climatic or
edaphic conditions. Prairies exist well within a
climate humid enough for tree growth, and where
trees are planted they grow if protected from
competition with the roots of the grasses. In
previous times, fires caused by lightning and the
grazing of big-game herds encouraged the
growth of the grasses in the treeless, wet prairies.
The prairie climate is not designated as a
separate variety in the Ko ¨ppen-Trewartha system. Geographers’ recognition of the prairie climate (Thornthwaite 1931; Borchert 1950) has
been incorporated into the system presented
here. Prairies lie on the arid western side of the
humid continental climate, extending into the
subtropical climate at lower latitudes. Temperature characteristics correspond to those of the
adjacent humid climates, forming the basis for
two types of prairies: temperate and subtropical.
Tall grasses associated with subdominant
broad-leaved herbs dominate prairie vegetation.
The grasses of the Argentine Pampa are said to
have once risen above the head of a man on
horseback. Trees and shrubs are almost totally
absent, but a few may grow as woodland patches
in valleys and other depressions. Deeply rooted
grasses form a continuous cover. They flower in
spring and early summer, the forbs in late summer. In the tall-grass prairie of Iowa, for example, typical grasses are big bluestem and little
bluestem; a typical forb is black-eyed Susan.
Because rain falls less in the grasslands than
in forest, less leaching of the soil occurs. The
pedogenic process associated with prairie vegetation is calcification, as carbonates accumulate
in the lower layers. Soils of the prairies are prairie soils (Mollisols), which have black, friable,
organic surface horizons and a high content of
bases. Grass roots deeply penetrate these soils.
Bases brought to the surface by plant growth are
released on the surface and restored to the soil,
perpetuating fertility. These soils are the most
productive of the great soil groups.
These soils are not uniform and reflect the
transitional nature of the climate. A succession
of soil types, from the humid forest margins
across the prairies and the steppes to the dry
lands, conforms to the changes in moisture and
vegetation cover (Fig. 6.10). On the rainy
margins of the prairie, a deep soil is formed
which is so abundantly supplied with organic
material that is dark-colored even in the B horizon. This is the black prairie soil (Fig. 6.11).
Near the dry margin, rainfall decreases to the
point that minerals dissolved near the surface
are carried down to the B horizon and no farther.
Two soil types share this process of having
mineral accumulations, chiefly lime, in the B
horizon. The first of these, occupying the dry
margins of the prairies, is known as the chernozem (Mollisol). The color of the chernozem is
even darker than the black prairie soil, and its
fertility is increased by the decreased effectiveness of the leaching process. The dry boundary of
the chernozem coincides with the prairie–steppe
boundary, where, because the depth of the moist
surface soil becomes less than about 60 cm, the
tall grasses give way to the short grasses. The
smaller supply of humus from the short grasses is
reflected in a change from the black color of the
chernozem to a chestnut-brown color; and the
more active evaporation and shallower penetration of the rain water result in the formation of a
continuous layer of lime salts much closer to the
surface than in the chernozem. This is the chestnut-brown soil (Mollisol).
Most of the regions of this type are either
plains or plateaus. The rainfall in the prairie is
usually sufficient to support permanent streams,
many of which are lined by galeria forests.
Most of the prairie has disappeared, replaced
by some of the richest farmland in the world
(Fig. 6.12). Some of the native animals of the
region enjoyed either an elimination of their natural enemies or an increase in the supply of food,
which made possible a sudden and large increase
in their numbers.
64
6 The Humid Temperate Ecoregions
planted around the farmsteads and villages, so
that today the buildings are all but hidden in
foliage during the summer.
In Canada and northern United States the
transition from grassland to boreal forest consists
of a narrow belt of poplar and aspen deciduous
forest. This belt is from 60 to 200 km wide.
The boundary between the prairie and the
forest is not so clearly related to climatic or
edaphic conditions. Prairies exist well within a
climate humid enough for tree growth, and where
trees are planted they grow if protected from
competition with the roots of the grasses. In
previous times, fires caused by lightning and the
grazing of big-game herds encouraged the
growth of the grasses in the treeless, wet prairies.
The prairie climate is not designated as a
separate variety in the Ko ¨ppen-Trewartha system. Geographers’ recognition of the prairie climate (Thornthwaite 1931; Borchert 1950) has
been incorporated into the system presented
here. Prairies lie on the arid western side of the
humid continental climate, extending into the
subtropical climate at lower latitudes. Temperature characteristics correspond to those of the
adjacent humid climates, forming the basis for
two types of prairies: temperate and subtropical.
Tall grasses associated with subdominant
broad-leaved herbs dominate prairie vegetation.
The grasses of the Argentine Pampa are said to
have once risen above the head of a man on
horseback. Trees and shrubs are almost totally
absent, but a few may grow as woodland patches
in valleys and other depressions. Deeply rooted
grasses form a continuous cover. They flower in
spring and early summer, the forbs in late summer. In the tall-grass prairie of Iowa, for example, typical grasses are big bluestem and little
bluestem; a typical forb is black-eyed Susan.
Because rain falls less in the grasslands than
in forest, less leaching of the soil occurs. The
pedogenic process associated with prairie vegetation is calcification, as carbonates accumulate
in the lower layers. Soils of the prairies are prairie soils (Mollisols), which have black, friable,
organic surface horizons and a high content of
bases. Grass roots deeply penetrate these soils.
Bases brought to the surface by plant growth are
released on the surface and restored to the soil,
perpetuating fertility. These soils are the most
productive of the great soil groups.
These soils are not uniform and reflect the
transitional nature of the climate. A succession
of soil types, from the humid forest margins
across the prairies and the steppes to the dry
lands, conforms to the changes in moisture and
vegetation cover (Fig. 6.10). On the rainy
margins of the prairie, a deep soil is formed
which is so abundantly supplied with organic
material that is dark-colored even in the B horizon. This is the black prairie soil (Fig. 6.11).
Near the dry margin, rainfall decreases to the
point that minerals dissolved near the surface
are carried down to the B horizon and no farther.
Two soil types share this process of having
mineral accumulations, chiefly lime, in the B
horizon. The first of these, occupying the dry
margins of the prairies, is known as the chernozem (Mollisol). The color of the chernozem is
even darker than the black prairie soil, and its
fertility is increased by the decreased effectiveness of the leaching process. The dry boundary of
the chernozem coincides with the prairie–steppe
boundary, where, because the depth of the moist
surface soil becomes less than about 60 cm, the
tall grasses give way to the short grasses. The
smaller supply of humus from the short grasses is
reflected in a change from the black color of the
chernozem to a chestnut-brown color; and the
more active evaporation and shallower penetration of the rain water result in the formation of a
continuous layer of lime salts much closer to the
surface than in the chernozem. This is the chestnut-brown soil (Mollisol).
Most of the regions of this type are either
plains or plateaus. The rainfall in the prairie is
usually sufficient to support permanent streams,
many of which are lined by galeria forests.
Most of the prairie has disappeared, replaced
by some of the richest farmland in the world
(Fig. 6.12). Some of the native animals of the
region enjoyed either an elimination of their natural enemies or an increase in the supply of food,
which made possible a sudden and large increase
in their numbers.
64
6 The Humid Temperate Ecoregions