Geoscience Reference
In-Depth Information
quently as you read this section. These biomes are organized
into four categories: (1) forest biomes, (2) grassland biomes,
(3) desert biomes, and (4) tundra biome. The alpine and arctic
ice biome is not discussed in detail because it simply consists
of landscapes covered by bare rock and glaciers and therefore
has no plants.
As you study the remaining 11 biomes, remember that the
boundary between vegetation communities is rarely distinct.
Instead, a transition usually occurs from one vegetation type
to another that may take place over several hundred kilome-
ters. Such a vegetation transition is called an
ecotone
because
it contains elements of two distinct plant communities. These
transitions occur because the boundaries between climate
regions are usually difficult to define precisely.
Tropical Rainforest Biome
The tropical rainforest biome
straddles the equatorial region between 23.5° N and 23.5° S
(Figure 10.6) and is very closely related to the tropical rain-
forest climate (
Af
) zones. It is also associated with small
areas of tropical monsoon climate (
Am
) that have a very lim-
ited dry season, such as in central Mexico. Average annual
precipitation in the tropical rainforest biome ranges from
180 cm to 400 cm (70.1 in. to 158 in.), with monthly rainfall
consistently greater than 6 cm (2.4 in.). These areas are al-
ways warm, with an average temperature ranging from 20°C
to 30°C (68°F to 86°F).
The tropical rainforest biome contains a staggering assort-
ment of trees and plants that form a very dense biomass. The
trees are broadleaf and experience no seasonal leaf fall because
of the consistently high temperature and abundant moisture;
thus, these trees, like the needle-leaf trees that you are prob-
ably familiar with, are
evergreen.
The rainforest also contains a
variety of plants called
lianas
and
epiphytes
. Lianas are woody
vines that use trees for support but are rooted in the ground.
Epiphytes, in contrast, are plants that are rooted in a nook of a
host plant and thus use the plant for support.
A mature tropical rainforest consists of four definable
layers that can be seen vertically (Figure 10.7). The upper-
most layer consists of randomly spaced emergent leaf crowns
that exist at the tops of the tallest trees, some of which may
be up to 60 m (200 ft) tall. Many of these trees are so large
that they have
buttress roots
(Figure 10.8) that help support
them. Directly below the emergent layer is the canopy, which
is the densest layer of the tropical rainforest and consists of a
near-continuous web of branches and leaves. This part of the
tropical rainforest is from 15 m to 40 m (50 ft to 130 ft) above
the forest floor.
Forest Biomes
Forest biomes are geographic regions where the vegetation is
dominated by trees. These vegetation assemblages occur in ar-
eas where a net surplus of available moisture occurs, either be-
cause of high average annual precipitation or because average
temperatures are sufficiently cool that little evaporation takes
place. Forest biomes occur from the tropical regions to the high
midlatitudes and can broadly be categorized as the tropical for-
est biomes, midlatitude forest biomes, and the boreal forest
biome. The tropical and midlatitude forest biomes are further
subdivided into distinct geographical zones.
Ecotone
The transition area where two or more ecosystems
merge.
Emergents
40-60 m (130-200 ft)
Canopy (densest layer)
15-40 m (50-130 ft)
Understory
5-15 m (15-50 ft)
Ground layer
0-5 m (0-15 ft)
(a)
(b)
Figure 10.7 The tropical rainforest.
(a) The layered structure of the rainforest creates a continuous canopy that densely shades the
forest floor. (b) Do you see the emergent trees in this tropical rainforest?
