Geoscience Reference
In-Depth Information
Figure 10.13 The boreal forest.
This scene is from Alaska.
Coniferous trees dot this landscape because it is near the northern
margin of this biome.
Figure 10.14 The tropical savanna.
This photograph shows
savanna in Brazil. Here, the vegetation consists of patches of
grass that are dotted by isolated trees and woody shrubs.
biome. Most of the trees in the boreal forest biome are co-
niferous needle-leaf evergreens such as spruce, pine, and fir,
with large regions covered almost entirely by one or two of
these species (Figure 10.13). These expanses of conifers are
occasionally broken by small stands of broadleaf deciduous
trees, specifically birch, poplar, and aspen. These trees are
adapted to fire in that they are the first trees to move into
a freshly burned area. Subsequently, they are succeeded by
the coniferous trees, which also include the needle-dropping
species tamarack and larch. The lower level of the boreal
forest biome is sparsely vegetated, consisting of low shrubs,
mosses, and lichens.
Although this region appears to be biologically productive,
it is not due to the short growing season and the persistently wet
soils in which the plants grow. The stress of this environment on
plant growth is best seen in the northern parts of the boreal forest
biome, where the growing season is very short and cold. Here,
the trees are quite scraggly and stunted in comparison to their
counterparts that live in the southern, warmer parts of the biome.
A good place to examine this biome is in Africa, where
tropical savanna covers about 13,000,000 km
2
(5,000,000 mi
2
)
of the continent. Recall from Chapter 9 that this region is most
directly influenced by the seasonal migration of the tropical
Hadley cell and associated ITCZ and STH pressure systems,
with a summer wet season and winter dry season. In response to
this distinct wet/dry cycle, the vegetation is a mix of forest and
grass (Figure 10.14). Forest patches are denser where the con-
ditions are more humid and thin considerably where it is drier.
During the wet season, the savanna springs to life as grasses
turn green and grow tall at the same time that leaves develop on
trees. When the dry season comes, however, the grass withers
and browns while the trees drop their leaves. A major factor that
affects the savanna biome is the occurrence of wildfires, which
are common in the dry season. These wildfires are critical to the
ecosystem because they reduce the dead vegetation and recycle
nutrients back into the system (see the Discover feature in this
chapter).
Midlatitude Grassland Biome
Midlatitude grasslands
are broad areas where the dominant vegetation is grass. The
climate of these areas is seasonal, with warm to hot sum-
mers and cool to cold winters. Average annual precipitation
ranges from 25 cm to 75 cm (10 in. to 30 in.). Several climate
regions fall within this general description, including the dry
portions of the humid continental hot-summer (
Dfa
,
Dwa
)
climates, the drier side of the humid continental mild-sum-
mer climates (
Dfb
,
Dwb
), and most of the cold midlatitude
steppe climate (
BSk
). In many cases, the relative dryness of
these climate zones is due to the continental effect in that
the regions are located far from moisture sources. In other
places, such as the interior of North America, distinct rain
shadows exist in the lee of large mountain ranges such as
the Rockies.
Grassland Biomes
Now let's turn our attention to the grassland biomes. These plant
communities are distinct because they consist largely of grass
with few trees. Two major grassland biomes are recognized:
(1) tropical savanna biome and (2) midlatitude grassland biome.
The Tropical Savanna Biome
The tropical savanna con-
sists of grasslands at low latitudes that contain isolated trees.
This biome is most closely associated with the tropical savanna
climate (
Aw
) and is present in Africa, South America, India, and
Australia (see Figure 10.6). In these regions, average annual
precipitation is from 9 cm to 150 cm (4 in. to 60 in.), and aver-
age monthly temperature is greater than 18°C (64°F).
