Geoscience Reference
In-Depth Information
cover, and having more than three months of pronounced drought per year. Tree
density and understory structure vary in each vegetation type. Seasonal forests in
Africa cover 12 percent of the total land area of the continent, although these for-
ested areas are rapidly decreasing. Seasonal forests are somewhat shorter, more
open, and less species rich in the northern subregion than in the south. In the East
African coastal forests, the forest is more susceptible to fire and easier to burn dur-
ing the dry season. Burning provides usable land for cultivation and most of the
seasonal forests in eastern Africa have been cleared for agriculture. The remaining
patches are effectively ''islands'' within a matrix of savanna-woodland, coastal
thicket, or farmland.
Forest Structure
The structure of African tropical seasonal forests differs from that of the Neo-
tropics. The woody vegetation of the forests regions has three layers. In the semi-
evergreen and deciduous forests, the upper layer is composed of tall trees up to 82
ft (25 m) that form a closed canopy. Lianas may grow as high as this layer. Beneath
is a loose layer of trees reaching 30-50 ft (10-15 m) in height. The understory is
composed of shrubs and vines that form a dense layer. Shade-tolerant grass species
grow sparsely on the forest floor among the leaf litter. Typical species in the north-
ern region are from the legume (Fabaceae), ebony (Ebenaceae), sopadilla (Sapota-
ceae), and soapberry (Sapindaceae) families. In the southern region, common tree
species include the families above, as well as trees in the mahogany (Meliaceae)
and cocoa/plum (Sterculiaceae) families. Several leguminous trees in the Caesalpi-
nioideae subfamily dominate the woodlands, such as miombo trees in Central and
South Africa.
Duration and intensity of rainfall and length of dry season vary throughout the
African continent, with corresponding differences in leaf fall, flowering, and fruit-
ing. For trees and shrubs, most leaf fall occurs at the start of the dry season and
coincides with the main fruiting period. Individual trees do not all lose their leaves
at one time, nor do all trees shed their leaves in unison.
Several distinctive forest types are found within this region, but they differ in
species composition or dominance. The Sudanian subregion consists of dry wood-
lands with deciduous
Terminalia, Combretum, Isoberlina,
and acacia trees reaching
15-20 ft (507 m) with tall baobabs (reaching heights of 50 ft [15 m]) towering above
them (see Figure 5.13). Like their palo borracho cousins in the Neotropics, baobabs
can be enormous with swollen trunks.
The Zambezian
Cryptosepalum
dry forests are found in Zambia. These are ever-
green forests confined to an area around the Kabompo River. These forests lie
between the rainforest of the Congo and the Zambezian woodlands and make up
the largest area of tropical evergreen forest outside the equatorial zone. The forest
shares species from both rainforest and woodland-savanna and is species rich.
The forests grow on infertile Kalahari sands with no permanent surface water.
Because of this, these dry forests have remained relatively uninhabited. The forests
