Geoscience Reference
In-Depth Information
Climate
The common climatic factor in all tropical seasonal forests is the occurrence of a
distinct dry period that can last from two to seven months. In African seasonal for-
ests, rainfall during the dry season averages less than 4 in (100 mm) per month.
During the rest of the year, rainfall is typically high, totaling 30-60 in (800-1,500
mm) or more. Rainfall totals and the duration of the dry season are the primary
determinants of seasonal forest type. In some areas, rainfall can be greater, specifi-
cally at higher elevations on the windward side of mountains. The rainy season
occurs in the summer and fall, with a cool dry season in winter followed by hot dry
season in spring. Temperatures are warm, with an average minimum of 68
-72
F
(20
-22
C) and an average maximum of 82
-92
F (28
-30
C). These areas do
not experience frost.
The global circulation pattern that heavily influences the African Seasonal For-
est Biome is the shifting Intertropical Convergence Zone (ITCZ). In Africa, a mari-
time airmass flowing southwest from the Indian Ocean meets an opposing hot, dry
continental airmass from the northeastern deserts. Where the two meet is a zone of
instability and high rainfall. These airmasses move seasonally from north to south
with the shifting ITCZ, varying from 5
Sin
July. These movements account for the seasonal distribution of rainfall in tropical
Africa. From December to March, during the dry season in the Northern Hemi-
sphere, a hot dry wind known as the Harmattan Winds blow from the Sahara De-
sert, reaching West Africa. The Southern Hemisphere experiences warm dry winds
from the Ethiopian Highlands.
-7
N latitude in January to 17
-21
Soils
Soils within the African region are varied. In some areas, soils tend to be very old
and nutrient poor, originating from ancient bedrock of the Precambrian Shield.
These oxisols are the dominant soils within the forests of the Zambezian subregion.
Other soil types include ultisols and the more fertile alfisols and inceptisols. In gen-
eral, soils of the seasonal forest are rather infertile and low in nutrients and miner-
als. This is due to the long-term processes driven by heat and moisture over
hundreds of millions of years, as well as centuries of burning. Fires in nearby sav-
annas have greatly influenced these forested landscapes and their soils. Although
the seasonal forest is not conducive to fire, uncontrolled and frequent burning has
changed many areas from dry forest to savanna.
Vegetation in the African Seasonal Forests
Tropical seasonal forests in Africa can be semi-evergreen or deciduous forests,
evergreen dry forest, or woodlands having at least 70 percent continuous tree
