The Afrotropical realm is one of the Earth's eight biogeographic realms. Most of the Afrotropical realm, except for Africa's southern tip, has a tropical climate. Within Africa, there are approximately 3.5 million square kilometers of rainforests.
The Afrotropical realm ecoregions.
The forest zone, a belt of lowland tropical moist broadleaf forests, runs across most of equatorial Africa's Intertropical Convergence Zone. The Upper Guinean forests of West Africa extend along the coast from Guinea to Togo. The Dahomey Gap, a zone of forest-savanna mosaic that reaches to the coast, separates the Upper Guinean forests from the Lower Guinean forests, which extend along the Gulf of Guinea from eastern Benin through Cameroon and Gabon to the western Democratic Republic of the Congo.
Tropical African forest is 18 percent of the world's total and covers over 3.6 million square kilometers of land in West, East, and Central Africa. This total area can be subdivided to 2.69 million square kilometers (74%) in Central Africa, 680,000 square kilometers (19%) in West Africa, and 250,000 square kilometers (7%) in East Africa.
Biodiversity and Endemic Species
The tropical environment is rich in terms of biodiversity. The Afrotropic has various endemic bird families, including ostriches (Struthionidae), the secretary bird (Sagittariidae), guineafowl (Numididae), and mousebirds (Coliidae). Africa has three endemic orders of mammals, the Tubulidentata (aardvarks), Afrosoricida (tenrecs and golden moles), and Macroscelidea (elephant shrews).
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Four species of great apes (Hominidae) are endemic to Central Africa: both species of gorilla (western gorilla, Gorilla gorilla, and eastern gorilla, Gorilla beringei) and both species of chimpanzee (common chimpanzee, Pan troglodytes, and bonobo, Pan paniscus). The peculiar okapi - a kind of forest giraffe - occurs only in the north-eastern corner of the Congo Basin; the pygmy hippo is restricted to West African rainforests; and the bonobo (pygmy chimpanzee) occurs only to the south of the great Congo River.
World Heritage Sites
Within Africa's 3.5 million square kilometres of rainforests there are just five world heritage sites, covering an area of 63,000 km2. Four of these are in the Congo Basin (Dja Faunal Reserve in Cameroon; Lope National Park in Gabon; Salonga National Park and Okapi Faunal Reserve in the Democratic Republic of Congo) and the fifth (Tai National Park in Cote D'Ivoire) is in the Upper Guinea forest block. There are also three sites along the Great Rift which are primarily rainforest habitats (Bwindi-Impenetrable, Virunga and Kahuzi-Biega National Parks), but these are covered elsewhere.
Climate and Rainfall
Among rainforest areas in other continents, most of the African rainforest is comparatively dry and receives between 1600 and 2000 mm of rainfall per year. Areas receiving more rain than this mainly are in coastal areas. The circulation of rainfall throughout the year remains less than in other rainforest regions in the world. The average monthly rainfall in nearly the whole region remains under 100 mm throughout the year.
Deforestation and Conservation
The rate of deforestation in Africa is less known than the rate of other tropical regions. The cultivation of various cash crops has led to forest depletion. West African countries depend on products like gum, copal, rubber, cola nuts, and palm oil as a source of steady income. Land use change spoils entire habitats with the forests. The conversion of forests into timber is another cause of deforestation.
The African Timber Organization member countries eventually recognized the cooperation between rural people and their forest environment. Customary law gives residents the right to use trees for firewood, fell trees for construction, and collect of forest products and rights for hunting or fishing and grazing or clearing of forests for maintenance agriculture.
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Causes of tropical deforestation.
Tropical forests once covered 15-20% of earth’s land surface. About half of this is now replaced by cropland, pasture, tree plantations, secondary forest, or wasteland. We are in a race between human destruction and human discovery.
The rainforests that remain in West Africa now greatly differ in condition from their state 30 years ago. In Guinea, Liberia, and the Ivory Coast, there is almost no primary forest cover left unscathed; in Ghana, the situation is much worse, and nearly all of the rainforest is being removed. Liberia loses 800 km2 (310 sq mi) of forests each year. Extrapolating from present rates of loss, botanist Peter Raven pictures that the majority of the world's moderate and smaller rainforests (such as in Africa) could be destroyed in forty years.
Recent estimates show that the annual pace of deforestation in the region can vary from 150 km2 (58 sq mi) in Gabon to 2,900 km2 (1,100 sq mi) in Côte d'Ivoire.
The Tragedy Of Deforestation | Climate Change: The Facts | BBC Earth
The Congo Basin
The rainforests of Africa span the equatorial region for approximately 7 degrees north to south. This forest region is dominated by the Congo Basin forest system in Central Africa which harbours approximately 54% of the total area and is the second largest swathe of rainforest in the world. The Congo Basin comprises a mosaic of flooded forests, bamboo swamps and rivers, spanning 6 countries and covering an area of over 2 million square km.
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The Congo Basin accounts for 60% of Africa's remaining lowland forest, with the last primary intact tracts of forest within the Congo Basin being located in the central and northern parts of the country. These forest tracts mostly comprise a habitat of Congolian lowland and swamp forests, together with a small fraction of montane forests located in the Albertine Rift.
Lowland Forests
The lowland forests of Africa are now confined to within a nearly continuous narrow equatorial belt which extends from the Atlantic, at Basse Casamance in Senegal in the west, to its eastern limit in western Uganda. The western extension of this forest belt is interrupted by the Dahomey Gap, a band of Guinean forest-savannah mosaic, around 300 km wide which continues to the coasts of Togo, Benin and Ghana. South of the Democratic Republic of Congo (DRC) the African rainforest extends into Angola, with its southern limit being confined primarily to river valleys.
Many of these existing East African forests would previously have been connected with each other and with the Ituri Forest (Parque Nationale du Virungas) across the Semliki River in the DRC with climate variation likely being partly responsible for these changes.
Nutrient Cycling and Forest Productivity
Analysis of soils of tropical regions shows them to be virtually devoid of soluble, mineral forms of nutrients. This is supported by the analysis of stream water draining tropical regions, which likewise reveals a scarcity of dissolved nutrients. Most tropical soils are clays with little soluble mineral content, and moderate to strong acidity which interferes with the ability of roots to take up nutrients.
Figure 5 shows a budget accounting that indicates nutrients are found mainly in living plant biomass and in the layer of decomposing litter; there is little nutrient content of the deeper soil, as there is in temperate-zone ecosystems. There are many organisms that are players in this decomposition process: termites, bacteria, fungi, various invertebrates. Of particular importance are micorrhizal fungi that invade the roots of trees to obtain nourishment. As we learned in the lecture on Microbes, these fungi gain carbon nourishment from the tree and they benefit the tree by providing a vastly expanded nutrient gathering network in the soils.
For example, one study in the Amazon rain forest, which used Ca-45 and P-35, found that more than 99% of the nutrients added to the system (in the form of isotopes) were retained in root mats. More commonly, about 60 - 80% of nutrients are retained by the roots, and thus made available to the tree.
When forests are burned, or the cut timber is removed as in logging, the nutrients that were in the tree biomass are either washed out in the case of burning or simply removed from the system. Because there was only a small stock of nutrients in the soil and most of the nutrients were in the biomass, there is little nutrient stock remaining to support regrowth of the forest. This is why slash and burn agriculture does not work for more than a few years after burning, and why the land is made very infertile and growing new vegetation is difficult.
Table 1 above shows 7 tropical forests, arranged roughly from least to most fertile soils. Note the range of above-ground biomass is about twofold. Yet the nutrient stocks vary by several orders of magnitude.
The greatest above-ground productivity is in the Ivory Coast forest, on soils of intermediate fertility. Look next at the root biomass, which correlates pretty well with the soil fertility. Root biomass is highest where soil quality is poorest, and vice-versa.
Tropical forests, covering 7% of the earth’s surface area, contain perhaps 50% of the world’s species.
Ecological Specialization and Diversity
Tropical species exhibit highly specialized ecological roles. The tropical forests themselves are multi-layered. A temperate forest usually has two or three main layers. In an undisturbed forest the trees are fairly similar in height, there usually is some ground vegetation, and in between there may or may not be a shade-adapted middle layer. We also see evidence of great ecological specialization, as illustrated for mammals living in trees and on the ground that are active at day versus at night.
Historical Climate Change
In more recent times (say over the last 1-5 Ma), the forests of the tropics have gone through many cycles of fragmentation and reunion, due to the same global climate changes that gave us repeated bouts of glaciations at higher latitudes. These repeated episodes of forest retreat and advance would provide numerous opportunities for forest-dwelling species to be isolated into separate small populations. This in turn allows genetic change and evolutionary specialization to occur.
During periods of glaciation, the earth’s average temperature was cooler, and this would result in less evaporation and thus less rainfall. During wetter periods associated with interglacials (time during the glacial retreats), forests would expand and rejoin. If the dry periods correspond roughly to periods of glacial advance, they would have lasted 50 to 100 thousand years, which likely is sufficient time for evolutionary divergence.
Deforestation Drivers
Why are we rapidly converting these lush, productive, and biologically diverse ecosystems to timber production, pasture land for cattle, and agriculture? Of course part of this conversion is to meet the needs for food and land of the rapidly growing number of peasant farmers in tropical countries (small-scale agriculture, Figure 11).
Traditional slash and burn agriculture releases the nutrients held in above ground biomass to produce a high yield from a small area right after the burning. However, within 2-3 years these areas lose much of their productivity, because there are few nutrients available in the soils of tropical forests (the nutrients are in the biomass). Even if such areas are left fallow (unused) for years to decades, the productivity does not return because the nutrients have been mostly lost from the system, as described earlier in this lecture. This traditional land use is thus not “sustainable” because the majority of the land is fallow at any one time and new areas of forests must be cut and burned.
Not just nutrients are lost when the forest is removed. The water cycle itself is disrupted, and the initial consequence is increased erosion because there is no vegetation to act as a “buffer” to hold the water in the plants and soils. Another likely consequence is a long-term and irreversible decline in available water in the region.
New Study on Central African Forests
A new study maps the different forest types present in Central Africa and pinpoints which are most vulnerable to the climate crisis. They store more carbon per hectare than the Amazon and host a higher concentration of large trees than any other continent. They are also under threat.
The researchers identified 10 types of forest, according to Nature. These include Atlantic coastal evergreens in Gabon and semi-deciduous forests at the northern edge of the Central African study area.
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