37 countries of tropical Africa have been studied, covering a total area of 21.9 million km2. Djibouti and Mauritania are the only two tropical countries of continental Africa not included in this group, which excludes the mediterranean countries of North Africa (Morocco, Algeria, Tunisia, Libya, Egypt) and the temperate countries of southern Africa (Republic of South Africa, Lesotho and Swaziland).
The 37 countries have been classified into five subgroups:
the “Northern Savanna Region” includes the six sudano-sahelian countries south of the Sahara: the Gambia, Upper Volta, Mali, Niger and Chad. Their total area is 4.24 million km2 and their population estimated at 29.6 million inhabitants in 1980;
“West Africa” includes the 9 countries of the northern coast of the Gulf of Guinea from Guinea-Bissau to Nigeria. These are: Benin, Ivory Coast, Ghana, Guinea, Guinea-Bissau, Liberia, Nigeria, Sierra Leone and Togo. Their total area is 2.12 million km2 and their population estimated at 113.8 million inhabitants in 1980;
“Central Africa” groups the 7 countries which are covered partly or totally by the large stretch of humid forest of the central western part of Africa. These countries are: Angola, Cameroon, Central African Republic, Congo, Gabon, Equatorial Guinea and Zaire. They are oriented economically towards the atlantic coast which is their almost exclusive economic outlet (with the exception of the Shaba province in Zaire, the mineral products of which are transported to the Indian Ocean). The total area of this group of countries is 5.33 million km2 and its total population estimated at 48.5 million inhabitants in 1980;
“East Africa and Madagascar” includes 13 countries which are characterized physically by the presence of the great lakes and the mountain ranges of the north-south tectonic axis of the large African fault and, economically, by their eastern orientation (the Red Sea and Indian Ocean). These countries are: Burundi, Ethiopia, Kenya, Madagascar, Malawi, Mozambique, Uganda, Rwanda, Somalia, Sudan, Tanzania, Zambia and Zimbabwe. Their total area is 8.81 million km2, with a total population estimated at 149.7 million inhabitants in 1980;
“Tropical South Africa” includes Namibia and Botswana, countries which lean economically towards the Republic of South Africa, with a total population of only 1.8 million inhabitants in 1980, and a total area of 1.4 million km2.
These 37 countries of tropical Africa represent approximately 45.5% of the total area of the 76 tropical countries covered by this study, but include only 17.8% of the population (343.5 million inhabitants).
Geologically, old siliceous or sandstone rocks are predominant in most parts of the continent. Anteprimary series are represented by granites, quartzites, conglomerates and metamorphic limestones. Manganese, gold and copper ores date from that period. Primary series are mostly dominated by sea limestones and sandstones of various eras which form the essential part of Mauritania and Sahara plateau. Continental deposits at the end of primary and the beginning of secondary era (limestones and sandstones) have their largest extension in Sahara and mostly in southern Africa. Secondary and tertiary eras have left large sea deposits, mostly limestones, in the coastal zones: Ethiopia, Senegal, Gabon, lower Congo, Angola, Mozambique, western Madagascar. Continental deposits are dominant in the core of the continent since the end of primary era with limestones, sandstones, clays and marls Climatic variations of the quarternary era have had the most important consequences in Africa and they are responsible, as much as the type of bedrock, for the present soil pattern of the continent. It is during the wet periods that the large subterranean water tables of the Sahara were constituted and during the dry periods that the large dune areas were formed. That the geological evolution of Africa is not completed is shown by the volcanic activity in eastern Africa and the earthquakes in the great lakes region.
The large extension of siliceous rocks (granites, sandstones, sands), the alternating periods of high humidity and drought, the dissolving power of water stimulated by high temperatures, the mechanical erosion increased by violence of rain storms, explain both the fragility and the low fertility of the african soils. In the equatorial and sub-equatorial zones with constant humidity, the dominant soils are light clays, generally acidic because of the extremely rapid process of organic decomposition. Where water is stagnant, humus accumulation generates black clayey soils; when it runs off, leaching of fertile particles with concentration of aluminium or iron hydroxides gives a red or yellow colour to the clays (ferrallitic soils). In tropical regions soils are leached by water in the rainy reason and alteration of crystalline rocks leads to the formation of laterites with aluminium hydrates coloured red by iron oxide. During the dry season high evaporation brings iron and aluminium to the soil surface concentrating them in a completely sterile, hard iron pan. This layer is formed not only on the surface but also at varying depths in the range between two limit levels of the water table. These totally sterile lateritic zones cover large areas in western Africa, on the plateaus on the southern Sudan region (Mali, upper Guinea, Upper Volta). All soils in tropical regions are indeed threatened by laterization under the combined influence of the alternating humid and dry seasons, and of bush fires which destroy the productive vegetation cover and facilitate soil hardening and erosion. In the northern Sudan zone recent deposits in basins (Niger, Senegal) contain much sand and little clay. These light soils are easy to work, fragile and of low fertility. Only more clayey soils accumulated in the depressions show good agricultural potential. Around the tropics, mechanical erosion is more important than chemical alteration and soils are incomplete, extremely poor in humus and infertile. Deserts are characterized by the complete absence of vegetal soil. In the intertropical zone, the only actually fertile soils are those of the volcanic areas: hills and plateaus of Nigeria, Cameroon, eastern Africa and eastern Madagascar.
The massive character and the existence of plateaus and large internal depressions, limited by high peripheral mountains are two important physical characteristics of the African continent. Plains show generally two facets: either as depressions filled with river deposits (bassins of Niger, Chad, upper Nile, Congo, Kalahari depression), or as large plain areas gently undulating and uniform. At high altitudes these areas appear as plateaus. On these tabular zones, generally made of old rocks, residual crystalline or gneissic mountains, sometimes crop up. Coastal plains, generally narrow, are sometimes interrupted by high mountains dominating littoral areas or widening in gulfs or marine plains.
High mountains are of various types. Between the large internal basins which correspond to the depressed parts of the old African shale, massive ranges occur: the Fouta Djalon mountains, the Guinean spine, the Adamaoua range, the Crystal mountains (Equatorial Guinea, Gabon, Congo) which introduce high limits between littoral zones and the basins of Niger, Chad and western Congo. In eastern Africa high undulating plateaus, up to 2 000 m elevation, occur and dominate depressions. They constitute most of the landscape of Tanzania, Uganda, Kenya and Ethiopia. Volcanic ranges rise along the main faults of the continent, mostly in eastern Africa and on the border between western and central Africa. Kilimandjaro is the highest point of Africa (5 895 m), preceding mount Kenya (5 202 m). Mount Cameroon (4 070 m) is the highest point of western Africa and the Tibesti mountains, also volcanic, is the most important mountain of Sahara (3 415 m).
In this system of domes and depressions, high littoral mountain ranges have also a definite influence on hydrography: rivers form in the external ranges and flow towards the interior basins. If they are insufficiently fed they disappear in the bottom of the basins (53% of the continental land area have no exit to the sea). If they are abundantly supplied they spread during the inundations into swamps or huge internal deltas in the most depressed areas of the basins (e.g. Niger and Senegal). To emerge from the depression where their intermediate portion stagnates, the rivers must again traverse peripheral ranges in order to reach the sea. Most of them are thus cut by two main sets of falls on their upper and lower portions. The important African rivers constitute therefore, a poor access to the internal part of the continent and a poor link between the regions which they cross. Although they are only partially navigable, they are, nowadays, important economically as most of them are powerful and have an enormous potential for electricity generation as well as constituting water reserves, which can be used in regions where insufficient rainfall makes cropping hazardous. The most powerful African river is the Congo (or Zaire) with a 80 000 m3 per second flow at its mounth, the longest is the Nile (6 500 km), the most important are the Niger in western Africa and, in southern Africa, the Zambeze. River flow regime depends on the rainy season, each climatic zone having its own hydrography. The most important rivers generally have a complex regime which changes from the source to the mouth. Some rivers have, however, a simpler pattern: the Senegal, Volta, Gambia, Logone and Chari rivers have a typically tropical regime.
The massive character of Africa, its general structure of internal basins and tectonic depressions, the high rainfall and water supply in equatorial and tropical zones and the difficult water flow to the sea, are responsible for the existence of large lake areas. The widest and deepest ones are found in east Africa. In the large faults of the Rift Valleys there are a certain number of lakes of which Tanganyika lake is second in the world for its depth (1 435 m). Lake Victoria located between the two lines of the lakes of the Rift Valleys is the largest in Africa (83 000 km2). On the border between western and central Africa, lake Chad has an average area of 14 000 km2, which varies much from one season to another in relation to water supply, and a depth not more than 2 metres. Located in the Sahelian region, it would disappear quickly were it not supplied by the Logone and Chari rivers. During the last 15 years or so, in the development of the potential of the large valleys, dams have been built up constituting reservoirs, sometimes of a considerable area: lake Volta in Ghana (9 000 km2), lake Kariba in Zimbabwe, lake kaingi in Nigeria and more recently lake Kossou in Ivory Coast.
Africa is a hot continent. Located between latitudes 37° N and 34° S, it lies, for its most part, within the intertropical zone and is confined to subtropical and mediterranean areas at its most northern and southern tips. The equator crosses it in the middle.
In the intertropical and subtropical Africa, climates are regulated by the apparent move of the sun between the two tropics in the course of the year and by the corresponding movement of the important atmospheric activity centres, equatorial minima and subtropical high pressures. This mechanism is responsible for the climatic zones occurring as strips approximately parallel to the equator: equatorial zone, northern and southern tropical zones, northern and southern desertic zones. This arrangement is particularly conspicuous in estern Africa, whereas it disappears for various reasons in the eastern and southern parts of the continent. However the topographic pattern, orientation of the coasts, distance from the sea, cold or warm sea currents and altitude, are responsible for important climatic variations.
In the intertropical part of Africa, rainfall distribution, rather than temperature, is responsible for the seasons. It follows the intertropical front, with a 200 to 300 km difference. The zone between latitudes 3° S and 8° N, located in the area of humid equatorial air, receives heavy rainfall during the whole year, dry seasons occuring only when the sun is at its extreme distance. However, in the northern part of the Congo basin, where the strict equatorial climate dominates, it is rainy throughout the year. Between latitudes 8° and 18° N and latitudes 3° and 15° S, rainfall distribution becomes tropical and divides the year into two seasons. The rainy season corresponds to the occurrence of the inter-tropical front and occurs in summer in the northern hemisphere, and in winter in the southern hemisphere. The rainy season becomes shorter as the distance from the equator increases and that to the tropics decreases in the following succession: humid tropical climate, typical tropical or Sudanian climate and Sahelian or subdesertic climate. The zone of continuous drought without rainy season starts at latitude 20° N in the northern hemisphere. In western Africa for instance Guinean littoral areas receive an average of 1 400 to 1 800 mm of annual rainfall, while Sudanian areas located around latitude 15° N do not receive more than 500–700 mm; in the Sahelian zone rainfall is lower than 500 mm, while 200 mm indicate the southern limit of the desert.
Temperatures are high throughout the year all over Africa, except in the high mountains. In the intertropical zone annual averages are never below 20° C. The annual temperature range is small and generally less than the daily one. It increases with latitude, that is with air dryness. The highest annual averages are found in the internal parts of Africa. They exceed 30° C in the southern Sahara.
Warm sea currents increase tropical humidity in the coastal areas while on the contrary cold currents accentuate droughts and desertic conditions (Mauritania, northern Senegal and Angola).
High mountains exposed to rainy winds receive much more rainfall than the surrounding lowlands. Examples are the eastern slopes of the Malagasy plateau, the southern part of the Fouta Djalon range and of the Guinean spine, and the Jos plateau. In eastern Africa very high mountains cross the equatorial zone: they have a lower average temperature and the tops of the mountains are relatively dry as the southeastern wet winds have released their rain over the lower slopes.
On the whole, Africa has a fairly uniform climate over large areas, particularly in the intertropical zone where alternating humid and dry seasons, determined by an annual shift of air masses on both sides of the equator, are characteristic of an unchanging seasonal pattern.
Although Africa is the continent with the highest population growth rate in the world, it remains particularly underpopulated. Moreover populations are distributed most unevenly. In Africa, more than anywhere else, people are very dependent on climatic and soil conditions: in this respect, two environments are particularly adverse that is, the equatorial forest and and desert. Equatorial forests only allow a population scattered in clearings. However, it must be noted that infrastructure development (roads, railways and logging tracks) has made the penetration of the forest areas possible in all coastal areas and particularly in western Africa. Deserts have only a sparse nomadic population, in a permanent search for grazing lands and water points for their animals.
Savannas and steppes have a higher population density which does not, however, exceed 8 inhabitants per km2 on the average and varies a lot according to regions. Savannas provide man with the best resources and feed the most important groups, despite a short rainy season reduced to a few months when farmers have to concentrate their efforts and carry out the most demanding and difficult tasks. Furthermore endemic diseases are particularly serious during a most trying humid and hot season.
In western Africa for instance, it is in the northern Sudan savannas, between latitudes 12° and 15° N that highest agricultural densities are found: more than 50 inhabitants per km2 in northern Nigeria, in the Mossi region of Upper Volta and in the Serer region of western Senegal, more than 20 inhabitants per km2 in the Niger valley. On the contrary, wooded savannas of the southern Sudan belt, between latitudes 9° and 12° N are thinly populated: forest galeries shelter glossines and water courses shelter simulies which transmit onchocercose. However, in this zone, mountainous areas constitute centres of high population density (more than 50 inhabitants per km2), having served as refuges, as is the case of the high plateaus of Cameroon, Nigeria, Togo and Ghana.
High population densities in the Haoussa region of northern Nigeria and in the Mossi region of Upper Volta, where natural environment is by no means exceptional, are related in part to the existence of ancient civilizations which developed on the basis of varied activities and strong political and social structures.
The borders of tropical regions are either almost empty when they constitute the transition to desert, or very densely populated. Sahelian steppes of western Africa have less than 5 inhabitants per km2 between the valley of the Senegal river and lake Chad, and even much less in the east of the lake. The most dense groups live in the coastal areas from Senegal to Nigeria, as well as on the eastern coast of Madagascar, despite its insalubrity. Africa north of the equator has more than two thirds of the total population of the continent. Population nuclei are generally peripheral except that of northern Nigeria and the Ethiopian plateaus. In western Africa, for instance, where the largest population of the continent is concentrated (150 million inhabitants), coastal countries have five times more inhabitants than the ones of the hinterlands in an area twice as small. South of the equator the region of the great lakes constitute the main population centre of the tropical part, with 30 million inhabitants.
Despite generally low population levels (15 inhabitants per km2) population growth is high through the rapid increase of birth rate and the significant decrease of the death rate (46 and 23‰ respectively i.e. both highest in the world). All African regions without exception, are subject to many migrations, sometimes old, very varied with regard to their extension in space, their rhythm, their duration and modalities. They generally start from areas with insufficient resources and a fairly closed economy towards richer areas more open to trade economy and where monetary gains more and more needed by the rural population are possible. The migrating people are not only farmers but also shepherds and traders. Most of them move to the towns. Accelerated urbanisation has been one of the most characteristic features of Africa for the last 25 years, harbours, mining and industrial centres being the most attractive place (harbours of the coasts of the Gulf of Guinea and of eastern Africa, mining centres of Zaire, Zambia, Ghana and Mauritania). The attraction exerted by towns on young generations, is a common feature: in Gabon, for instance, urbanisation rate is higher than 30%. Population increase in some coastal towns reaches commonly 6 to 7% per year, that is almost a doubling of the population every ten years. Dakar thus increased from 214 000 inhabitants in 1955 to 457 000 in 1965 and 798 000 in 1976. 25% of the Ivory Coast population and around 30% of the Senegalese live in towns. But there is still no more than 10% of urban population in Mali and Mauritania, 6% in Upper Volta and 5% in Niger.
The 37 countries of tropical Africa covered by this study had, in 1980, a total population of 343.5 million inhabitants increasing at an average annual rate of 2.95%. The agricultural population is around 242.3 million inhabitants increasing at an average annual rate of 2.09%. It is in the “east Africa and Madagascar” region that the agricultural population is increasing most rapidly (annual rate of 2.23%) whereas the rate is 2% in the sahelian region and slightly less in the other regions. Among the world's three main tropical regions, Africa has the highest annual net increment of agricultural population. This rate is 1.53% in tropical Asia and only 1.14% in tropical America.
1.1 Natural woody vegetation
a) Until 1950 descriptions of African vegetation were made mostly by:
taxonomists, on one hand, trying to define types of vegetation associations from known botanical surveys (which they had carried out themselves in most cases), within geographically limited territories (countries or groups of countries under the same colonial administration). They had worked out few regional syntheses and practically no inter-regional ones;
geographers, on the other hand, more interested by the physiognomic aspect of vegetation and, at the same time, more concerned by the climatic characteristics to be in a position to detect eco-floristic differences within the extremely complex tropical vegetation which would have allowed for a sound definition of vegetation types.
The compilation of the whole knowledge acquired was most difficult because of the large diversity of the African vegetation and the localized knowledge of the various geographers, taxonomists and foresters. Another difficulty originated from language, as many terms used did not have a precise definition with their translation into other languages proving often unsatisfactory (such as the English words “bush” and “scrub”). Much confusion prevailed in the nomenclature of African vegetation and it was impossible to compile a general vegetation map understood by all for lack of a universally adopted definition of vegetation types.
b) On the initiative of the Scientific Council of Africa South of the Sahara (C.S.A.) a body of the Commission for Technical Cooperation in Africa South of the Sahara (C.C.T.A.), a meeting of specialists in African phytogeography was held at Yangambi (Zaire) in 1956. Its main purpose was to propose a nomenclature adapted to the vegetation of tropical Africa and useful for the description and mapping of African vegetation. The end result was the so-called “Yangambi classification” which was used in particular for the “Vegetation Map of Africa South of the Tropic of Cancer”, published in 1959 with the assistance of Unesco on behalf of “l'Association pour l'Etude Taxonomique de la Flore d'Afrique tropicale” (AETFAT).
This classification is based on the physiognomic aspect of vegetation, which has often an ecological meaning. Moreover, account was taken, to a large extent, of established practices concerning some names of formations, such as savanna and steppe, even if these concepts do not correspond to the same vegetation types as in other continents.
Two main categories were singled out: closed forest formations and mixed forest-grassland formations. The presence of a continuous grass layer allowing for the spreading of fires, distinguishes the latter from the former whatever the specific composition. The first category is subdivided into climax and edaphic formations;
forests at low and medium altitudes: moist forest (“forêt dense humide”) evergreen or semi-deciduous, dry deciduous forest (“forêt dense sèche”) and thicket (“fourré”);
forests at high altitudes: moist montane forest (“forêt dense humide de montagne”), dry montane forest (“forêt dense sèche de montagne”) and bamboo forest (“forêt de bambous”);
edaphic formations: mangrove, swamp forest (“forêt marécageuse”), periodic swamp forest (“forêt périodiquement inondée”) and riparian forest (“forêt ripicole”).
Mixed forest-grassland formation were divided in:
woodland (“forêt claire”);
savanna woodland (“savane boisée”);
tree savanna (“savane arborée”);
shrub savanna (“savane arbustive”).
When perennial herbaceous vegetation is replaced to a large extent by annual plants generally less than 80 cm high, the word steppe is used with the following denominations: wooded steppe (very rare), tree steppe (“steppe arborée”), shrub steppe (“steppe arbustive”), dwarf shrub steppe (“steppe buissonnante”), and succulent steppe (“steppe succulente”).
Though this simple classification does not reflect the whole diversity of existing formations, in particular of those of southern tropical Africa, it is, however, quite logical and allows for an easy dialogue between specialists. It is a bilingual classification which permits the description of the vegetation of tropical Africa, an important step forward insofar as it removes the complete confusion which prevailed concerning the denominations used. The Yangambi meeting recommended the compilation of an extensive atlas of the main vegetation types which would contain a series of diagrams, schematic structural representations, photographs and botanical surveys. Unfortunately this atlas was never drafted, which explains in particular the difficult definitions of the transition types which are many in the mixed forest-grassland formations. This classification was used for the 1/10 000 000 scale map published in 1959 on behalf of AETFAT. Explanatory notes were added to this map which describe the various types of forest and other vegetation south of the equator. There were contributions from many experts, in particular: A. Aubréville, P. Duvigneaud, A.C. Hoyle, R.W.J. Keay, F.A. Mendonça and R.E.G. Pichi-Sermolli. The map was compiled from 1/10 000 000 maps produced for the various countries on the basis of existing information. The explanatory notes were drafted by R.W.J. Keay and translated into French by A. Aubréville. The map “aims at showing the vegetation as it is and not the presumed climax types”. The name chosen for a zone “refers to the most widespread natural or semi-natural types found within the zone”, which assumes the existence of associated types. “Where individual units of the regional ecosystem are large enough to map separately, this has been done”. Small components of catenas due to minor variations in relief, as well as areas of cultivation and various stages of forest regrowth, are not shown.
The map shows the following tropical woody vegetation types:
montane evergreen forests: this is a forest of Juniperus and Podocarpus with Olea, Ocotea, Schefflera and Pittosporum;
moist forest at low and medium altitudes: this is a high forest of mixed broadleaved species which presents an evergreen and semi-deciduous facies;
forest-savanna mosaic: zone where patches of moist forest (not confined to stream sides) are not surrounded by savanna of tall grasses. These zones are bordering areas of closed forests of western Africa, of the Cameroon-Congolese block and of Madagascar;
coastal forest-savanna mosaic: zones resembling the preceding ones “except that the forest patches and the savanna grasses tend to be less tall”. The total extent is limited (Ghana, coast of Kenya and Tanzania);
dry deciduous forest: two types are represented on the map: (i) formations with Baikiaea plurijuga present in Zambia, Zimbabwe and Botswana; they seem to be intermediate between the Brachystegia and the Acacia-Commiphora wooded steppes and “are sometimes regarded as woodland rather than forest”; (ii) formations with Adansonia are present on the western coast of Madagascar;
undifferentiated relatively moist woodlands with dominance of the genera Isoberlinia, Brachystegia and Julbernardia: herbs are very dense and tall and fires are severe. North of the equator they form a belt between the forest-savanna mosaic and the Isoberlinia woodlands. They extend up to the north of lake Victoria and even to Kenya. South of the equator they are limited to the northwestern region of Angola, bordering the moist closed forests;
northern woodlands with Isoberlinia doka and I. dalzielii: they form a belt north of those of the previous type and occur from Mali and northeastern Guinea to south-western Sudan;
southeastern woodlands with abundant Brachystegia and Julbernardia are commonly known as miombo. They occupy the western and southern parts of Tanzania, the southern coast of Mozambique, Malawi, the southern tip of Zaire, eastern Zambia and northern Zimbabwe;
southwestern woodlands: similar to the preceding type that include a number of distinctive species such as: Marquesia acuminata, Cryptosepalum pseudotaxus and Guibourtia coleosperma. They occupy the most part of central and eastern Angola and western Zambia;
undifferentiated dry types of wooded and tree savannas: a considerable number of floristic types are included in this category, most of which are located between the preceding types and the wooded steppe with Acacia and Commiphora. These forests are scattered south of the equator from the Angolan coast to that of Mozambique and from the north of lake Victoria to northern Botswana. North of the equator they constitute a continuous belt from Senegal to lake Victoria occupying the intermediate zone of Mali, the southeast of Niger and northern Upper Volta, northern Nigeria, southern Chad, intermediate part of Sudan and northern Uganda;
Ethiopian types: these woody savannas are constituted by an heterogeneous mix of low deciduous trees and shrubs of which Boswellia papyrifera is a characteristic species. These often merge into the Oxytenanthera bamboo thickets; they are found in western Ethiopia and in the eastern tip of Sudan;
woodlands with abundant Colophospermum mopane: often occurring in ill-drained valleys of large rivers, they are 15 metres or more high but stands are often lower or stunted. They occur between latitudes 15° and 25° S from southern Angola to southwestern Mozambique covering northern Namibia and northern Botswana, southern Zambia and southern Zimbabwe;
wooded steppes with abundant Acacia and Commiphora: “vegetation classified under this heading cover large tracts between the desert and subdesert types on the one side and moister woodland types on the other. The appearance of the vegetation depends on the relative abundance of the trees and shrubs. In some places the trees… form open or closed woodland and thickets; in other places the trees are widely scattered”. This zone covers a large strip in northern Senegal, southern Mauritania, the central part of Mali, Niger, Chad and Sudan, eastern Ethiopia, Somalia, northeastern and southeastern Kenya and northeastern Tanzania. Vegetation of western Botswana can be related to this category;
thickets: certain types of dense vegetation composed of evergreen or deciduous shrubs are classified in this category. Three types of thickets are mapped: “Itigi” thickets in central Tanzania, with a height reaching 5 metres and with Pseudoprosopis and Baphia burtii as dominant species, xerophytic thickets with Euphorbia and Didierea in southwestern Madagascar and montane evergreen thickets of eastern Africa and Ethiopia with Acokanthera schimperi and Buxus hildebrandtii.
c) In 1966 the World Forest Atlas (Weltforstatlas) was published at Reinbeck under the direction of Professor C. Wiebecke containing a map of “vegetation forms” of Africa at 1/15 000 000 scale. This very simple map gives clearly and accurately the limits of broad vegetation types of Africa. Limits of closed forests, woodlands, wooded and tree savannas are clearly defined as well as those of montane formations and of mangroves. Its use is limited only by its scale. The legend separates the following woody formations in the tropical zones:
moist and dry closed forest (“Tropischer Feucht und Trockenwald”);
tropical montane forest (“Tropischer Gebirgswald”);
areas of degraded closed forests (“Degradierter oder Devastierter Wald”);
swamp forests of Congo and Oubangui valleys (“Sumpfwald”);
dry thickets of Ethiopia and southwestern Madagascar (“Trockenbush”);
savannas with Colophospermum mopane;
shrub savannas (“Strauchsavanne”);
wooded steppes (“Baum-und Strauchsteppe”).
d) In 1976 Prof. J. Schmithüsen from Sarrebrück University published a biogeographic atlas which contains a vegetation map of Africa at 1/25 000 000. This map is mainly based on humidity and dryness criteria. It distinguished humid closed forest, dry closed forest, humid and dry savannas. The region of the southern woodlands is called “tropical deciduous dry forest with evergreen understorey”. Montane vegetation of eastern Africa shows up very clearly. Madagascar is represented by a very detailed map at 1/5 000 000 where the various vegetation facies are well singled out.
e) Finally there is under publication a vegetation map of Africa at 1/5 000 000 compiled by Prof. F. White on behalf of AETFAT with support from Unesco. It distinguishes 80 vegetation types and mosaics (compared to 33 for the 1/10 000 000 scale map of 1959). Forest formations are relatively less differentiated, in particular the humid types. On the contrary grasslands, shrublands and arid and semi-arid types, are abundantly detailed as well as forest-grassland mosaics.
This map appears less forest oriented than the former one, despite its larger scale. Western Africa is not analyzed in great detail and the generalizations for this part of the continent do, only to a small extent, reflect the physiognomic diversity of forest formations.
An estimation of the areas of the various vegetation types and mosaics was made on the draft of this map for the 37 countries covered by this study. Results are summarized in the following table:
Areas of vegetation types and mosaics for the 37 countries of tropical Africa according
to Unesco/AETFAT vegetation map of Africa at 1/5 000 000 scale
(compiled by F. White)
|Map code||Denomination Unesco/AETFAT map||Areas|
|in thousand ha||%|
|1||Lowland rain forest: wetter types (a) Guineo-Congolian (b) Malagasy||115 810||5.29|
|2||Guineo-Congolian rain forest: drier types||70 915||3.24|
|3||Mosaic of 1a and 2||16 000||0.73|
|4||Transitional rain forest||1 950||0.09|
|5||Malagasy moist montane forest||5 780||0.26|
|6||Zambezian dry evergreen forest||4 165||0.19|
|7||Malagasy dry deciduous forest||5 245||0.24|
|8||Swamp forest||26 685||1.22|
|9||Mosaic of 8 and 1a||20 500||0.94|
|Sub-total forests||267 050||12.20|
|Forest transitions and mosaics|
|11||Mosaic of lowland rain forest and secondary grassland (a) Guineo-Congolian (b) Malagasy||206 015||9.41|
|12||Mosaic of lowland rain forest, Isoberlinia woodland and secondary grassland||7 460||0.34|
|13||Mosaic of lowland rain forest, secondary grassland and montane elements||1 750||0.08|
|14||Mosaic of lowland rain forest, Zambezian dry evergreen forest and secondary grassland||4 640||0.21|
|15||West African coastal mosaic||740||0.03|
|16||East African coastal mosaic (a) Zanzibar-Inhambane (b) forest patches (c) Tongoland-Pondoland||41 135||1.88|
|17||Cultivation and secondary grassland replacing upland and montane African forest||2 285||0.10|
|18||Cultivation and secondary grassland replacing upland and montane Malagasy forest||15 395||0.70|
|19||Undifferentiated montane vegetation (a) afromontane (b) Sahelomontane (c) Malagasy J = Juniperus procera forest, M = mixed forest||49 605||2.27|
|21||Mosaic of Zambezian dry evergreen forest and wetter miombo woodland||33 540||1.53|
|22||Mosaic of dry deciduous forest and secondary grassland (a) Zambezian (b) Malagasy||53 775||2.46|
|Sub-total forest transitions and mosaics||416 340||19.01|
|25||Wetter Zambezian miombo woodland (dominated by Brachystegia Julbernardia and Isoberlinia||112 450||5.14|
|26||Drier Zambezian miombo woodland (dominated by Brachystegia and Julbernardia||87 585||4.00|
|27||Sudanian woodland with abundant Isoberlinia||118 030||5.39|
|28||Colophospermum mopane woodland and scrub woodland||54 245||2.48|
|29||Undifferentiated woodland (a) Sudanian (b) Ethiopian (c) North Zambezian (d) South Zambezian (e) transition to Tongaland-Pondoland bushland||177 650||8.11|
|30||Sudanian undifferentiated woodland with islands of Isoberlinia||31 120||1.42|
|Sub-total woodland||581 080||26.54|
|Woodland mosaics and transisions|
|32||Jos Plateau mosaic||875||0.04|
|33||Mandara Plateau mosaic||740||0.04|
|34||Transition from South African scrub woodland to Highveld grassland||7 060||0.32|
|35||Transition from undifferentiated woodland to Acacia deciduous bushland and wooded grassland. (a) Zambezian (b) Ethiopian (c) The Windhoek Mountains||43 555||1.99|
|36||Transition from Colophospermum mopane scrub woodland to Karoo-Namib shrubland||6 050||0.28|
|Sub-total woodland mosaics and transitions||58 280||2.67|
|37||Acacia polyacantha secondary wooded grassland||1 615||0.07|
|Bushland and thicket|
|38||Evergreen and semi-evergreen bushland and thicket: East Africa||24 200||1.11|
|40||Deciduous Itigi thicket||605||0.03|
|41||Deciduous Malagasy thicket||4 235||0.19|
|42||Somalia-Masai Acacia-Commiphora deciduous bush and thickets||143 370||6.55|
|43||Sahel Acacia wooded grassland and deciduous bushland||81 600||3.73|
|44||Kalahari Acacia wooded grassland and deciduous bushland||33 205||1.52|
|Sub-total bushland and thicket||287 215||13.13|
|Bushland and thicket mosaics|
|45||Mosaic of East African evergreen bushland and secondary Acacia wooded grassland||14 790||0.68|
|46||Mosaic of Malagasy deciduous thicket and secondary grassland||2 890||0.13|
|47||Mosaic of Brachystegia bakerana thicket and edaphic grassland||15 395||0.70|
|Sub-total bushland and thicket mosaics||33 075||1.51|
|49||Transition from Mediterranean Argania scrubland to succulent semi-desert shrubland||605||0.03|
|51||Bushy Karoo-Namib shrubland||23 795||1.09|
|54||Semi-desert grassland and shrubland: (a) Northern Sahel (b) Somalia-Masai||157 890||7.21|
|56||The Kalahari/Karoo-Namib transition||12 975||0.59|
|Sub-total semi-desert vegetation||194 660||8.89|
|59||Edaphic grassland on volcanic soils||1 950||0.09|
|60||Edaphic and secondary grassland on Kalahari Sand||18 150||0.83|
|61||Edaphic grassland in the Upper Nile basin||14 250||0.65|
|Sub-total grassland||34 350||1.57|
|Edaphic grassland mosaics|
|62||With Acacia wooded grassland||18 150||0.83|
|63||With communities of Acacia and broadleaved trees||8 805||0.40|
|64||With semi-equatic vegetation||17 815||0.81|
|Sub-total edaphic grassland mosaics||44 770||2.04|
|65||In tropical Africa||2 690||0.12|
|66||In South Africa||135||0.01|
|Sub-total altimontane vegetation||2 825||0.13|
|Desert (67–73: Sahara; 74: Namib)|
|67||Absolute desert||99 345||4.54|
|68||Coastal desert (a) Atlantic (b) Red Sea||2 015||0.09|
|69||Desert dunes without perennial vegetation||20 435||0.93|
|70||Desert dunes with perennial vegetation||14 790||0.68|
|71||Regs, hamadas, wadis||74 610||3.40|
|72||Saharomontane vegetation||1 145||0.05|
|74||The Namib desert||16 200||0.74|
|Sub-total desert||228 540||10.43|
|75||Herbaceous swamp and aquatic vegetation||6 115||0.28|
|76||Halophytic vegetation||3 965||0.18|
|Sub-total azonal vegetation||20 295||0.92|
|Inland waters||18 821||0.86|
|Total 37 countries||2 189 531||100.00|
1.1.1 Description of the vegetation types
Closed broadleaved forests (NHC)
a) Humid tropical forests on dry land (predominantly of mixed broadleaved trees). The Guinean region (as defined by Chevalier and Emberger, and Lebrun) or Guineo-Congolese region (Monod, Troupin) includes humid closed forests and Savannas (generally with gallery forests) which are considered as derived from the former. Climax vegetation is consisting of humid forests which can be separated into several types according to their humidity and characteristic species.
Tropical moist forest in Africa constitutes two main blocks, separated by a savanna zone reaching the Gulf of Guinea:
the Cameroon-Congolese block in the east, from the Gulf of Guinea to the region of the eastern great lakes between latitudes 6o N and 7o S, covers the south of Nigeria, Cameroon and Central African Republic, the whole of Equatorial Guinea and Gabon, the southern and northern parts of Congo, the northern part of Zaire and Angola. The northern limit with savanna, is fairly sharp while in the south the forest extends in the form of long galleries towards the savanna zone;
the western block, much smaller, covers the southern part of Ghana, Ivory Coast, Liberia and Sierra Leone and part of upper Guinea; in addition, remnants of dryer forest types with a poorer flora, cover part of Guinea-Bissau up to the southern part of Senegal (Casamance) to reach latitude 12° N. On the whole, the northern border of the block which reaches latitude 9° N in Guinea is higher in latitude than that of the Cameroon-congolese block, due to oceanic influence.
From a phytogeographic viewpoint the following regions have been distinguished (Troupin, Schnell):
the western forest region (from Casamance to Nigeria), subdivided between:
the ombrophilous district with a climax of moist evergreen forest (mostly rainy areas with a very short dry season);
the mesophilous district, with a climax of semi-deciduous forests;
the montane district with a particular facies;
the derived savanna district with remnants of semi-deciduous forest, characterized by the absence of some species such as Chrysophyllum perpulchrun;
the Senegambia district with small remnants of semi-deciduous forest with an impoverished flora;
the wet montane district (above 1 000 m) characterized by the abundance of Parinari excelsa which becomes dominant above 1 500 m;
the Cameroon-Congolese block with three districts:
the (fragmented) southeastern coastal block.
Climatic conditions corresponding to the occurrence of the moist forest are an annual rainfall of more than 1 350 mm and a dry season not exceeding 2 to 3 months (Aubréville). However, in lower Guinea where rainfall is more than 3 000 mm closed forests left are in the form of remnants (Babadou valley) but the dry season lasts 5 months. In Gabon the length of the dry season (more than 6 months in some areas), is compensated for by a high relative humidity and a minimum of evaporation because of a permanent cloud cover during the dry season.
The western block is less rich floristically than the Cameroon-Congolese one. It includes:
the evergreen forests which occupy the southernmost regions, incline towards the south in the eastern part of Ivory Coast and then extend north in the east to reach the southern part of upper Guinea. Among the representative species the following ones can be mentioned: Lophira alata, Turraeanthus africana, Tarrietia utilis, Uapaca guineensis, U. esculenta. The following types are differentiated: forests with Mapania (a species of undergrowth) occurring on shale soils (associated with Khaya ivorensis which has now almost disappeared); forests with Turraeanthus on sandy soils with little clay;
the semi-deciduous forests occupy areas with less rainfall intermingled with avergreen forests to form a transition zone where local factors (valleys, plateaus) play a determining role in the distribution of both types. In the north they mingle in mosaics with wooded savannas and extend in the form of gallery forests. Terminalia ivorensis, T. superba, Triplochiton scleroxylon and several species of Celtis, Aningeria altissima, Entandophragma angolense, E. cylindricum, E. utile, Mansonia altissima, Pterygota macrocarpa occur in this forest but being light-demanding many of them tend to colonize secondary growth of evergreen forests. Several types can be distinguished:
in Ivory Coast: the most humid forests with Chrysophyllum perpulchrum, the average ones with Celtis, the less humid ones with Khaya grandifolia and Bombax buonopozense;
in Ghana: Celtis-Triplochiton scleroxylon forest and less humid forests with Antiaris africana and Chlorophora excelsa;
in Guinea Bissau “sub-humid closed forests” of the southeastern part of the country with Afzelia africana, Ceiba pentandra and Chlorophora excelsa and the “semi-dry forests” with Khaya senegalensis, Parinari excelsa and Antiaris africana (also in Casamance).
In the Cameroon-Congolese block it is possible to separate, on the one hand, the central forest district with essentially closed evergreen forests, intermingled with semi-deciduous ones and, on the other hand, surrounding areas where semi-deciduous forests are predominant and are prolonged by gallery forests. They include many secondary forest areas with frequent dominance of Triplochiton scleroxylon.
Evergreen forests of the central part of the Congo basin are dominated by Brachystegia laurentii. Around the basin are Gilbertiodendron dewevrei forests (Zaire, Congo, Cameroon, Gabon, Central African Republic). This species does not extend beyond latitudes 5° N and 5° S and its limits closely follow river courses. Julbernardia seretii is often found in association with Gilbertiodendron, the two species almost exclusively constituting the medium and upper storeys.
Semi-deciduous forests are characterized by a mixture of species dominated by Celtis spp., Chrysophyllum perpulchrum, Antiaris welwitschii, Canarium schweinfurthii, Entandophragma cylindricum, E. angolense, Guarea cedrata, C. thompsonii. They are particularly rich in commercial species in southeastern Cameroon, in Central African Republic and in northern Congo.
The Biafran forest (according to Letouzey) is a Cameroonian variant of the Cross River basin with a very high rainfall, constant throughout the year without dry season. Caeselpiniaceae are most abundant.
The Aucoumea klaineana forest of Gabon presents various forms. Along the coast, Aucoumea is associated with Sacoglottis gabonensis; inland it is associated with Scyphocephalium ochocoa (central forest of Gabon); finally it disappears completely in the northeastern region where Celtis, Terminalia superba and Triplochiton scleroxylon are the dominant species.
The Mayombe forest (in mountainous areas around 600 m elevation in southern Congo, lower Zaire and in the Angolan province of Cabinda) is partly semi-deciduous. Most important commercial species of the upper storey are Gossweilerodendron balsamiferum and Terminalia superba.
Patches of closed forest south of the Cameroon-Congolese block are scattered within drier types of vegetation and their occurrence is often linked either with the short distance from the coast and the humid sea winds (western Angola), or with deep valleys in a north-south direction (Shaba plateau in Zaire), or with elevation (fringing mountains of the southern tropical African plateau: Malawi, Tanzania, Zimbabwe-Mozambique border and eastern Zambia). Forests of western Angola are semi-deciduous. Ceiba pentandra and Khaya anthotheca are amongst the frequently occurring species. Further south of Angola, semideciduous forests are also found in altitude with Celtis spp. and Chlorophora excelsa. Forests of lower Shaba constitute a transition type between the Guinean and Sudano-Zambezian zones. Montane forests of southern tropical Africa are semi-evergreen. Their altitude varies between 1 700 to 2 600 m. Ocotea usambarensis, Entandrophragma excelsum, Podocarpus usambarensis, P. milanjianus are among the most important species of these forests which have a somewhat similar species composition to that of the Guinean-Congolese forests, characterized in particular by the occurrence of Podocarpus at high altitude (like in the high mountains of eastern Africa) while Widdringtonia spp., typical of the southern mountains, extend rather far to the north. Heavily encroached by man, they are often found in the form of remnant patches.
Eastern closed forests include several types. The lake Victoria belt forests are semideciduous forests with Celtis spp.; the forests of the southern tip of Sudan occur in the form of galleries and remnants on the border with Uganda and are also semi-deciduous, with Celtis spp.; montane forests of Kenya, Tanzania, Uganda, Rwanda and Burundi are stratified according to altitude: at lower altitudes they are composed almost exclusively of broadleaved species dominated by Ocotea usambarensis, at intermediate ones they present a mixture of broadleaved species dominated by Olea africana and of coniferous species (Juniperus procera and Podocarpus spp.), at medium elevations they contain bamboo stands (Arundinaria alpina), particularly abundant in Kenya but absent in Tanzania, and finally, higher elevations are covered by a stunted forest dominated by Rapanea rhododendroides. The forest remnants of the eastern coastal region of Kenya and Tanzania include species common with the Cameroon-Congolese forests and “vicariant” 1 ones such as Mesogyne spp., of which one is endemic in Tanzania and the other in Sao Tome island. These similarities can be explained by the continuity in the old times between these forests and the Cameroon-Congolese block.
Moist forests of Madagascar on the eastern coast of the island contain more than 150 different species, none being dominant. Trees are often badly shaped and do not exceed 30 m in height. A stratification can be made according to altitude with the following types: coastal forest with Afzelia bijuga and Calophyllum inophyllum, forest on the hills and cliffs reaching 800 m with Canarium and Dalbergia replacing gradually the species of the coastal forest, forests at higher elevations, which are the most important ones in the country, and finally the plateau forest between 800 and 1 300 m, of which only patches remain. The effect of man's interference on the African moist forest can be presented schematically in three phases:
the pre-agricultural phase, still represented by the pygmies: man earns his living essentially by gathering food and hunting. He has practically no destructive action on the ecosystem and the vegetation;
the ancient agricultural phase: farming populations (Bantous) clear the forest to establish subsistence crops. They respect however precisely defined fallow periods and generally affect only a limited part of the forest areas except when their density is high;
the modern agricultural phase: the development of plantations of export crops particularly (coffee, cocoa, bananas, oil palm, etc.) and the population growth are responsible for an increased pressure on forests which are made more accessible by the opening of logging roads. Large tracts are thus cleared and replaced by secondary low thickets and young forests which cannot evolve towards the reconstitution of the original forests because of the short fallow period.
These three phases, presented in an historical sequence, all exist in fact nowadays. The most important aspect is the shifting nature of agriculture with the abandoning of the subsistence crops after 2 to 3 years, and the consequent considerable wastage since a relatively small population can clear large tracts of forests. To this problem, related to the shortlived natural fertility of tropical soils after clearing, must be added the recent propensity for land ownership leading to a piecemeal destruction of forest areas. It seems as if every farmer tries to settle as far as possible from his neighbours, in order to be in a position to extend his “ownership” in the future (see paragraph 2.1.1).
Secondary growth evolves gradually from herbaceous and shrubby types to thickets and forests of increasing height. The various stages of this evolution can be presented according to the following schematic succession (Aubréville, Lebrun, Adam):
herbaceous and low shrubby vegetation with tree seedlings;
dense thicket with light-demanding shrubs and young trees: Trema guineensis, Macaranga hurifolia, Alchornea cordifolia and the most common “umbrella tree” (Musanga cecropioides);
young secondary forest, where the young light-demanding fast-growing trees (Terminalia superba, Triplochiton scleroxylon, Ricinodendron heudelotii, Pycnanthus angolensis) dominate and gradually eliminate the low light-demanding species of the previous stage. The undergrowth still receives much light and is dense and has abundant shrubs and creepers;
high secondary forest where the more or less discontinuous canopy is closing gradually, with a still very thick undergrowth. Less light-demanding species appear in the undergrowth but, although this forest evolves towards a facies of original forest (if it is not cleared again) its specific composition may be altered. All depends on the occurrence in the neighbourhood of seed bearers of the species of the original forest. Some protected species may be favoured in relation to others, such as Chlorophora excelsa and Parinari excelsa. On the other hand, those which have been logged for their timber may disappear almost completely (e.g. Entandophragma spp. and Khaya ivorensis). Human impact, which has been important for long in Africa, and the frequent clearings in secondary forests of varied age often give to African closed forests the pattern of an “astonishing patchwork” (Lebrun and Gilbert), which makes the distinction of floristic associations in these forests very difficult.
b) Edaphic closed forests
In western Africa important swamp forests occur in upper Guinea, Liberia, Ivory Coast and Ghana. These forests are dominated by Symphonia globulifera and Mitragyna ciliata. In the Congo basin swamp forests extend over huge areas (Zaire, Congo and Cameroon). The similarity of these swamp forests with those of western Africa is noticeable. In eastern Africa swamp forests exist at medium altitude (1 000 m) around lake Victoria. All these swamp forests bear some similarity with those of the Amazon basin and the Guyanas where Symphonia globulifera also predominates. Accompanying species are different but like in Africa, ferns and Marantaceae are abundant in the undergrowth.
Riparian forests found in the Congo basin as well as in Ivory Coast and in upper Guinea have a similar floristic composition, with Cathormion altissimum and Cynometra schlechteri as characteristic species. In the estuaries these forests gradually merge into mangrove, mixing with Rhizophora racemosa and Pterocarpus santalinoides.
Seasonally inundated forests of the Congo basin (Zaire and Congo), on land submerged 3 to 4 months per year, cover huge areas. Particular species are Oubanguia africana, Scytopetalum pierreanum and especially Guibourtia demeusei.
Mangroves are found on the western coast of Africa from the Senegal river to the northern part of Angola. Their total area is relatively limited compared with the American and Asian mangroves. Large estuaries and deltas allow for their relatively large extension in Gambia and Sebegal, in Guinea Bissau, in Guinea and mostly in Nigeria and Cameroon. In Gabon and Angola their area is limited but they are almost inexistent in Liberia, Ivory Coast and Ghana. There is some floristic similarity with the mangroves of the atlantic coast of tropical America and of the Caribbean islands. Rhizophora racemosa is the dominating species, often associated with R. mangle whereas R. harrisonii, Avicennia nitida, Laguncularia racemosa, Conocarpus erectus, Annona palustris, Pandanus spp., may be present. These mangroves never develop as much as those of America and, particularly, Asia. Their maximum height is rarely more than 15 m and their utilization is very limited.
On the eastern coast of Africa mangroves are even more reduced. In Kenya and Tanzania their development is limited to some estuaries and to Zanzibar island. Rhizophora mucronata and Ceriops tagal are the two main species. In Mozambique their area is larger but the stands have a small size. In Madagascar they occupy some spots of the northeastern and northwestern coast. Avicennia marina, Rhizophora mucronata, Bruguiera cylindrica, Ceriops tagal, Lumnitzera racemosa, are the most important species; a close floristic similarity exists with Asian mangroves.
c) Dry closed forests
This denomination has been applied to various vegetation types differing in their structure and in their history (climax, or more or less secondary or degraded facies), as well as by their location within phytogeographic units. The term is limited to those forests without a continuous grass layer. They are often likely to be primary forests since they have not suffered from destruction by man and fire.
North of the Guinean region, in the Soudanian zone, patches of dry forest can be found, with Anogeissus and Albizia, or with Parkia and Pterocarpus erinaceus, or with Isoberlinia doka and Uapaca somon. Sometimes they may not be relics, since it can be assumed that forest cover was reconstituted thanks to the likely interruption of disturbance by man and the action of fire. They occur in Senegal, Mali, Upper Volta, Ivory Coast, Central African Republic and southern Sudan. Many species belonging to the flora of the moist forests can be found in these patches.
South of the Guinean region, the Zambezian zone contains remnants of dry forest, known in Shaba as “muhulu”, within the savannas and woodlands. These forests correspond seemingly to the remnants of an original climax forest. Their extension is very limited. Erythrophleum suaveolens, Diospyros koyleana, Anthocleista squamata can be quoted among the species.
Finally, a dry closed forest exists on the eastern coast of Madagascar with Hildegardia erythrosiphon as main species.
1 i.e. species of the same genus occurring in similar ecological conditions in different regions.
Open broadleaved forests (NHc/NHO)
They are constituted essentially of the mixed forest-grassland formations in which the tree stratum is more or less open and the grass layer continuous. Woodlands (“forêts claires”) with fairly close crowns are separated from wooded and tree savannas. It is sometimes difficult to clearly distinguish between these formations. Woodlands and wooded savannas in particular, are not easy to distinguish separately on aerial photographs or satellite images.
a) Woodlands and wooded savannas
They are given several names: forest savannas, tropophilous forests etc. The herbaceous layer is floristically different from that of the tree savannas. They occur both in the Sudanian and Zambezian regions and, in both cases are separated from closed forests by a savanna zone. South of the equator they form large forest areas, while in the north they constitute scattered patches in the savannas. Essentially they contain Leguminoseae tree species, many of them gregarious. Northern and southern woodlands have a certain number of common genera with “vicariant” species.
Woodlands north of the equator are essentially forests with Isoberlinia doka, Uapaca togoensis and Monotes or Anogeissus and Boswellia. Total tree height is not more than 15 m.
Woodlands south of the equator are richer floristically and constitute more varied groups. Two main categories can be differentiated:
woodlands with Brachystegia, Isoberlinia, Parinari and Uapaca, located between 1 000 and 1 800 m altitude. They are often referred as “miombo” or “miombo woodland”. They cover large tracts in southeastern and eastern Angola, southern Zaire (Shaba), and Zambia, central Zimbabwe, southern Tanzania and Malawi, northern Malawi and Mozambique;
Colophospermum mopane woodlands located at lower altitudes. They are found more south, in southern Angola, northern Namibia and Botswana, in the periphery of Zimbabwe and in western Mozambique.
The 1959 version of the AETFAT map provides a breakdown of these various types of woodlands. In Angola, woodlands have a large extension; three main types occur: Brachystegia spiciformis and Copaifera bauniana associations, Brachystegia longifolia and Syzygium guineense associations, and Brachystegia tamarindoides and Monotes loandensis associations. They often reach 20 m in height and are sometimes so dense and the herbaceous undergrowth so sparse that fires cannot spread. In Shaba, woodlands are even higher, sometimes reaching 30 m; crowns are touching but the light cover allows for enough illumination to reach the ground for the development of the grass layer; Julbernardia paniculata and Brachystegia spiciformis are dominant everywhere but several types can be distinguished: with Berlinia giorgii and Marquesia macrousa (transition with closed forests), with Brachystegia and finally with Guibourtia coleosperma and Copaifera bauniana on the Kalahari sands. In Zambia and Malawi woodlands occupy depressions, Brachystegia microphylla is predominant on deep soils, whereas B. utilis occupies rocky outcrops; miombo is often less developed and less high than in Angola and Shaba. In Zimbabwe Brachystegia spiciformis is dominant at medium altitudes (1 600 to 1 800 m) and is replaced at lower altitudes by Brachystegia boehmii. Man has had a strong degradation impact on the woodlands of this country.
b) Tree savannas
They can be schematically divided in two groups: tree savannas on the periphery of humid forest areas and those corresponding to the “thinned” forms of woodlands and woody savannas of southern Africa. The first ones occupy the Sudanian zone, evolving towards shrub types to the north, and the belt of lake Victoria, central Zaire and northeastern Angola. The latter ones are disseminated in Shaba, in southern Tanzania, in Zambia, in Mozambique, in Zimbabwe, in southern Angola and in northern Botswana.
These savannas are essentially grassland formations maintained in this state by fires, without which vegetation would evolve towards more wooded formations, woodlands and even dry closed forests. Various authors conclude that they are generally secondary formations deriving from a degradation of forest types destroyed by man and fire. But even in this case, it is possible to assume that these savannas are very old as is the impact of man and fire on vegetation, a fact confirmed by the adaptation of animals to the environment (giraffes, herbivores). The floristic composition of tree savannas, varies with climate, soil and man's interference.
The Sudanian zone extends from Senegal to Sudan and tree savannas are characterized by the occurrence of Vitellaria paradoxa and Uapaca togoensis, whereas in the north the abundance of Combretum and Acacia is an indication of the proximity of the Sahelian zone. Most frequent tree species are: Bombax costatum, Anogeissus leiocarpus, Pterocarpus erinaceus, Parkia biglobosa, Daniellia oliveri, Khaya senegalensis, Lophira lanceolata, Detarium senegalense.
Tree savannas of central Zaire are characterized by Gramineae such as Hyparrhenia, and by Acacia polyacantha. Acacia albida and A. sieberiana are also found. Other savannas are characterized by the ramineae Themeda triandra and Andropogon spp., associated with Combretum and Acacia.
Tree savannas of northeastern Angola are dominated by Adansonia digitata, Sterculia setigera, Euphorbia conspicua, Acacia welwitschii, Guibourtia gossweileri.
Finally, tree savannas of the zone of Zambezian woodlands have the same composition as these latter of which they constitute a degraded form.
Coniferous forests (NS)
They are found in northeastern Africa: Sudan, Somalia, Kenya and Ethiopia. Two main types of coniferous forests can be differentiated:
Juniperus procera forest, which in old times occupied the whole high Ethiopian plateau, at altitudes often more than 2 500 m and up to 3 200 m. This forest, largely destroyed by man, still exists in Ethiopia on relatively large areas, as well as in Somalia and Sudan. In Kenya it occurs very seldom. Juniperus is often found in pure stands, but may also be mixed with Podocarpus gracilior and P. milanjianus. At lower altitudes Olea chrysophylla, Pygeum africanum, Pittosporum abyssinicum are often disseminated;
Podocarpus forest occupies generally lower altitudes (1 500 to 2 600 m). According to areas some Podocarpus are dominant: Podocarpus gracilior in Ethiopia, P. milanjianus in Kenya and Sudan. Podocarpus stands may be pure or mixed with Olea hochstetteri, Syzygium spp., Ocotea usambarensis, etc. To the south (south-western Kenya, Tanzania, Uganda, Rwanda, Burundi) Podocarpus trees represent a lower proportion of the canopy of mixed broadleaved forests, which have been classified in this study as predominantly broadleaved forests (NHC).
Scrub formations (nH)
Formations considered under this denomination in this study, are those which have more that 10% crown cover of woody species between 50 cm and 5 to 8 m in height, being understood that they may contain scattered trees. Because of this physiognomic definition, it is most difficult to differentiate between these corresponding formations especially in view of their close mixture with tree savannas and of the lack of information related to the cover percentage of stunted savannas and steppes. However, a list of the vegetation types included in this category can be made.
a) Montane formations
They occur above 2 500 m in the mountains of eastern Africa (Burundi, Malawi, Rwanda Uganda, Madagascar). They consist of sclerophyllous scrubs such as arborescent ferns and many species of heath (Ericaceae). They are found also on the hills of western Africa (Sierra Leone) where Kotschya ochreata dominates. In Guinea they have been almost completely destoyed by man.
b) Coastal formations
They occur in particular along the coast of some countries of western Africa (Senegal, Guinea-Bissau, Liberia, Ivory Coast, Ghana). Chrysobalanus spp., and Combretum micranthum are the most frequent.
c) Subdesertic climax formations
They correspond to a shrub or tree steppe dominated by Acacia. They occur in all subdesertic zones of the Sahelo-Sudanian region, south of the Sahara, up to the eastern horn of Africa (Somalia, Ethiopia and Kenya).
d) Dwarf scrub formations of Kalahari (Botswana, Namibia)
Acacia spp., Boscia albitrunca, Rhigozum brevispinosum and Terminalia sericea are the dominant species in these types which extend over large areas.
e) Degraded forms of tree savannas
It is these formations that are the most difficult to classify with regard to the criteria used to define scrub formations in this study. Crown cover percentage is particularly difficult to assess. Two regions are particularly concerned: the Sudanian zone south of the Sahara (Senegal, Mali, Niger, Chad, Nigeria, Sudan) and the Zambezian zone in southeastern Africa (Mozambique, Malawi, Zambia) where these formations correspond generally to degraded forms of the Colophospermum mopane tree savanna.
f) Thickets of the plateaus of eastern Africa
Called “itigi”, they are found mostly in Tanzania and Zambia. Bussea, Baphia, Euphorbia and Burttia are the main genera.
1.1.2 Present situation of the woody vegetation
Present areas (tables 1)
The following tables summarize the main area estimates at country level derived from existing information and documents (see chapter 2). All estimates refer to the end of 1980.
● Table 1a: Areas of closed broadleaved forests
In these tables areas of humid and dry closed broadleaved forests are combined. Dry forests are not extensive in the countries of western Africa. In Ivory Coast their area has been estimated at 465 000 ha and classified in NHCf2i (closed broadleaved forests unproductive for physical reasons) since they correspond generally to sacred groves where logging is prohibited. In Guinea-Bissau and Senegal they are called “semi-dry” forests. Their occurrence in southern Mali and Upper Volta is likely but because of their reduced area they have been included in open formations (NHc/NHO). In northern Togo and Benin some patches of deciduous dry forests occur on the slopes of the plateaus.
Dry forests with Acacia of Ethiopia and Sudan cover relatively large areas. Patches of dry forest of very limited extension can be found in Malawi, Tanzania, Mozambique, Zimbabwe and Zambia. The low sclerophyllous forests of the western slopes of the eastern range of Madagascar have almost completely disappeared. As a whole the dry forests cover a total area probably close to 2 million ha. They are most threatened by clearings and fires which are gnawing away their fringes.
Areas of humid closed broadleaved forests include also mangroves which have all been classified as closed forests unproductive for physical reasons (NHCf2i) and cover some 3.4 million ha.
The productive closed broadleaved forests (NHCf1) are almost exclusively humid closed forests of mixed broadleaved species. Central Africa contains 85% of them with 49.5% for Zaire alone. These percentages are even higher, if they refer only to undisturbed forests (NHCf1uv): 94.5% and 67.5% for central Africa and Zaire respectively.
Closed broadleaved forests where logging is not permitted (NHCf2r) - essentially those included in national parks-represent only 4.25% of the total area of closed broadleaved forests, of which there is 2.65 % for Zaire alone; this latter country is an exception in central Africa were practically no significant areas of closed forests have been earmarked in national parks or integral reserves.
Total area estimates for closed broadleaved forests is slightly higher than estimates made previously (198 million ha by Lanly/Clement). The 16 million ha difference derives mostly from:
the inclusion to a larger extent of dry forests within the open formations in the first study, whereas in this study 2 million ha of dry forests have been identified;
the account made in this study of gallery forests and patches of closed forests in the savanna zone (around 6 to 7 million ha approx.);
forest patches of small areas (10 to 100 ha) scattered in the cleared zones of western Africa in particular (3 to 4 million ha);
an underestimation of mangroves and other unproductive forests in the first study (e.g. Raphia stands in the first study).