Chapter 12. Africa: ecological zones

Figure 12-1. Africa: ecological zones

Figure 12-1 shows the ecological zones of Africa, as identified and mapped by FRA 2000. Table 12-1 contains area statistics for the zones by subregion, and Table 12-2 indicates the proportion of forest in each zone by subregion.

TROPICAL RAIN FOREST

This zone covers the central part of Africa on both sides of the equator as well as the southeastern coast. The climate is more or less tropical. Rainfall ranges from 1 000 mm to more than 2 000 mm per year. If there is a dry season, it does not exceed three to four months and always occurs in winter. Temperature is always high, generally more than 20°C, except in the mountains.

The greater part of the zone was formerly covered with rain forests and swamp forests. Today, little undisturbed rain forest remains and secondary grassland and various stages of forest regrowth are extensive. Compared to the rain forests of South America and Asia, African forests are relatively poor floristically.

The most extensive formation is the Guineo-Congolian lowland rain forest, concentrated in the Congo Basin. It is a tall, dense forest, more than 30 m high with emergents up to 60 m and several strata. Some species are deciduous but the forest as a whole is evergreen or semi-evergreen. The large trees include Entandrophragma spp., Guarea cedrata, Guarea thompsonii, Lovoa trichilioides, Maranthes glabra, Parkia bicolor, Pericopsis elata and Petersianthus macrocarpus. Small patches of moist evergreen and semi-evergreen rain forest occur with a single dominant, usually Brachystegia laurentii, Cynometra alexandri, Gilbertiodendron dewevrei, Julbernardia seretii or Michelsonia microphylla, all Leguminosae.

Table 12-1. Africa: extent of ecological zones

Subregion	Total area of ecological zone (million ha)
	Tropical						Subtropical					Temperate					Boreal			Polar
	Rain forest	Moist	Dry	Shrub	Desert	Mountain	Humid	Dry	Steppe	Desert	Mountain	Oceanic	Continental	Steppe	Desert	Mountain	Coniferous	Tundra	Mountain
Central Africa	291	112	13	1		19
East Africa	21	68	79	268	103	76
North Africa					497	20		26	48		11
Southern Africa	26	187	192	106	76	22	8	8			31
West Africa	70	106	86	226	222	10
Total Africa	409	473	370	601	898	147	8	35	48		42
TOTAL WORLD	1468	1117	755	839	1192	459	471	156	491	674	490	182	726	593	552	729	865	407	632	564

Note: Data derived from an overlay of FRA 2000 global maps of forest cover and ecological zones. The subregion Africa - small islands is not included in the table because of incomplete information.

Table 12-2. Africa: proportion of forest by ecological zone

Subregion	Forest area as proportion of ecological zone area (percent)
	Tropical						Subtropical					Temperate					Boreal			Polar
	Rain forest	Moist	Dry	Shrub	Desert	Mountain	Humid	Dry	Steppe	Desert	Mountain	Oceanic	Continental	Steppe	Desert	Mountain	Coniferous	Tundra	Mountain
Central Africa	65	44	74			23
East Africa	6	15	32	5		9
North Africa								23			7
Southern Africa	34	28	42	7		15	16	7			3
West Africa	47	35	74	1		6
Total Africa	57	31	48	4		11	16	19			4
TOTAL WORLD	69	31	64	7	0	26	31	45	9	2	20	25	34	4	1	26	66	26	50	2

Note: Data derived from an overlay of FRA 2000 global maps of forest cover and ecological zones. The subregion Africa - small islands is not included in the table because of incomplete information.

The drier periphery of the zone has transitional forest types. In West Africa these evergreen or semi-evergreen forests include Afzelia africana, Aningeria altissima, Aningeria robusta, Chrysophyllum perpulchrum, Cola gigantea, Khaya grandifolia and Mansonia altissima. Other important species are Triplochiton scleroxylon, Celtis mildbraedii, Holoptelea grandis, Sterculia spp., Trilepisium madagascariense and Chlorophora excelsa.

Mangroves extend along the muddy, sheltered coasts of the Gulf of Guinea from Angola to Senegal. They include Rhizophora racemosa, Rhizophora harrisonii , Rhizophora mangle, Avicennia africana, Avicennia nitida, Laguncularia racemosa and Acrostichum aureum.

TROPICAL MOIST DECIDUOUS FOREST

This zone lies on the Great African Plateau to the south of the Guineo-Congolian Basin, mostly at an altitude of 900 to 1 000 m but in some places up to 1 500 m, as well as along the southeastern coast of Africa and in the central part of Madagascar. The dry season is always pronounced, lasting up to six months. There is a single rainy season, in summer, but there is pronounced regional variation. Annual rainfall for the zone varies between 800 and 1 500 mm, but can reach 2 000 mm locally.

Dry evergreen forest is widely distributed on Kalahari sands, featuring species of Marquesia, Berlinia and Laurea. Semi-evergreen forest of the Guineo-Congolian type is mainly confined to Angola. On the eastern coastal plain, forest is the climax but has been largely replaced by wooded grassland and cultivation.

Everywhere else the most characteristic vegetation is woodland - wetter Zambezian miombo woodland to the south and Sudanian woodland to the north. Zambezian woodlands are characterized by several species of Brachystegia (B. floribunda, B. glaberrima, B. taxifolia, B. wangermeeana, B. spiciformis, B. longifolia, B. utilis) with canopy heights sometimes up to 30 m. Associated species include Marquesia macroura, Pterocarpus spp., Julbernardia spp. and Isoberlinia spp. Sudanian woodlands, generally lower, are characterized by several species of Acacia and by Isoberlinia doka. Other characteristic species include Acacia dudgeoni, Acacia gourmaensis, Antidesma venosum, Faurea saligna, Lophira lanceolata, Maprounea africana, Maranthes polyandra, Monotes kerstingii, Ochna afzelii, Ochna schweinfurthiana, Protea madiensis, Terminalia glaucescens and Uapaca togoensis.

In Madagascar, the primary vegetation is a dry deciduous forest or thicket, but the most extensive vegetation is now secondary grassland. Nevertheless, some areas of forest remain, especially along the coast, with Dalbergia spp. on lateritic soils; Tamarindus indica on sandy soils; and Adansonia spp. and Bathiaea sp. on calcareous plateaus.

Mangroves occur along sheltered coasts of the Indian Ocean, dominated by Rhizophora mucronata, Avicennia marina and Sonneratia alba. Other tree and shrub species include Ceriops tagal, Bruguiera gymnorrhiza and Xylocarpus obovatus.

TROPICAL DRY FOREST

Farther from the equator and the wet southeastern coast, rainfall decreases and the dry season is always long six to seven months. Rainfall varies between 500 and 1 000 mm. Temperature is always high, with the mean temperature of the coldest month about 20°C. Similar conditions are found in Ghana (Accra) and Angola (Cabinda).

Woodland is the predominant vegetation type under these drier conditions. In the Zambezian region there is drier miombo, mopane (Colophospermum mopane) woodland or Sudanian woodland in the southern valleys and depressions and scrub woodland in the southern lowlands with Acacia caffra, Acacia davyi and Acacia luederitzii. In the Sudanian region, woodland species include Acacia albida, Acacia macrostachya and Acacia nilotica. In the Sudan, woodland species typically include Anogeissus leiocarpus and various species of Combretum. Where cultivation is possible, most of the land is bush fallow. Near Accra, Ghana, some patches of dry semi-evergreen forest with Diospyros abyssinica and Millettia thonningii remain. In Cabinda, Angola, the prevalent vegetation is wooded grassland with Adansonia digitata and many individuals of Anacardium occidentale and Mangifera indica, two introduced trees. A conspicuous tree of this zone is the baobab (Adansonia digitata) with its bizarre big trunk.

TROPICAL SHRUBLAND

In the Sahelian zone, the Kalahari and the southwestern part of Madagascar, rainfall becomes lower while temperatures are still high. Rainfall is always less than 1 000 mm and reaches scarcely 200 mm in the drier parts. The mean temperature of the coldest month is generally more than 20°C, except in the Kalahari where temperatures are lower (to 10°C). Even though Somalia lies across the Equator the climate is semi-arid to arid, with annual rainfall between 400 and 750 mm and very high temperatures.

In these very dry areas, spontaneous vegetation is generally pseudo-steppe, scrub woodland or thicket. In the Sahelian zone, wooded grassland (mainly with Anogeissus and Acacia species) is located in the south and semi-desert grassland in the north. Somalia has predominately deciduous shrubland and thicket with Acacia and Commiphora species. In the Kalahari, stunted scrub woodland with acacia (Acacia karroo) and shrub pseudo-steppe forms the landscape. In Madagascar, some dry deciduous forest still occurs in the northern part of the zone but the most characteristic vegetation type in the western part is deciduous thicket with Didiereaceae.

TROPICAL MOUNTAIN SYSTEMS

The main mountain systems are the Cameroon highlands, the mountains of Kenya, the Kivu ridge and the Ethiopian highlands. Some lower and isolated hills occur, such as the Fouta Djalon, Jos and Mandara Plateaus in West Africa, Hoggar in the Sahara and Windhoek Mountain in southern Africa. Madagascar has a high central range.

The climate is similar to that of the surrounding lowlands but with lower temperatures and, often, higher rainfall. Above 800 to 1 200 m, temperature decreases and vegetation changes, defining submontane, montane and high-elevation ecofloristic zones.

The vegetation is extremely diverse and varies with climate. On most mountains the lowermost vegetation is forest. Between the lowland forest and the rather different (in physiognomy and flora) montane forest, there is a submontane transition zone. In many places, however, fire and cultivation have destroyed the vegetation of this transition zone. Montane forest, generally above 1 500 to 2 000 m, is lower in structure than lowland and submontane forests. At the upper part of the montane level is an Ericaceous belt followed, above 3 000 m, by alpine vegetation.

In western Africa, on the Kivu ridge or the wetter slopes of the Ethiopian highlands and East African mountains, the trees of the upper stratum are 25 to 45 m tall with middle and lower layers. Characteristic species include Aningeria adolfi-fredrici, Chrysophyllum gorungosanum, Cola greenwayi, Diospyros abyssinica, Drypetes gerrardii, Olea capensis, Podocarpus latifolius, Prunus africana, Syzigium guineense subsp. afromontanum and Xymalos monospora.

Bamboo (Arundinaria alpina) forest or thicket occurs between 2 300 and 3 000 m on most of the high mountains in East Africa and sporadically on some of the mountains of Cameroon.

In Madagascar, the original vegetation in the mountains was moist montane forest with Tambourissa and Weinmannia species, sclerophyllous montane forest with Dicoryphe and Tina species on the eastern slopes and drier, "tapia" (Uapaca bojeri) forest on the western slopes. These forests have been replaced over extensive areas by secondary grassland.

In other areas shrubland and thicket is the prevalent vegetation.

SUBTROPICAL HUMID FOREST

This zone is restricted to a narrow zone along the east coast of southern Africa, roughly between 25° and 34°S. It has moderately high and well-distributed rainfall and, except in the extreme south, is frost free. Annual rainfall is 800 to 1 200 mm and the mean temperature of the coldest month is 7° to 15°C. Mean annual temperatures diminish from 22°C in the north to 17°C in the south. Further inland, climate changes rapidly over short distances.

In most of the zone the natural vegetation is evergreen or semi-evergreen forest, the most luxuriant stands approaching rain forest stature and structure. The canopy varies in height from 10 to 30 m. About 120 species occur, although more than 30 are not usually present in any one stand. Endemic species include Atalaya natalensis, Anastrabe integerrima, Beilschmiedia natalensis, Brachylaena uniflora, Cola natalensis, Commiphora harveyi, Cordia caffra, Diospyros inhacaensis and Manilkara concolor. Today, where the original vegetation has not been completely replaced, land cover often consists of a mosaic of forest, scrub forest, bushland, thicket and secondary grasslands. Where rainfall is too low to support forest, the most widespread climax vegetation is evergreen and semi-evergreen bushland and thicket.

SUBTROPICAL DRY FOREST

This zone includes parts of North Africa and South Africa with a Mediterranean climate. There is a pronounced dry season in summer. Most of the rainfall (400 to 1 000 mm per year) occurs in winter although in the eastern regions of South Africa it is more evenly distributed (subtropical humid). The annual temperature varies but the mean temperature of the coldest month, in the lowlands, is always more than 7°C.

In northern Africa, the climax vegetation is forest, with Quercus suber, Quercus faginea, Quercus ilex and Pinus pinaster in the most humid parts under marine influence and Tetraclinis articulata, Q. ilex and Pinus halepensis in more continental situations. In many places, as a result of degradation by overgrazing these forests have been replaced by scrub.

In South Africa, the prevalent vegetation of this zone is fynbos, sclerophyllous shrublands 1 to 4 m high, with the main shrub genera Protea, Cliffortia, Muraltia, Leucospermum, Restio, Erica and Serruria. The only tree species, silver tree (Leucadendron argenteum), is found on the slopes of Table Mountain.

SUBTROPICAL STEPPE

This transitional belt lies in the Marrakech and Agadir Basins in Morocco and the lower inland plateaus in Algeria and Tunisia. Rainfall varies from 200 to 500 mm with a long dry hot season of 6 to 11 months. The mean temperature of the coldest month is always more than 7°C. Vegetation in this zone is a tree pseudo-steppe with Acacia gummifera, Ziziphus lotus and Pistacia atlantica. In Morocco (Sous) the typical vegetation type is Argania spp. forest.

SUBTROPICAL MOUNTAIN SYSTEMS

In northern Africa, the Atlas Mountains dominate the landscape and extend over 3 000 km. Their altitude reaches 1 500 m in Tunisia, 2 500 m in Algeria and 4 165 m in Morocco. The Rif Atlas experiences a humid climate because of proximity to the Atlantic Ocean. Rainfall approaches 1 000 mm, with a short summer drought. Further inland, the dry season is always pronounced and the climate becomes semi-arid to the south.

In South Africa, the largest highland area is the Highveld region, more than 1 000 m in altitude, bordered by the Drakensberg, reaching more than 3 000 m. The mountain ranges in the Cape region also belong to this ecological zone. The climate is humid with a tropical regime. Rainfall varies from 500 to 1 100 mm with a short winter dry season. Winter temperatures are only somewhat low, more than 7°C up to 1 500 m. In the northern Atlas Ranges, the lower slopes are covered by mixed forest with deciduous oaks or Quercus ilex associated with Pinus pinaster or P. halepensis. Above 1 600 m this forest gives way to Cedrus atlantica forest. In the southern, drier ranges is Juniperus thurifera forest.

In southern Africa an evergreen montane forest with Podocarpus and Apodytes species grows on the Drakensberg slopes. In the Cape region, a forest with conditions resembling those of temperate forest, Podocarpus spp., Ocotea spp. and Olea capensis, grows on the slopes of the Outeniekwaberge, facing the sea.

BIBLIOGRAPHY

Ecological Laboratory of Toulouse ( LET). 2000. Ecofloristic zones and global ecological zoning of Africa, South America and Tropical Asia, by M.F. Bellan. Rome, FAO.

Hamilton, A. 1989. African forests. In H. Lieth & M.J.A. Werger (editors). Tropical rain forest ecosystems: biogeographical and ecological studies. Ecosystems of the world, Vol. 14b. Amsterdam, Elsevier.

Walter, H. 1985. Vegetation of the Earth and ecological systems of the geo-biosphere. Third, revised and enlarged edition. Berlin, Springer-Verlag.

White, F. 1983. The vegetation of Africa - a descriptive memoir to accompany the UNESCO/AETFAT/UNSO vegetation map of Africa. Natural Resources Research, No. 20. Paris, UNESCO.