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African Climatic Conditions

A number of factors influence the climate of the African continent. First, most of the continent—which extends from 35° S to about 37° N latitude—lies within the tropics. Second, the near bisection of the continent by the Equator results in a largely symmetrical arrangement of climatic zones on either side. This symmetry is, however, imperfect because of a third factor—the great east–west extent of the continent north of the Equator, in contrast to its narrow width to the south. In consequence, the influence of the sea extends farther inland in Southern Africa. Moreover, a quasi-permanent subtropical high-pressure cell (the Saharan anticyclone) develops in the heart of northern Africa, while in Southern Africa the belt of high pressure on land weakens during the time of high sun (the season when the Sun is overhead—in December and January in the south). A fourth factor consists of the cool ocean currents, which chill the winds that blow over them and thereby influence the climate of the neighbouring shores. Fifth, because of the extensive plateau surfaces of the continent and the absence of high and long mountain ranges comparable to, for example, the Andes in South America or the Himalayas in Asia, climatic zones in Africa tend to shade into one another, rather than change abruptly from place to place. Finally, the high mountains have climatic zones of their own that vary with altitude.

While these factors help to account for the broad climatic patterns of the African continent, there are nevertheless numerous local variations to be found from place to place within the same climatic zone. Urban areas, for example, have climates that often differ in many respects from those of the surrounding countryside. Typically experiencing higher average temperatures, urban areas also frequently have less wind and lower relative humidity; there is too little relevant data from Africa, however, to permit a detailed study of urban climates.

The most important differentiating climatic element is rainfall; this, together with several other climatic elements, depends upon the characteristics of the dominating air mass. The air masses of relevance to the African climate may be broadly classified as maritime tropical, maritime equatorial, continental tropical, maritime polar, and continental polar. Of these, the least important are the continental polar air masses, which may occasionally bring intense cold to northern Egypt in December and January, and the maritime polar air masses, which are associated with rain-bearing depressions over the northern and southern extremities of the continent during the winter. With the exception of these, the continent is affected both by a continental tropical air mass to the north and by maritime tropical and maritime equatorial air masses to the south.

These northern and southern air masses meet at the intertropical convergence zone (ITCZ). The hot, dry continental tropical air mass, which is present in the upper levels of the atmosphere, descends to the ground only at the convergence zone. Less hot than the continental tropical are the maritime tropical and maritime equatorial air masses, which originate from the Indian and South Atlantic oceans, respectively; they differ only in that the maritime equatorial air mass is unstable and brings rain while the maritime tropical air mass, when fully developed, is stable and does not normally bring rain unless it is forced to rise by a high mountain.

In July the ITCZ—following the sun—moves northward toward the area of low pressure over the Sahara; there the maritime and continental tropical air masses converge, with the maritime air masses swinging inland from the sea. There is no rainfall on the northern side of the convergence zone, since the region is completely under the dry continental tropical air mass originating over the Sahara. At the ITCZ itself, however, precipitation is prolonged and intense as air converges between the maritime and continental air masses and is forced aloft. Immediately south of the convergence zone, rainfall is heavy because of the unstable nature of maritime tropical air over a heated land surface. South of the Equator, at yet greater distance from the convergence zone, the maritime air masses are less-heated, thick, and stable, and they bring hardly any rainfall, except over some of the East African highlands. Only the southern tip of South Africa receives rainfall at this time, from winter cyclones.

During the period of low sun in the Northern Hemisphere (from December to January, when the sun has moved to its southern limit), the situation described above is reversed. The convergence zone moves southward, dipping into Southern Africa. At this season the whole of northern Africa (except the Atlas Mountains) is under the dry continental tropical air mass, while Southern Africa receives rainfall except in the Cape region and on the southwest coast, where the maritime air mass remains stable offshore over the cool Benguela Current.

Climatic regions When considered in detail, the movement of air masses and their effects provide the basis for a division of the continent into eight climatic regions. These are the hot desert, semiarid, tropical wet-and-dry, equatorial (tropical wet), Mediterranean, humid subtropical marine, warm temperate upland, and mountain regions.

The hot desert region consists of the Sahara and Kalahari deserts, which are always under the influence of dry continental tropical air masses, and the northern Kenya–Somali desert, the aridity of which is principally caused by the stable nature of the maritime air masses that pass over it throughout the year. The stability of these maritime air masses is induced by their passing over the cool body of water offshore. In addition to aridity, the desert climate is characterized by high mean monthly temperatures; the diurnal (daily) temperature range is, however, greater than the annual range of the mean monthly temperature.

Semiarid climatic regions fringe the desert areas and include the greater part of the land south of the Zambezi River. They differ from true desert regions in being just within reach of the ITCZ in the course of its seasonal movement and therefore receiving more rainfall. Temperatures are about the same as those in the desert regions.

The tropical wet-and-dry region is often called the savanna climatic region; this implies, incorrectly, that all areas with savanna vegetation have this type of climate. This region covers a little less than half of the total surface area of the continent, extending toward the Equator from the semiarid areas. The great distinguishing feature of this climatic region is the seasonal character of its rainfall. During the period of high sun, the maritime air masses produce up to six months of rainfall, the length of the rainy season depending on nearness to the Equator. The rest of the year is dry. In a few places—for example, on the coast of Mauritania and Senegal—there is also a little rainfall in the period of low sun. As in the desert and semiarid climatic zones, mean monthly temperatures show less variation than daily temperatures. In western Africa the period of low sun corresponds to the harmattan season. The harmattan is a warm, dry, northeasterly or easterly wind that blows out of the southern Sahara and is frequently laden with large quantities of sand and dust.

Regions with the equatorial, or tropical wet, type of climate, or variants thereof, are the wettest in Africa. There are two peak periods of rainfall corresponding to the double passage of the ITCZ. Because areas with an equatorial climate are constantly covered by warm maritime air masses, variations in their monthly and daily temperatures are less pronounced than in the tropical wet-and-dry regions.

Marked variations in the rhythm of equatorial climate sometimes occur. For example, the rainfall may be monsoonal and the second rainy season may be all but nonexistent. But the most notable anomaly can be observed on the western African coast from around Cape Three Points, Ghana, eastward to Benin, where, although the bimodal rainfall regime prevails, the total annual precipitation is less than 40 inches (1,000 millimetres). Among the many explanations that have been suggested are that the presence of a cold body of water offshore chills the lower layers of the maritime air mass and makes it stable, that the body of cold air that forms offshore diverts the incoming airstreams to the west and east of the anomalously dry area, that there is a strong tendency for the winds to blow parallel to the shore during the rainy seasons, that the absence of highlands deprives the region of orographic (mountain) rainfall, that fluctuations in the offshore moisture-bearing winds occur during the rainy season and reduce rainfall, and that local meteorological peculiarities of thunderstorms contribute to the reduction in rainfall.

In the northern and southern extremities of the continent, there is a dry summer subtropical, or Mediterranean, type of climate. Rain falls only in winter (December–January in North Africa, June–July in Southern Africa), although in some localities it may fall in autumn (September in North Africa, April in Southern Africa). Mean monthly temperatures are lower than in tropical climates, dropping to about 50 °F (10 °C) in winter, while summer (June–July in North Africa, and December–January in Southern Africa) temperatures may sometimes exceed those of tropical climates. Clear blue skies are characteristic.

The humid subtropical marine climate is restricted to the southeast coast of Africa. This region is characterized by rainfall throughout the year, but it is heaviest in summer. In South Africa, south of KwaZulu-Natal, the winter rainfall is more pronounced, and the temperatures are a little lower than in the north. Thus, at Port Elizabeth there are six months when temperatures are below 62 °F (17 °C), while at Durban mean monthly temperatures do not fall below 64 °F (18 °C).

The warm temperate upland climatic region is found on the Highveld of Southern Africa. Its rainfall regime is similar to that of the tropical wet-and-dry climate, but temperatures are greatly modified by the altitude; frost, for example, occasionally occurs in Lesotho. Toward the coast the climate shows maritime characteristics, and there is a tendency toward winter rainfall.

The mountain climatic region includes the high mountain areas of Ethiopia and the lake region of East Africa. In some respects the climate is similar to the warm temperate upland climate, except that temperatures are even lower and snow occurs on the tops of the highest peaks, such as Kilimanjaro. The rainfall regime is similar to that of the adjacent lowland areas.

Plant life

African vegetation develops in direct response to the interacting effects of rainfall, temperature, topography, and type of soil; it is further modified by the incidence of fire, human agriculture, and grazing and browsing by livestock. Of the total land area of the continent, forests cover about one-fifth; woodlands, bushlands, grasslands, and thickets about two-fifths; and deserts and their extended margins the remaining two-fifths.

Ecological relationships Until about two million years ago Africa’s vegetation had always been controlled by the interactions of climate; geology, soil, and groundwater conditions (edaphic factors); and the activities of animals (biological factors). The addition of humans to the latter group, however, has increasingly rendered unreal the concept of a fully developed “natural” vegetation—i.e., one approximating the ideal of a vegetational climax. Nevertheless, in broad terms, climate remains the dominant control over vegetation. Zonal belts of precipitation, reflecting latitude and contrasting exposure to the Atlantic and Indian oceans and their currents, give some reality to related belts of vegetation. Early attempts at mapping and classifying Africa’s vegetation stressed this relationship: sometimes the names of plant zones were derived directly from climates. In this discussion the idea of zones is retained only in a broad descriptive sense.

As more has become known of the many thousands of African plant species and their complex ecology, naming, classification, and mapping have also become more particular, stressing what was actually present rather than postulating about climatic potential. In addition, over time more floral regions of varying shape and size have been recognized. Many schemes have arisen successively, all of which have had to take views on two important aspects: the general scale of treatment to be adopted and the degree to which human modification is to be comprehended or discounted.

Once, as with the scientific treatment of African soils, a much greater uniformity was attributed to the vegetation than would have been generally acceptable in the same period for treatments of the lands of western Europe or the United States. Quite the opposite assumption is now frequently advanced. An intimate mosaic of many species—in complex associations and related to localized soils, slopes, and drainage—has been detailed in many studies of the African tropics. In a few square miles there may be a visible succession from swamp with papyrus, through swampy grassland and broad-leaved woodland and grass, to a patch of forest on richer hillside soil, and finally to succulents on a nearly naked rock summit.

The span of human occupation in Africa is believed to exceed that of any other continent. All the resultant activities have tended, on balance, to reduce tree cover and increase grassland, but there has been considerable dispute among scholars concerning the natural versus human-caused development of most African grasslands at the regional level. Correspondingly, classifications have differed greatly in their principles for naming, grouping, and describing formations: some have chosen terms such as forest, woodland, thornbush, thicket, and shrub for much of the same broad tracts that others have grouped as wooded savanna, savanna, and steppe. This is best seen in the nomenclature adopted by two of the most comprehensive and authoritative maps of Africa’s vegetation that have been published: R.W.J. Keay’s annotated Vegetation Map of Africa South of the Tropic of Cancer (1959) and its more widely based successor, The Vegetation Map of Africa (1983), compiled by Frank White. In the Keay map the terms savanna and steppe were adopted as precise definitions of formations, based on the herb layer and the coverage of woody vegetation. The White map, however, discarded these two categories as specific classifications. Yet any rapid demise of savanna in its popular and more general sense (i.e., as dry tropical grassland or mixed woods and grassland) is doubtful.

The vegetational map of Africa and general vegetation groupings used in this article mainly follow the White map and its extensive annotations, although some 100 specific types of vegetation identified on the source map have been compressed into 14 broader classifications.

Vegetational zones

Lowland rainforest African lowland rainforests occur along the Guinea Coast of western Africa and in the Congo basin. The full development of this tropical formation requires continuously warm conditions and an annual rainfall exceeding 50 to 60 inches (1,270 to 1,520 millimetres) distributed fairly evenly over the year. The vertical limit is about 3,500 to 4,000 feet. This multistoried, highly diverse, extensive, and potentially self-perpetuating assemblage has been described by some as the source of virtually all tropical floristic diversity. No other part of the world sustains a greater biomass (total weight of organic matter in a given surface area) than lowland tropical rainforests. Even though the speciation (proliferation of distinct types of plant) within the African rainforests is notably poorer than that of its counterparts in Southeast Asia and the Amazon basin of South America, these forests sustain a huge multiplicity of life-forms, occupying different strata (generalized levels of plant height) and niches (separate, small-scale habitats).

Characteristically, tropical rainforest is composed of a ground story, from 6 to 10 feet tall, of shrubs, ferns, and mosses; a middle story of trees and palms 20 to 60 feet in height; and a dominant top canopy consisting of trees up to 150 feet high with straight unbranched trunks, buttressed roots, and spreading crowns of perennial leafage. The large branches of these crowns provide niches for epiphytes, including orchids, ferns, and mosses. Lianas tie trees to one another, parasitic species cling to trunks and branches, and strangler figs (Ficus pretoriae) put down aerial taproots. Nevertheless, these are not “impenetrable” jungles. It has been suggested that some early European travelers and pioneer botanists may have exaggerated the difficulties of human penetration because they journeyed along atypical waterways and along tracks where disturbance of the original vegetation had thickened the regenerating ground layer. In true rainforests, grasses are adventitious (occurring in consequence of fortuitous intrusions). Elephant grass (Pennisetum purpureum) can grow abundantly in areas where the vegetation has been disturbed, providing good fodder for grazing animals when young but quickly becoming rank, coarse, and a refuge for insects. Cogon grass (Imperata cylindrica) is a troublesome grass on depleted and fire-seared ground.

Eastern African forest and bush

Lowland forests and evergreen bushland form a long belt of land some 125 miles broad along the Indian Ocean. From various causes—notably the monsoonal climate, freely draining soils, and long historical impact of humans—these forests are much more limited in their structure (physical form), speciation, and robustness. On more favoured terrain—such as estuarine fringes, the seaward flanks of the islands of Zanzibar and Pemba, and hill masses athwart the rain-bearing southeast monsoon—forest and a close broad-leaved woodland are still dominant. Where land is in a rain shadow, in areas of unfavourable geology (e.g., raised coral reefs), and near cities and small ports, thorny bush, succulent shrubs, and scrawny grassland prevail. Nevertheless, the region now sustains a number of economically important domesticated trees—both indigenous and exotic—such as the coconut palm, cashew, mango, and (especially on Zanzibar and Pemba) clove.

Mangrove swamp

Mangroves include a variety of species of broad-leaved, shrubby trees (10–40 feet high) that fringe muddy creeks and tidal estuaries. They require warm saline water—hence their distribution along tropical coastlines. Often they form nearly impenetrable stands, for which the easiest access is by sea. The trunks and roots are termite-resistant, and they have long been favoured as a building material and for making charcoal.

Broad-leaved woodland and grassland

This classification constitutes one of the most extensive composite categories now recognized and includes much of the land formerly labeled as savanna. Two broad bands extend across the continent, one from about 7° to 12° N latitude and the other from about 8° to 22° S latitude. Structure and floristic composition vary greatly with the increase of latitude, both in the north and the south. Annual rainfall averages 35 to 45 inches, with marked seasonality of occurrence and considerable fluctuations from year to year, both in total rainfall and in the onset of rainy periods. The woodlands of western Africa strikingly resemble those south of the Equator. In both areas, undulating wooded interfluves on light soils successively alternate with swampy, clay-based valley grasslands (called fadamas in Nigeria and dambos in Zambia and Malawi) in a topographically linked sequence of soils called a catena.

Trees, 30 to 50 feet high, are typically deciduous and often fire-resistant, since much of this land is burned annually. Common western African species include types of Isoberlinia (a spreading leguminous tree of the pea family), Daniellia (a leguminous tree with white bark), and Lophira (a tree with strap-shaped leaves that is said to yield the most durable timber in the region). Other hardwoods, forming distinct communities, are Combretum and Terminalia, which are better suited to the drier areas. Prevalent southern equivalents include Brachystegia (a leguminous hardwood, the bark of which formerly was used to make cloth) and Julbernardia (another plant of the pea family resembling Isoberlinia). Over much of the interior of Tanzania, in areas of reduced rainfall and poorer soils, a light-canopied, sustained woodland called Miombo forest rises above a rather scrawny ground layer. This is an excellent habitat for bees, and honey has long been gathered there.

Because of periodic burning, tall grasses have become dominant over large expanses of plateau land, which sometimes contains few, if any, of its original trees. The tall, coarse red grass Hyparrhenia can form prominent stands, but it makes poor grazing land and often harbours insects that spread disease. Much better for the pastoralists are induced swards of Themeda.

For centuries humans have selectively retained certain economically important tree species in areas cleared for farming; the effect has been to create what is called “farmed parkland,” in which a few favoured trees rise above the fields. Examples include the shea butter nut tree (Butyrospermum), common in Ghana and Côte d’Ivoire; Acacia albida, found in Senegal and Zambia; and the truly domesticated baobab (Adansonia digitata), which is perhaps the most widely distributed.

Thorn woodland, grassland, and semidesert vegetation

Toward the margins of the tropics, the vegetation cover becomes lower and thinner as the fluctuating transition to desert vegetation ensues. In the same progression the concept of an annual rainfall (nominally 5 to 20 inches) yields to the reality of extreme unreliability in both incidence and expectation. Under such restraints a definitive “boundary” with the desert becomes meaningless. Moreover, there appears to have been a trend toward declining precipitation in the last half of the 20th century, and human impact certainly has enhanced the natural deprivation of plant life in the marginal regions. The southern margin of the Sahara—roughly between the latitudes of 15° and 20°—is called the Sahel (Arabic: Sāḥil; meaning “shore” or “edge”), the word being extended by implication to comprehend the fluctuating margins of the great sand seas of the Sahara to the north. The southern equivalent covers much of the Kalahari, which is often called a desert but is more properly a thirstland.

Thorn woodland displays a predominance of xerophytic, sometimes succulent or semisucculent trees, such as acacia, Commiphora (the myrrh tree), or Boscia (an evergreen hard-leaved tree). The occurrence of the bunched and thorny desert date (Balanites) seems to accompany land impoverishment. A relatively luxuriant shrub layer, often forming dense thickets, is found in conjunction with succulents, such as aloes, Sansevieria (a fibrous species), and Adenium, or desert rose (a succulent shrub with smooth gray bark, a huge water-storing base, and beautiful red or pink flowers), and smaller euphorbias.

Farther toward the desert, tree growth and perennial grass—surviving in narrow strips along watercourses—separate much larger areas of sparse annual grasses (Cenchrus in western Africa, Eragrostis south of the Equator, and Chrysopogon on the margins) and scattered low shrubs, often mainly acacias. Shrubs may often be salt-tolerant. While shrubs may die from inadequate moisture, they are little affected by the rare fires that occur.

Afromontane vegetation

All high mountains exhibit azonality; i.e., their vegetation differs from that found in the climatic zones from which they rise. The differences manifest themselves as progressive modifications, which are usually well stratified and reflect altitude-dependent climatic changes. Generally, as elevation increases, temperature decreases (to the point where frost and even glaciation can occur) and precipitation increases (although above a certain level precipitation decreases markedly). Mountainous terrain can retain ancient climatic conditions—making possible, for example, the survival of relict species—and the relative inaccessibility of the higher elevations to humans has helped preserve more of the vegetal patterns of the past.

Vegetation strata typically are skewed with regard to slope orientation (aspect). This is mainly due to a contrast between exposure to rain-bearing winds and shadowing from them but may also reflect long-term history. If lower slopes rise abruptly from the base, as they often do in Africa, then a distinct boundary between vegetation formations may be clearly distinguished; if the rise is gentle, vegetations merge (as in the western Kenyan highlands). (All the circumstances mentioned above are represented in the African mountain systems, but for purposes of illustration the vegetational map identifies only areas of altitudinal modification. Thus, some areas that are included are not tropical, such as parts of the Red Sea Hills and the mountains of South Africa and Lesotho.)

Altitudinal modifications of vegetation are clearly discernible on the high East African peaks near the Equator (e.g., Kilimanjaro and Mounts Kenya and Elgon), and a rich forest belt—much reduced upslope by human activities, except where the land has been reserved—clothes the zone that receives the maximum rainfall and is free of frosts (up to about 5,000 to 6,000 feet). Such mountains have great human importance as watersheds and as repositories of native plants.

Desert vegetation

The Sahara has one of the lowest species densities in the world, and a sustained vegetation cover (which can include trees and bushes) occurs only in the massifs and oases. Elsewhere the vegetation is discontinuous and consists of two main types: perennials with huge root systems and sparse aerial parts, often protected by waxy cuticles, thorns, and hairs; and ephemerals with slight root systems and little foliage but with the ability to flower profusely immediately after occasional storms and then to seed quickly and abundantly. The stony and rocky expanses give more hold for plants than do the vast areas of shifting sands. In some areas with slightly more rainfall, grass tufts may grow 50 yards apart. Aristida is the dominant grass, and for brief periods it can yield a nutritious forage called ashab.

The Namib is one of the world’s driest deserts. The area along the coast, however, is almost always foggy, and succulent shrubs (such as aloes) manage to survive on this moisture. The Namib also contains the strange tumboa, or welwitschia (Welwitschia mirabilis), which may live 100 years or more.

Karoo-Namib shrubland

In this drought-prone land, soils are often shallow, even saline. The low shrubs that grow there can be divided into two groups: woody plants, such as species of Acacia and Pentzia and the saltbush (Atriplex); and succulents, including aloes, euphorbias, and Mesembryantheum. Aristida and Themeda are characteristic grasses. Every year the blossoms of bulbous plants lay short-lived carpets of colour. Being both drought-resistant and high in minerals, many of the shrubs can provide useful grazing for goats and sheep.

Highveld grassland

The grassland classification is restricted to regions with 10 percent or less woody plant cover. The Highveld meets this definition and probably owes much to unaided nature for its creation and perpetuation, since fires caused by lightning strikes are relatively frequent. Its extent has always been fairly precisely defined: areas with more than 15 inches of rainfall during the summer. Highveld vegetation, though modified considerably by human activity, traditionally has been differentiated into sweet veld (dominated by Themeda) or sour veld (Andropogon and Eragrostis), the latter making poorer pasturage.

highveld grassland

Highveld grassland near Heidelberg, South Africa, southeast of Johannesburg. Gerald Cubitt—Bruce Coleman Ltd. Learn More!

Mediterranean vegetation

This zone is determined chiefly by its climate, which is characterized by very dry summers and mild, rainy winters, but it has long been much differentiated by its inhabitants. Large tracts have been degraded into maquis (macchie), garigue, or dry semidesert (steppe) vegetation. Maquis consists of dense scrub growths of xerophytic (drought-resistant) and sclerophyllous (leathery) shrubs and small trees, which are often fire-resistant. Garigue characteristically is found on limestone soils and has more woody growth, including evergreen and cork oaks (Quercus suber). The higher slopes of the Atlas Mountains once carried large stands of pine and cedar, but they have been much depleted. Typical grasses, progressing from the coast to the desert, are Ampelodesmos, Phalaris, and Stipa.

Cape shrub, bush, and thicket This region constitutes the southern counterpart of the Mediterranean zone, although (with the exception of the Atlas Mountains) it is richer in its vegetation potential. There were once considerable enclaves of true evergreen bushland, which have reverted to shrubland (fynbos). Sclerophyllous foliage and proteas abound. Although grassy tracts occur on the mountains, they are characteristically unusual lower down. Beyond the Cape Ranges, fynbos grades into karoo.

Madagascar

Physically and biologically, Madagascar has long formed a separate entity. White has identified eastern and western regions of endemic (unique) vegetation. In the eastern centre, about one-sixth of the plant genera and more than three-fourths of the thousands of species are regarded as endemic. The Madagascar rainforest has shorter trees and a somewhat drier climate than its equatorial counterpart and contains its own dwarf palms (Dypsis) and bamboos (Ochlandra). The western deciduous forest stands in the rain shadow; some of its trees resemble Mediterranean oaks. The southern thickets have prominent euphorbias and species of the Didiereaceae family. The island has much degraded secondary forest (locally called savoka) along the eastern and northern coasts.

Sudd In addition to the major types of vegetation described above, a special vegetation called sudd (literally meaning “barrier”) occurs in the great Nile, Niger, and Zambezi drainage systems of the African interior plateau. Sedges (especially papyrus), reeds, and other water plants—including the floating Nile cabbage (Pistia stratiotes)—form masses of waterlogged plant material that are largely unproductive and are a nuisance to fishing and navigation. Pistia has become an unwelcome invader of Lake Kariba, the body of water formed by the impounding (1959) of the Zambezi River in the Kariba Gorge.

Long-term changes in vegetation

Africa’s basic vegetational zones are believed to have existed in approximately the same climatically controlled series and with the same characteristically developed species for a long period of time; indeed, some ancient African plant families—such as the cycads, which evolved some 200 million years ago—still have living representatives. Nonetheless, the continent’s vegetation has been altered continuously by geologic and climatic changes and by the movement of the caloric (heat) Equator. The past million years have been a time of unusually rapid changes, with major consequences for Africa’s vegetation.

The vegetational history of Africa is of great scientific relevance. Studying the lichens growing in the high East African mountains, for example, may yield a better understanding of the continent’s climatic trends, and a knowledge of past conditions in the Sahel might help explain what influence natural phenomena have had on the disastrous droughts of the region since the late 1960s.

Geologic influences

The two most important geologic modifications of vegetation have been the very ancient separation of Madagascar from the mainland, which gave rise to the distinct speciation of the island’s flora, and the long-continuing faulting and volcanism along East Africa’s huge rift system that has thrown up high ranges (e.g., the Ruwenzori between Uganda and the Democratic Republic of the Congo) and great volcanoes (Kilimanjaro) and has thus created and reshaped Afromontane flora.

Climatic influences

The repercussions of the great Pleistocene Ice Ages of Europe have constituted the most notable climatic influence on African flora in relatively recent geologic history. These consist of a succession of colder periods marked by glacial advances, interrupted by warmer, drier interglacials; the last series of these ended between about 5,000 and 10,000 years ago. Tropical Africa experienced contemporaneous fluctuations in its climate, although it is misleading to infer any simple equivalences between these fluctuations and the European periods of glacial advances and retreats.

During the wetter times (pluvials) in Africa, equatorial forests spread, separating northern woodlands from their southern counterparts (with consequent species differentiation); mountain vegetation descended onto the plateaus; and there is evidence that the Saharan climate was greatly ameliorated, much to the advantage of humans. During the warmer, drier interpluvials the existing vegetation was degraded in many zones. Dunes spread from the Sahara and over the Kalahari, for example, and their fossilized alignments—now vegetated—can be traced across the thorny woodlands and grasslands of Niger, Nigeria, Namibia, and Botswana.

Human influences

The greater part of the reduction of Africa’s natural vegetation has happened in the last 2,000 years—probably since the late 19th century for the tropical portions—the time during which humans have been most numerous and active. Pastoralism, agriculture, the rapid growth of human and livestock populations, the expansion of cities and towns, and the external demands for primary resources have made ever-greater demands upon the land for sustenance and perceived economic betterment. Much is known of the detailed processes of vegetation modification along the Mediterranean, since they have been observed and studied since Classical times, and a good deal is also known from the more than three centuries of study of the Cape area of South Africa, but until the late 19th century very little was understood about these processes in tropical Africa. Indeed, the timescale of actual human impact on African vegetation may be causally linked to the awareness of it by Europeans.

Within the tropical forests and woodlands, fire undoubtedly has been the great human agent of clearance and degradation, of far greater efficacy than felling, bark-ringing, or uprooting—at least until the introduction of modern plantation agriculture and logging. Hunters, pastoralists, and cultivators have all fired the land for centuries and have gathered wild foodstuffs, thatch timber for construction, and fuelwood from the volunteer (i.e., uncultivated or self-generating) vegetation. The long-term effects of such activity bear directly upon the debated question of the origin of the savannas.

In earlier times, African cultivators found the fabric of the tropical rainforest comparatively difficult to modify substantially. In the 20th century, however, it was greatly reduced in extent (such as in Sierra Leone), patched and frayed (Nigeria), and exploited for timber exports (Gabon). Moreover, many of tropical Africa’s largest cities and busy seaports are in this zone. The most diverse and seemingly inexhaustible floral realm in Africa has therefore become a cause for widespread concern.

Conserving the vegetation

Perceptions of the need for environmental conservation in Africa held by those outside the continent are sometimes expressed in terms that seem opposed to the legitimate priorities and aspirations of African peoples (in meeting which agriculture and livestock management must remain crucial). It is not surprising that projections based upon the assumptions from these external sources frequently end in pessimism. A more constructive approach is to identify ways in which to more fully integrate wild plant life, crops, and animals, which can be expressed in the concept of productive countryside. The capacity and precision of resource surveys have been greatly enhanced by remote sensing, and this has been coupled with the worldwide transmissibility of information. Research and interest in agroforestry have expanded and become institutionalized. Above all, however, confidence must be put in the capacities of many millions of African farmers to expand agriculture while working toward reintegration with wild plant life.

How Climate Changes is Affecting Africa