Africa Geography the Continent of Africa Is with Ca. 30.4 Million Km²

Home , Afromontane, Atlas Mountains, Congo Basin, Kalahari Desert

Africa

Geography

The continent of Africa is with ca. 30.4 million km² the second largest land mass. Madagascar adds an additional 0.6 million km². From its most northerly point, Ras Ben Sakka in Tunisia at 37° 21’ N to its most southern point, Cape Agulhas in South Africa at 34° 51’ S is a distance of ca. 8000 km and the greatest E-W distance is ca. 7400 km. Unlike in other large continents, there are no great mountain ranges with continuous high altitude; most of the only 10 high mountains over 4000 m are isolated volcanos. The highest mountain on the continent is Mt. Kilimanjaro, a volcano at 5895 m. In the NW the continent is bordered by the Atlas Mountains which receive rain from the Atlantic but also have a rain shadow side next to the Sahara. This desert, the largest in the world with ca. 9 million km², extends from the Atlantic Ocean to the Red Sea and from the Mediterranean Sea to the Sahel zone, a broad transition from desert to savanna and dry forest. Further extensive deserts are situated in southern Africa, with the Namib Desert along the Atlantic coast and the Kalahari Desert in the interior. The most extensive uplands are the Ethio- pian Highlands in the E, loosely connecting with mountains in Kenya and Tanzania; W of these are some high mountains bordering the Rift Valley, such as Rwenzori (5109 m). This valley is a long system of tectonic rift (an incipient ocean) along which large elongated lakes have formed. Intermittent mountainous terrain extends from the Rift Valley S to the South African Cape (Table Mountain). Most of the W half of the continent is lowland, with Mt. Cameroon (4095 m) the only notable exception. The Congo Basin is a very large, level basin forming the drainage of the largest river by volume in Africa, the Congo. After the Amazon Basin in South America, it is the largest area of tropical lowland rainforest in the world. The continent can be divided into three major geographical regions relevant to conifers: The Atlas Mountains and the Mediterranean coast, the Afromontane region from Ethiopia to South Africa, and Madagascar. A smaller area is formed by Mt. Cameroon and the surrounding highlands; it is the most isolated in terms of distance to other areas with conifers. The most substantial biogeographical barrier separating the continent from Eurasia is the Sahara; this desert also separates the Atlas Mountains and the Mediterranean coast from sub-Saharan Africa. Within sub-Saharan Africa, the major barriers to conifer distribution are the Kalahari Desert and the Congo Basin. The most important conduit for conifer distribution is the archipelago-like arrangement of higher altitude areas and mountains that constitutes the eastern African Afromontane region. Towards the southern tip of Africa, a temperate climate allows species from the Afromontane forest to descend to near sea level, but near the Cape and especially up the W coast increasing aridity turns the vegetation from treeless fynbos to desert. africa 459

Conifers in Africa

Families Genera Species Species + infra Countries with Area of occupancy Endemic taxa conifers in km² 4/8 10/70 34/615 41/794 25/54 14,625/31,000,000 22 Numbers after ‘/’ are global or continent totals.

Compared to other continents, conifers are poorly represented in Africa. Vast areas of the continent are devoid of conifers. The main distribution of conifer species is in the Atlas Mountains and throughout the eastern African Afromontane region, with smaller areas with conifers around Mount Cameroon and in Madagascar. The southern coast of the Mediterranean has a few species in the less dry areas. Elsewhere, conifers are sparsely distributed across the region between the Kalahari Desert and the Congo Basin. In the Sahara, there is only is a single population of Cupressus dupreziana. There are no conifers in the Congo Basin, the Kalahari Desert, the highland areas west of Mount Cameroon, the Horn of Africa south of the coastal Cal Madow range and in the entire western parts of Madagascar. Conifers are also absent from savanna and woodland belts in Southern Africa and south of the Sahara; and from the Karoo and other plateaus in South Africa. The two principal influences determining conifer distribution in Africa relate to ecology and history. Many native conifer species in Africa have limited seed dispersal capability. Few species have effectively winged seeds and these only occur in the Atlas Mountains (Abies, Cedrus and Pinus species). Some species have virtually no dispersal mechanism. The Podocarpaceae in Africa and the sole Juniperus species ( J. procera) are bird dispersed. Of these, only J. procera and Podocarpus milanjianus have spread widely and have maintained these broader distributions. The extreme conditions in the deserts and semi-deserts of the Kalahari, Namib, Sahara and the Horn of Africa explain the absence of conifers in these regions. The unusual and solitary occurrence in the Sahara of Cupressus dupreziana is a classic example of a relict population (see page 159). Explaining the absence from the Congo Basin is more difficult. Conifers, mainly Podocarpaceae and Agathis, do occur in lowland tropical rainforests, for example in Borneo. There are no obvi- ous geographical barriers for conifer distribution into this basin from the surrounding regions, especially from the east. This suggests that ecological conditions have been and are the limiting factor. In lowland tropical rainforest environments, conifers are restricted to nutrient deficient soils, for example, white sands (e.g. the kerangas of Borneo) and ultramafic soils. The sedimentary nature of the Congo Basin with numerous flooding rivers heightens soil fertility throughout the region. The growing season is virtually year-round and conditions are without edaphic limita- tions. Under these conditions, competition with angiosperms is severe, limiting opportunities for establishment. These limiting conditions however do not apply in the vast stretch of tropical highlands west of Mount Cameroon. Here, suitable habitat appears to be common yet is devoid of naturally occur- ring conifers. This lack of conifer species must have an historical explanation. Fossil evidence indicates a presence of conifers until the end of the Pliocene (Morley in Turner & Cernusak, eds. 2011); presumably they went extinct during the Pleistocene. Introduced conifers from similar habitats in Africa (e.g. Afrocarpus mannii) grow well in the region. Further underlining the pecu- liarity of this absence is that the genus Afrocarpus, distributed widely in the Afromontane region, is only represented by A. mannii on the island of São Tomé off the West African coast. Podocar- pus in West Africa is only represented by the most widely distributed species P. milanjianus but even this species does not occur west of Mount Cameroon. Extinction cannot be excluded as an explanation as there is fossil evidence of podocarpaceous pollen in West Africa beyond the pres- ent occurrence of members of the family Podocarpaceae.