Broadleaved

The woodland savanna region lies in the central and southern parts of the country, excluding the flood region in the south-east. The vegetation is of mixed grass types with bushes and trees, the proportions being dependent on rainfall, soil type, and fire frequency and intensity. Rainfall is confined to a few months followed by a long hot dry season.

High rainfall (> 900 to 1 000 mm) woodland savanna

The high rainfall woodland savanna extends into most parts of Bahr el Ghazal and Equatoria provinces. The trees are tall and broadleaved and thorny ones are rare. Coarse tall tussocks of perennial grasses predominate and hence fires are generally more fierce than in the low rainfall woodland savanna. Forest species easily recolonise areas with high rainfall when fire protection is provided. Two types have been described:

  • Anogeissus-Khaya-Isoberlinia woodland is found on ironstone soils with rainfall of 900 to 1 300 mm. It is far from homogeneous due to variations in climate, different types of soil that occur from ridge top to valley bottom and the effect of different degrees of human disturbance.

    The driest parts, with a rainfall of 900 to 1 000 mm, have such species as Sclerocarya birrea, Diospyros mespiliformis, Tamarindus indica, Balanites aegyptiaca with a good deal of Combretum ghasalense and many Acacia species. Mahogany (Khaya senegalensis) is sporadic on the ridges and near seasonal watercourses. With a slightly increased rainfall, these more drought-resisting species become rather rare. The ridge tops are denuded of soil in some places to form almost bare ironstone pans. But where the soil is deeper, they carry vegetation characterised by tangles of evergreen shrubs and lianas centred on termite mounds, with one or two larger trees on these mounds or in the centre of the tangle. The larger trees include Khaya senegalensis and Parkia oliveri. On the slopes down from the ridges, bu (Daniellia oliveri) is found, and, on clay soils, Anogeissus leiocarpus. In the valley bottoms, there tends to be open grassland or scattered trees of Terminalia laxiflora or T. macroptera. This belt of vegetation, of which mahogany and bu are the most valuable trees, extends from south of Aweil through Wau, Rumbek, Yirrol and Tali Post almost to Juba, and similar vegetation is found east of the Nile near Mongalla. To the south and west of this belt is a broad zone in which species of Isoberlinia are dominant over large areas, particularly on the lighter soils. At its northern drier limit, the Isoberlinia tends to be mixed with other species and is especially characteristic on the edge of the ironstone outcrops, where large blocks of ironstone have been broken off. Further south, Isoberlinia is found pure or almost pure over very wide areas, avoiding only the clay soils of depressions. It sometimes has an understorey of Monotes kerstingii or Uapaca sansibarica. These Isoberlinia woodlands are sensitive to human interference and where there has been much cultivation, they are replaced by more mixed vegetation of which Burkea africana and Terminalia mollis are typical species. At its wetter edge, Isoberlinia woodland is replaced by species of derived woodland such as Terminalia glaucescens and Albizia zygia. Many other large trees are found in smaller quantities and there are many species of small trees such as Combretum spp., Grewia mollis and Acacia seyal var. multijuga. Butyrospernum niloticum becomes dominant with the appearance of parkland, where other species have been removed during shifting cultivation and the latter preserved for the value of its fruit.

    • Woodland derived from rain forest is found on ironstone and red loam soils in the south of the Equatoria provinces with over 1 300 mm of rainfall annually. There is a good deal of evidence that it has replaced rain forest destroyed by shifting cultivation and fires during the last few hundred years. Small areas of remaining rain forest and gallery forest have been described separately as belonging to "closed broadleaved forests". Situated along the Zaire border from Li Yuby to the Aloma Plateau, it reappears again in the western foothills of the Imatong Mountains and in the Kinyeti valley. The Boma Plateau, although at a higher altitude, has generally similar vegetation. Characteristic species of larger trees in this woodland are Terminalia glaucescens, Albizia zygia, Vitex doniana and, though not in all areas, Acacia campylacantha. Anogeissus leiocarpus is common in places west of the Nile, growing up to 30 meters high. Smaller trees include Combretum binderianum, Grewia mollis, Annona chrysophylla, Bridelia scleroneuroides and Dombeya quinqueseta. The number of species per hectare is less than in the typical ironstone woodland, as can be expected in a vegetation type of comparatively recent origin. Especially in the wetter parts, occasional trees of Chlorophora excelsa, Khaya grandifoliola and Erythrophleum guineense are found.

    Low rainfall (<900 to 1 000 mm) woodland savanna

    This type covers most of central Sudan and areas east of the flood region. In the drier parts, the trees are nearly all thorny and of low stature, with species of Acacia predominating. Broadleaved deciduous trees become predominant in the wetter parts, but there is not as great a variety of species as in the high rainfall woodland savanna, and thorn trees are usually present. There are also more annual grasses than perennials and a variable proportion of herbs. The following subdivisions have been distinguished.

    • Terminalia-Sclerocarya-Anogeissus-Prosopis savanna woodland. Situated in southern Darfur and Kordofan Provinces with rainfall of over 600 mm, this type of vegetation is only distinguished from the "high rainfall woodland savanna" by the different soil type, the slightly poorer variety of trees and the mixture of annual grasses with the perennials. The trees attain 10 to 15 meters in height, and include Terminalia laxiflora, Sclerocarya birrea, Anogeissus leiocarpus and Prosopis africana. Tamarindus indica is usually associated with the dominants and in the drier parts small quantities of Guiera senegalensis, Acacia senegal and Dalbergia melanoxylon may be found.

    • Anogeissus-Combretum hartmannianum savanna woodland. This type of vegetation, though occurring on dark cracking clays, is confined to well-drained sites, such as sloping ground with rock fragments frequently found near the surface, and over 800 mm of rainfall. Combretum hartmannianum and Anogeissus leiocarpus trees (10 to 15 m high) are nearly always mixed with a few specimens of Acacia seyal, alternating with evergreen trees such as Sterculia setigera.

      • Acacia seyal-Balanites savanna. This savanna is found on cracking clays with over 600 mm of rainfall. But in the flood region, with over about 700 mm of rainfall, level sites cannot absorb all the water falling on them and the surface soil is waterlogged or flooded. Here the trees disappear and are replaced by open grassland. Patches of higher clay in the flood region do, however, carry typical savanna of this type and it appears again towards the fringes of the ironstone country. Acacia seyal is distributed throughout, usually more or less mixed with Balanites aegyptiaca. In the northern parts Acacia mellifera thickets occur as an understorey to the Acacia seyal. In the central parts, Acacia senegal is common, though it is rarely found in pure stands over extensive areas as it is in the sand country. Further south again Acacia fistula appears with Acacia drepanolobium on areas liable to flooding. In these peculiar areas Acacia campylacantha and Dichrostachys glomerata also form part of the local vegetation. On the southern fringe of the area Combretum hartmannianum and Entada sudanica begin to occur, mixed with Acacia seyal, the former being especially common when rock underlies the clay at a few meters below the surface.

      • Vegetation of rocky hills. Throughout central Sudan small rocky hills, sometimes grouped (Nuba Mountains, Ingessana Hills), protrude through the superficial deposits of the plains. These hills have a characteristic vegetation generally more moist in character than the vegetation of the surrounding plain and showing erosion catena development. They may be found in areas with rainfall of 400 to 1 000 mm. Hills composed of massive boulders often have Ficus populifolia, Ficus glumosa and other Ficus. Where the rock is broken into smaller fragments with pockets of soil between, Boswellia papyrifera, Sterculia setigera, Lonchocarpus laxiflorus, Combretum harmannianum, Terminalia brownii, Anogeissus leiocarpus, Stereospermum kunthianum, Adenium spp. and Pterocarpus lucens occur. Bamboo, Oxytenanthera abyssinica, is often found. A hard surface zone frequently occurs at the base of the hills. Here Acacia nubica, Lannea humilis and Sclerocarya birrea are present. The Dom palm, Hyphaena thebaica, occurs on more permeable soils. Along seasonal watercourses Acacia albida, Ziziphus spina-christi, Cordia abyssinica and Khaya senegalensis are characteristic, the latter two in wetter areas.

      • The Baggara and Raqaba catenas. These repeating patterns of soil types are found over considerable area in southern Kordofan and Darfur provinces. They consist of alternating patches of clay and, in some cases, also sand. The stabilised sands of the Baggara catena carry the same shrubs and trees as generally found on sandy soils in the low rainfall woodland savanna. However, the non-cracking clays are usually bare of trees except at their edges, where Acacia mellifera, A. seyal, A. hebecladoides and Lannea humilis may be found. The water of these soils drains into rainwater pools. Around them occur trees such as Anogeissus leiocarpus, Tamarindus indica, Diospyros mespiliformis, Mitragyna inermis, Crataeva adansonii, Celtis integrifolia, Acacia sieberiana and A. campylacantha. The dark-cracking clays of the Raqaba catena have typical Acacia seyal-Balanites savanna. The other areas of the catenas generally do not bear woody vegetation but are mostly grasslands.

        • Vegetation of Jebel Marra. The lowest zone of Jebel Marra, up to 2 000 meters, is largely cultivated with Cordia abyssinica, Thespesia garkeana and Ficus spp. In the middle zone, between 2 000 and 2 500 meters, Olea laperrini is the most common tree on older soils, with Acacia albida on immature volcanic tuffs. Ficus palmata is common in valleys and sheltered places, with Salix safsaf near watercourses. The tree vegetation is, however, very sparse and most of the zone is open grassland. Above 2 500 meters, there is short grassland with very scattered Olea trees. Jebel Marra was, until fairly recently, much more densely populated than it is today and it is possible that much of the forest there has been destroyed in past centuries by cultivation and grazing.

        • The high land of the flood region. Situated in south-eastern Sudan with a rainfall of 750 mm or more, these sand or sandy-loam plains, which may only be a meter or two above the general level, are not flooded in normal years. Four types of forest are distinguished:

          • Palm type, with Hyphaena thebaica generally and Borassus aethiopium locally dominant;
          • A poorly developed broadleaved type, similar to that of the high rainfall woodland savanna, but poorer in species;
          • The mixed Acacia dominated type in which Acacia sieberiana is the dominant species; and
          • Some areas of Acacia seyal-Balanites savanna occurring on clay soils.

        The "intermediate land" and the swamp of the same flood region cannot be considered as bearing woody vegetation. Only occasionally are trees found at their upper level (Acacia spp., Balanites aegyptiaca, Mitragyna inermis, Crataeva adansonii, Ziziphus mauritiana and Aeschynomene elephroxylon).

last updated: Wednesday, October 6, 2004