Shrubs

Scrub types occur in the "semi-desert zone" (the northern half of Kordofan and Blue Nile provinces, the whole of the Khartoum province and about three-quarters of Kassala province and some parts of Darfur adjoining Kordofan). The rainfall is 75 to 300 mm, confined to July and August, and comes as a few local storms or scattered showers with a variable and unreliable pattern. The vegetation is a variable mixture of grasses and herbs, either without any shrubs or more frequently with widely scattered ones about 2 meters high. The grass is not dense enough to allow fires to burn through it annually. Only three sub-divisions have been classified as bearing woody vegetation:

    - Acacia tortilis-Maerua crassifolia desert scrub is seldom homogeneous because of advanced geological erosion. The characteristic tree is Acacia tortilis, which usually has a flat-topped crown and several stems. In the drier parts it is confined to drainage lines, but as the rainfall increases it spreads out from these and in places occupies quite large areas. Some fairly thick even stands occur east of Khartoum. A common associate is Maerua crassifolia while Acacia raddiana, Salvadora persica and Leptadenia pyrotechnica are found in sandy wadis and Capparis decidua, Ziziphus spina-christi and Balanites aegyptiaca in clay drainages. Acacia nubica and Calotropis procera indicate heavy overgrazing.

    -Acacia mellifera-Commiphora desert scrub replaces the previous vegetation type in northern Kordofan and northern Darfur. Ground cover varies greatly. Generally, the bushes occur as only a scattering but they become thicker and more uniform on hollows between "gozes". Particularly near rock outcrops, but also away from these, Commiphora africana and other species of Commiphora are the most abundant bushes, growing in association with Acacia mellifera. Others in varying quantity are Maerua crassifolia, Boscia senegalensis, Acacia raddiana and Lannea humilis. Leguminous shrubs are often present. Generally, but not always, there are more annual grasses than perennials. Blepharis sp. is widely seen in places where the grazing pressure is not too heavy.

    -Acacia glaucophylla-Acacia etbaica scrub is found on the lower slopes of the southern part of the Red Sea Hills. On slopes facing the sea Acacia tortilis is replaced by Acacia glaucophylla and A. etbaica, but on the coast, although this area receives winter and not summer rainfall, Acacia tortilis is again dominant. Delonix elata and Moringa aptera are species rare elsewhere in the Sudan and there are a number of succulents such as Euphorbia cuneata and E. thi, Dracaena ombet occurs in some of the valleys, and Phoenix reclinata near wadis.

    Acacia mellifera thornland occurs on clay in the northern Sudan with rainfall between 400 and 600 mm and in the south-eastern Sudan with a rainfall range between 600 and 800 mm. In the latter area, the rainfall, instead of being largely concentrated in two or three months in summer, is spread out over a longer period. In addition, part of the Toposa area (south-east Equatoria) has been classified as belonging to this vegetation type. Its characteristic tree species is Acacia mellifera, which is found in dense, almost impenetrable thickets separated by areas of open grassland. The Acacia thickets and the grassland appear to alternate in time in a regular cycle. Associated with the Acacia mellifera are Canaba glandulosa, C. rotundifolia and Boscia senegalensis. Acacia seyal is absent, except on water-receiving sites. East of Jebel Dair, there is a peculiar area of low Dalbergia melanoxylon.

    Acacia senegal savanna is found with rainfall of between 300 and 450 mm on stabilised sands in Northern Darfur and Northern Kordofan provinces. This shrubby savanna is characterised by the occurrence of Acacia senegal in almost pure stands that may cover large areas. These often follow cultivation and it appears that, at least in part of its range, Acacia senegal may essentially be a secondary species, the result of re-colonisation after agricultural activity. To a large extent, these stands are then preserved, as they are a source of gum arabic. In the drier parts, the trees associated with Acacia senegal are Leptadenia pyrotechnica, Boscia angustifolia, Acacia albida and A. raddiana. In wetter parts Combretum cordofanum and Guiera senegalensis are frequent. In depressions, on sandy soils into which a little clay has been washed, the baobab tree, Adansonia digitata, is often found, together with Acacia nubica.

    Combretum cordofanum-Dalbergia-Albizia sericocephala savanna woodland occurs on sand in the centre of Darfur and Kordofan provinces with rainfall between 450 and 600 mm. The dominant trees, usually of small size (under 5 meters) and non-thorny, are Combretum cordofanum and Guiera senegalensis on the softer sands, Dalbergia melanoxylon on harder sands, Albizia sericocephala and Terminalia brownii on sands with a hard red layer due to iron salts and, towards the south, with more rainfall, Terminalia laxiflora and Sclerocarya birrea. All these rarely attain great size. Acacia senegal is also present, but not in the extensive pure stands to be seen farther north.

last updated: Tuesday, January 13, 2004