Open forests

Broadleaved

Woodlands

The woodland types consist of stands with two or three layers of trees and shrubs between 8 and 15 m in height. They have a rather open canopy of deciduous trees. The herbaceous cover is variable and contains forbs as well as grasses.

Baikiaea woodland occurs in pure stands in the Chobe District in the north-eastern part of the country. This species grows on deep sands, where it forms the climax vegetation. It may also be found in association with Burkea africana, Pterocarpus angolensis, Erythrophleum africanum, Ricinodendron rautanenii and Guibortia coleosprma. Sometimes these latter species form natural communities with Baikiaea plurijuga. These stands might be considered as a variant from which Baikiaea is sometimes absent because of previous extensive logging operations. The tree and shrub layers are often stratified, and the latter consists primarily of Baphia massaiensis, Bauhinia macrantha, Ochna pulchra and Commiphora mossambicensis, together with the regeneration of the canopy layer.

Baikiaea woodland lies between the 600 mm and the 700 mm isohyets, from the Chobe River in the north to the Ngwezumba River in the south. Areas outside the Chobe National Park where Baikiaea is dominant include the Kachikau enclave and the Kasane Forest Reserve. South of the Ngwezumba River and east of the Mababe depression, the tree canopy opens up and Burkea africana becomes more prominent. Fire, logging and a high concentration of elephants and buffalo on the northern limits of these woodlands have played an important role in their ecological evolution.

Nearly pure monotypic woodlands occur between the 550 mm and 600 mm isohyets on poorly drained soils, such as vertisols and soils of lacustrine origin. A narrow belt of this Colophospermum mopane woodland exists to the east of the Mababe depression and there are other small pockets of this type that is characterised by having bare or nearly bare ground beneath the trees. A poorly developed understorey of shrubs consists of the same species in shrubby form, of Ximenia americana, Grewia spp. and Combretum spp. Some stands are mixed, with isolated trees of Acacia spp., Combretum imberbe and Lonchocarpus capassa. Densities vary greatly over short distances and change to savanna woodland, tree savanna and shrub savanna very rapidly.

Savannas

Where edaphic or climatic conditions become unfavourable, the tree canopy opens up, grasses increase and shrubs and small trees become more abundant in the understorey.

Between the open areas of the south and the woodlands of the north there is a broad belt of mosaic-like vegetation consisting of units of tree savanna, of scattered patches of shrubland or grassland and small units of woodland. In many areas the tree savanna occurs on the ridges with the shrubland or grassland in the depressions. This pattern is characteristic of the transitional zone, although the juxtaposition of the units is more obvious in some areas than in others. Among the commonly occurring tree species are Baikiaea plurijuga, Burkea africana, Terminalia sericea, Ricinodendron rautanenii and Combretum spp. Locally common are Acacia spp., Boscia albitrunca and Lonchocarpus nelsii. Shrubs include Grewia spp., Dichrostachys cinerea, Bauhinia macrantha, Combretum spp., etc. The grass cover is variable, both in density and composition. On finer soils in certain depressions, there are pockets of Acacia woodland, the species being mainly A. giraffae, A. mellifera and A. erubescens.

Tree savanna, with a usually continuous and fairly well developed grass cover, occurs particularly in the east of the country, but also as a buffer zone between the Kalahari shrublands and denser woody vegetation types, such as transition savanna. Trees are scattered throughout the area and include mostly stunted forms of Kirkia acuminata, Acacia spp., Kigelia pinnata, Syzygium guineense, Garcinia livingstonei, Terminalia sericea, Colophospermum mopane (sometimes in open pure stands) and, in a dispersed way, many species that occur throughout the transition zone and in the woodlands. Terminalia prunioides and stunted shrub-like trees of Acacia tortilia are mainly found in scattered groups in heavily grazed grassland, especially north of Kanyu and sough of Gweta. The shrub layer includes species that form shrublands under the more arid conditions of the south (see also "shrubs"). On hilltops, particularly in the Goha, Gubatsa and Shinamba Hills, Albizia hanganyoensis, Steganotaenia araliacea and Adansonia digitata become characteristic.

Although the shrub-tree savanna is predominantly herbaceous, woody components are more frequent than in the shrubland. The greatest development of this type of vegetation occurs between the shrublands and grasslands of the south and the open savanna toward the north. In many areas, the growth of woody plants is greater on the sandy ridges. Among the shrubs Terminalia sericea is very common. Others include Grewia flavescens, Boscia spp., Rhus spp., Acacia spp., Colophospermum mopane and Zizyphus mucronata. Frequent trees, generally less than 12 m high, are Acacia giraffae, Croton gratissimus, Pterocarpus angolensis and Burkea africana. These species occur also in the shrub storey, often showing coppice development as the result of damage. This vegetation type extends into the Kalahari desert.

last updated:  Wednesday, April 28, 2010