E2. Mopane scrub woodland

Colophospermum mopane scrub woodland occurs in discrete patches throughout all the habitats of the valley floor. These distinct islands of vegetation, which are clearly visible as white patches on aerial photographs, appear to arise due to local soil conditions.

Scrub mopane soils are typically compacted pinkish grey to light grey sandy silt loams over an impermeable calcareous or sodic clay loam B horizon. The xerophytic conditions created by the impermeability of the soil, and the relative alkalinity of the B horizon produce a hostile environment for herbaceous plant species. The resulting paucity of herbaceous ground cover exacerbates the erosion problems in this habitat and, as with vegetation type E1, scrub mopane soils are dissected by numerous drainage channels and erosion ditches.

As in E1, Colophospermum mopane is dominant in this vegetation type, but here its growth is stunted, and it rarely exceeds 3m in height. Characteristic associated shrubs are Commiphora spp., Maerua angolensis (as a shrub) and Lannea humilis. Climbers include Maerua juncea, Hippocratea indica and Cissus spp. The herbaceous layer is sparse. Typical grasses are Aristida spp., Sporobolus cordofanus, Sporobolus panicoides and Eragrostis viscosa. Characteristic subshrubs and herbs include Zanthoxylum chalybeum, Cadaba kirkii, Jasminum stenolobum (as a ground herb or suffrutex), Decorsea schlechteri, Scadoxus multiflorus, Plectranthus tettensis and Cyphostemma spp. Ipomoea kituiensis is locally common.

There are two major factors which cause the shrub growth form of Colophospermum mopane in the valley: browser damage (see above) and/or soil conditions. Where scrub mopane trees are intimately associated with tall mopane trees, as in vegetation type E1 on the alluvial soils in the east of the park, stunted growth appears to be largely due to browser-pollarding. Elsewhere, both near the river and throughout the rest of the park, comparatively large, discrete areas of scrub woodland occur. This is a distinct vegetation type in which edaphic factors are more important than browsing pressure in influencing vegetation physiognomy (Dye & Walker, 1980).