The dominance of the genus Baikiaea (Fabaceae, subfamily Caesalpinioideae) makes the KS teak forests distinct from other forest types and woodlands in Zimbabwe. The dominant Baikiaea plurijuga species is endemic to Kalahari sand. The Baikiaea forests are functionally different because of their great depth, high permeability, low water-holding capacity and the extreme nutrient poverty of Kalahari sand. Another feature of these forests is that all of the dominant tree species on Kalahari sand belonging to Caesalpinioideae family are endomycorrhizal (Hogberg and Piearce, 1986).
A number of vegetation types can be recognised in the protected KS teak forests and these have been described by Wilkinson (1941), Mitchell (1960), White (1962), Anon (1964), Fanshawe and Savory (1964), Calvert (1970), Fanshawe (1971), Von Breitenbach (1972) and Rushworth (1975). These vegetation types include:
i. Closed Baikiaea forest or woodland. This vegetation type displays a fairly closed overstorey with a distinct shrub understorey and moderate grass cover. The Baikiaea forest or woodland is the dominant type covering over 90 percent of the Zambezi teak forest ecosystem. The forest occurs primarily on the dominant Kalahari sand ridges. In this vegetation type Baikiaea plurijuga is the principal upperstorey species. Its other upperstorey associates are Pterocarpus angolensis, Schinziophyton rautanenii, Guibourtia coleosperma, Afzelia quanzensis, Combretum collinum, and Erythrophleum africanum. The sub-canopy species are Peltophorum africanum, Erythrophleum africanum, Combretum apiculatum, Strychnos pungens, Croton gratissimus, Pseudolachnostylis maprouneifolia, Swartzia madagascariensis, Diplorhynchus condylocarpon, Peltophorum africanum and Terminalia sericea. The understorey species are dominated by Bauhinia petersiana, Baphia massaiensis, Grewia flavescens, Grewia monticola, Dichrostachys cinerea, Commiphora species and Acacia fleckii.
ii. Open Baikiaea woodland. This woodland has a markedly discontinuous upperstorey. The dominant tree species in the upper canopy include Guibourtia coleosperma, relic Baikiaea plurijuga, Pterocarpus angolensis, Burkea africana and Erythrophleum africanum. This vegetation type includes open scrub or clumps of coppice regeneration of Baikiaea plurijuga and Guibourtia coleosperma.
iii. Dwarf teak woodland. This woodland type is characterised by a dense cover of multi stemmed Baikiaea coppice. Often the overwood is variable, from almost pure stands of Pterocarpus angolensis with occasional Burkea and Erythrophleum to scattered and isolated trees of other species.
iv. “Sinanga” woodland. This vegetation type is characterised by a broken upper canopy of Baikiaea with occasional Entandrophragma caudatum and/or Afzelia quanzensis over a dense thicket understorey of Combretum species. This vegetation type is typically found in Gwampa and Gwaai forests.
v. Miombo woodland. This woodland type is dominated by Brachystegia spiciformis with or without Julbernardia globiflora. The understorey is often open with scattered shrubs and small trees and sparse to moderate grass cover. This woodland type is typical of parts of Mafungabusi, Sikumi, Ngamo, Gwaai and Bembesi forests where shallow sandy loam soils are predominant. Species composition in the upperstorey comprises Brachystegia spiciformis, Julbernardia globiflora, Brachystegia boehmii, Combretum collinum, Combretum molle, Combretum zeyheri, Pterocarpus angolensis and Terminalia sericea. The sub-canopy is composed mainly of Combretum apiculatum, Burkea africana, relic Baikiaea plurijuga and Guibourtia coleosperma, Croton gratissimus, Pseudolachnostylis maprouneifolia, Swartzia madagascariensis, Diplorhynchus condylocarpon and Peltophorum africanum. The understorey species are Grewia monticola, Ochna pulchra, Dichrostachys cinerea, Grewia flavescens, Friesodielsia obovata and Vangueria infausta.
vi. The Kirkia acuminata woodland occurs on shallow loamy soils underlain by rocks near the soil surface. The principal species in the upperstorey is Kirkia in association with Brachystegia boehmii, Peltophorum africanum, Sclerocarya birrea and Pterocarpus angolensis. The sub-canopy comprises species such as Combretum apiculatum, Gardenia resiniflua, Albizia harveyi, Croton gratissimus and Ziziphus mucronata. In the understorey are species such as Grewia flavescens, Bauhinia petersiana and Commiphora mollis.
vii. Colophospermum mopane woodland. Colophospermum is the principal upper canopy species on clayey soils. In the upperstorey, where it is not in a pure stand, it is in association with species such as Combretum imberbe, Combretum hereroense, Acacia nigrescens, Sclerocarya birrea, Acacia nilotica, Loncocarpus capassa and Combretum apiculatum. Sub-canopy species include Terminalia prunoides, Albizia harveyi and Loncocarpus capassa. The understorey is occupied by Grewia monticola
viii. Grasslands. There are two types. One associated with the vlei depression system and seasonally impeded drainage and the other found on apparently well-drained sands and primarily associated with repeated fire. The development of the grass, herb and shrub layer within the KS teak forests varies according to the overstorey layer. Where the tree canopy is densest there is minimal development of grasses, herbs and shrubs. The most common grasses on the Kalahari sand ridge include Aristida species, Pogonarhria squarrosa and Digitaria species. Most of the grass species found in this zone are coarse (sourveld grasses) that are palatable only in the growing season. The vlei depressions support a dense cover of a range of grasses including species harvested for thatch and brooms. Common species include Aristida, Sporobolus, Cyperus, Eragrostis, Pogonarrhia and Perotis. Vleis support significant quantities of thatch grass species, e.g. Hyperrhenia dissolute and Hyperrhenia filipendula.
The structure of the tree vegetation in the protected KS teak forests appears superficially to be relatively uniform over large areas, suggesting a broad similarity in key environmental conditions. Woody plants probably contribute more than 95 percent of the above ground biomass in the forests, with grasses and herbs making up the remainder. The forests typically comprise an upper canopy, a subcanopy layer that is often absent, an understorey of shrubs and saplings and a ground layer of grasses, herbs, forbs and suffrutices. The uniformity in appearance is due in part to the remarkably similar physiognomy of the canopy trees that are dominated by Baikiaea plurijuga.
Differences in forest structure are more apparent on the local scale. The origin of the differences appears to include edaphic factors, mainly soil moisture and soil nutrients (Campbell et al,. 1988); the effects of fire (Calvert, 1986); wildlife impact (Calvert, 1986; Wood, 1986) and past and present land use and anthropogenic disturbances (Huckabay, 1986; Wood, 1986).
The density of woody plants in the forests varies widely, 800-2 700 stems ha-1, and the density varies by forest reserve (FC’s inventories in Fuller, Mzola, Gwaai/Bembesi). Density seems to be related to rainfall, effects of fire, impact of wildlife, logging and anthropogenic disturbances. Tree height appears to be related to moisture availability and soil depth. Canopy dominants, such as Baikiaea plurijuga, Pterocarpus angolensis, Burkea africana, Ricinodendron rautanenii and Brachystegia spiciformis, growing on deep (more than 8 m) Kalahari sand soils, can reach up to 18 m but in general few trees grow taller than 15 m.
The recorded basal area of trees in Fuller, Gwaai, Lake Alice, Mzola and Bembesi ranges from 81.4 to 112 m2 ha-1 (Forestry Commission, 1998; 1993; 1996; 1993). Basal area appears to increase with increasing annual rainfall.
The KS teak forests have a marked seasonality of plant production, growth and reproduction, and highly variable rates of decomposition. These functions are driven directly or indirectly by the strongly seasonal rainfall, a large biomass of big herbivore in some of the forests, human disturbances and frequent fires. Seasonally limited moisture for plant growth and nutrient poor soils influences the patterns of plant production in these forests.
Most woody species and shrubs found in the KS teak forests are deciduous, shedding their leaves during the dry season. The shedding of leaves peaks in July–August. The timing of leaf fall as well as the length of the period when trees are leafless varies from year to year. The driving factor appears to be the prevailing weather conditions and the genetic characteristics of the tree species. In draught years leaves are shed early in the dry season. Where trees are able to abstract subsoil moisture they tend to retain their leaves longer into the dry season.
No complete studies of woody plant production have been done in the protected forests. However, shoot production is known to be dependent on carbohydrates and nutrient reserves stored from the previous growing season (Rutherford, 1984). For most tree species in these forests the current growth is completed early in the growing season. Stands composed of predominantly young or small trees produce more current growth per unit biomass than stands dominated by large trees. There is a lack of data on annual increments in biomass of the woody plants in the deciduous KS teak forests.
