Though much has been written about the various types of savannah forests, a short account of them will not be superfluous here. First it may be pointed out that savannah forests vary so much in their composition and distribution that a survey is practically impossible, and if made, would have to be revised annually. Savannah is a particularly aggressive form of forest growth. Given the opportunity, it at once occupies more land and on its own ground the inferior types oust the superior wherever there is an opening. Any extensive area of cultivation, if abandoned and allowed to regenerate its tree growth, lets in secondary forests containing species which are less exacting as to soil and moisture conditions, and the depreciation of these conditions to the farming methods stops the competitive growth of the trees which formerly occupied the area. Where the area cleared is sufficiently small and the period short enough, the conditions are retained, and are able to close up the clearing with little loss by degeneration.
The fringing forests, belts of evergreen and deciduous trees on the banks of streams, are continually hard pressed by the broad wedges of open savannah between them. Fires take toll of them every year and cultivation not only pushes them down stream from the source but interrupts and cuts them up into islands exposed to threats of extinction.
Savannah is very topographical, and though, within broad limits, it can be divided into two main types, Tree and Bush savannah, the two are so distributed or blended that the differences are often unrecognisable and not able to be recorded in a survey.
Tree savannah is recognised as a tall type having a light, closed canopy, with a sprinkling of under-shrubs and a sparse growth of grass.
Bush savannah is a more open growth of less height, more spreading and lower branched form, with no canopy, and many stunted shrubs and a dense growth of grasses which are normally burnt out each year. There is every variation of this latter type from Combretum and Guiera scrub of barren soils or stony plateaux to the type which closely approaches that of Tree savannah.
The following are some of the recognised sub-types:—
Park Savannah.—This is a natural or artificial formation, the latter a product of agriculture, and is a two-storeyed growth composed of large type trees scattered about and dominating an undergrowth of smaller trees. Type species are Parkia filicoidea, Butyrospermum Parkii, Afzelia africana, Tamarindus indica, Acacia albida, etc. In the artificial product the large trees often owe their presence to their value, having been allowed to stand when the land was cleared for farms, the lower storey being a subsequent growth whose height depends on the period that has elapsed since the farms were abandoned. Natural Park Savannah is well represented by the superior size of Afzelia africana or Paradaniellia Oliveri over the lower storey of Terminalia and other species which cannot attain the height or proportions of these large trees.
Fringing Forests.—These are the narrow belts of forest along stream banks, and are either intrusion of evergreen and other trees from a lower latitude or the remnants of broader belts which have dwindled to a mere fringe. Typical species are Khaya senegalensis, Eugenia guineensis, Adina microcephala, Diospyros mespiliformis, etc., with a number of evergreen shrubs, typically Jasmines. The narrower the belt the lower branched are the trees.
“Kurimi.”—This is the formation found in depressions, where there may or may not be a stream, not necessarily flowing in the dry season. It is an enlarged edition of the fringing belt and more nearly represents the evergreen forest. Trees of large dimensions with long, clean boles are found amidst a luxuriant undergrowth of shrubs and younger trees, which if undisturbed will attain full maturity. “Kurimi” is the chief source of timber in the Northern Provinces and from it such type species as Khaya grandifolia, Chlorophora excelsa, Albizzia Brownei, A. fastigiata, Diospyros mespiliformis, and frequently Oil Palms are obtained.
“Fadamma.”—This is represented by the broad river valley perennially inundated, or a depression of varying size in flat country where water lies during the rains. Typical species are Mitragyne africana, Paradaniellia Oliveri and Borassus flabellifer, the Fan Palm. There is either a dense growth of grass or, in the case of temporary lakes, an area of cracked mud.