Pure forests are rare in savannah and Acacia Seyal and Isoberlinia species are type cases. Gregarious clumps of various sizes are, however, one of the commonest features and a large number of trees is concerned. Examples are the Acacias Seyal, arabica, albida, Senegal and campylacantha; Anogeissus, Isoberlinia, Bauhinia, Anona, Bombax, Terminalia, Combretum, Detarium, Gardenia, Parinarium, Pseudocedrela, Stereospermum, Ximenia, Uapaca, Boswellia, Monotes, etc.
Savannah also shows many examples of a dominant species, the presence of which are indications of the composition of a forest. The presence of this or that species sets a standard of comparative quality to an area by which it may be valued from various standpoints.
Over wide areas of savannah there are to be found many evidences of previous peoples in the shape of walled ruins, foundations of corn stores, grindstones and heaps of hand-picked stones indicative of cultivation. The dating of such remains gives the approximate age of the forest growth, and from this evidence it would appear that certain types of savannah deteriorate considerably with age and that farming will regenerate them to a certain extent. A comparison of 20-30 year old forest with the untouched older forest shows the former to be denser, more healthy and of straighter and more even growth, while the latter contains a large number of very old trees in a state of decay, with great open crowns, liable to be blown down by storms, their branches burned by fires and riddled with fungus. The new growth which is to take their place consists mostly of grasses, stunted shrubs and small trees of species which are incapable of forming a canopy. As mentioned above, provided the area farmed is not too large, the support given it by the surrounding forests is sufficient to ensure effective regeneration, otherwise a poorer type takes hold.
In conclusion a short account of the growth and characteristics of savannah forests is given. As would be expected, the trees, being subject throughout life to extremely hard conditions, counter with defensive measures for self-preservation. Most of them are prolific seeders and many have the habit of retaining their fruits or seeds on the tree for many months, sometimes right round to the next flowering season. Germination of seeds is a matter of chance when the tree is liable to ground fires from October to April or May. The life of a seedling, too, is precarious, since at the age of six months it may be subject to a devastating fire. In later life a fire, occurring in April, may, if the grass is high, burn the bark to ashes on the outside, destroying all fruits or seeds, killing the twigs and small branches and apparently destroying all life. Yet, a month later leaves will spring from all but the burnt tips, the tree losing a year’s height growth, but in a position to put on another year’s stem girth. Wounds on the trunk of a tree heal over with the formation of a hollow or rotten core. Trees 4-5 feet in girth have been felled and their stems found to consist of a mere shell two or three inches thick. The hollow is filled either with the workings of termites or the fermenting sap which is forced up from the ground level and will pour out of the stump. Savannah trees are adapted to overcome most of these adverse conditions. Heavy crops of fruit and seed, rapid ripening of seeds or fire-resisting fruit coats and various means of distribution are means to ensure germination. Deep or thickened tap roots of seedlings are put down in the first year so that though the seedling may be levelled by fire it will appear with renewed vigour soon after. A very short flowering period for the individual tree is a marked characteristic of many species. Fire has a quickening effect on the flowering and leaf bearing of trees to a marked degree. A grass fire in November may produce flowers and leaves in December on trees which, if unburnt, would not have flowered till February. As a rule, no tree will flower again if the first flowers have been burnt off, though it will, of course, bear leaves. One or two exceptions, very rare, have been noticed by the author, but these were due to the peculiarity that some trees exhibit of bearing flowers on some of their branches and not on others, so that the flowerless twigs were probably excited by the fire into bearing flowers after it. Leaves are readily replaced after attacks by locusts which generally take place early in the season. The end of the cold weather and first sign of heat is the spring of growth. Few trees have to wait for rain before they produce all the signs of maturity except the complete growth of their leaves. The bark of young seedlings, subject to fire, is very thick and corky and that of old trees is still more so, while the inner layers are fibrous and full of sap. Almost all species will coppice very well, many throwing up shoots up to 10 feet high in a season, others only a foot or two. Root shoots are also very common and a large number of what are apparently seedlings in the forest are root shoots. Most trees are anchored very firmly to the ground by lateral roots far longer than the height of the tree, a protection against storms. A tree which is completely ringed will often endeavour to join up the cut, meanwhile continuing its season’s foliage, while the smallest connection with the root is sufficient for it to live for many years. Most new leaves, as a protection against the sun, are reddish in colour, many a brilliant crimson, the green being produced through all gradations of colour from the red. Others are hairy, scaly or covered with a bloom, the protective coverings wearing off with maturity. The proportion of sapwood to hard is large and many species show marked differences in this proportion according to their locality, some showing no heartwood in the north where it is produced in the south. The weights of woods are often considerably heavier in the north than is the case for the same species in the south. There are some localities where wide areas are infested with termites to such an extent that, during the dry weather their whole surface, even to the tips of the twigs, is completely covered with the earth carried by the termites. Although there are exceptions, it is observed that apparent damage is negligible and that the old scales are removed from the bark, exposing the fresh surface.
The savannah species are, then, well equipped to withstand fire, drought, insects, wounds, storms, and damage by man and it can be imagined how readily they will supplant better types, since the harder the conditions the better adapted are the trees to meet them. The wide distribution of most species renders them more aggressive, and an individual species that will grow into a fine tree under the best conditions can still grow under the worst, though it may differ in form and feature so as to be hardly recognisable as the same species.
If, as it is supposed, savannah forests are depreciating of their own accord, some factor other than climate, which has not been demonstrated by records, must be at work. It is sufficient to point to the increase and spread of the population from the towns to the forests to find a reason for the displacement of good forest by lower grades. Shifting cultivation encourages a worse type each time it is practiced and late fires are very destructive.
For valuable assistance given in identifying the trees and shrubs in this volume, the author is indebted to Dr. A. W. Hill, Director of the Royal Botanic Gardens, Kew, and Mr. J. Hutchinson of the Herbarium. To Dr. J. M. Dalziel he is indebted for the “Hausa Botanical Vocabulary,” a valuable short-cut to further research. Other books referred to are the “Flora of Tropical Africa” and Kew Bulletin, Additional Series IX., neither of which has been used other than for purposes of classification, it being the author’s aim to describe and illustrate the material as it appears to be, and not only as it actually is from the scientific point of view, avoiding botanical terms except where they have no alternatives.
H. V. L.
March, 1925.