A. L. GRIFFITH
Silviculturist, East African Agricultural and Forestry Research Organization, Kenya
This article is a condensation of the chapter in Africa from a forthcoming FAO monograph on acacia's and related species. The chapter has special significance the working party an forestry in savanna are set up by the first session of FAO's African Forestry Commission. There are three other relevant FAO publications:
Tropical Silviculture, Volume I (Forestry and Forest Products Study No. 13) which contains an interesting section on the dry deciduous forest, of which the savannas constitute an important part;Tree Planting Practices in Tropical Africa (Forestry Development Paper No. 8) particularly as to seed, nursery, and planting operations, and the section on selection of species; and
Choice of Tree Species (Forestry Development Paper No. 13) especially as to tropical lowland climates.
THE dry woodlands of Africa, south of the Sahara, occur in two zones and together occupy an area of over 5 million square miles (13 million square kilometers).
The first area is a long comparatively narrow strip in the Northern Hemisphere which extends roughly from the Atlantic Ocean in the west to the Indian Ocean in the east in the Horn of Africa. This strip occupies about 2 million square miles (5.2 million square kilometers) and is about 3,500 miles from west to east (5,600 kilometers) and about 600 miles (1,000 kilometers) from north to south. It lies roughly between latitude 5°N. and 23°N. Its vegetational characteristic is the predominance of Acacia species particularly in the center and the east. The only indigenous Prosopis species of Africa (P. africana) occurs throughout most of the- belt.
The second area is in the Southern Hemisphere and is a roughly circular area of some 3 million square miles (7.75 million square kilometers) in Central and South Africa. It occurs mainly in Tanganyika, Nyasaland, Northern and Southern Rhodesia, the Congo, and in Angola and Mozambique. It lies roughly between latitudes 5°S. and 17°S. Its vegetational characteristic is the lack of importance of Acacia species (except for Acacia albida) and the predominance of the Brachystegia-Isoberlinia-Julbernardia complex of many species.
In these areas the woodlands are usually of vital importance to the local population (as in Somalia) or to large local industries such as the Copper Belt of Northern Rhodesia and the Congo.
These African woodlands have been considered by the Commission for Technical Co-operation in Africa South of the Sahara (CCTA) and the Scientific Council for Africa (CSA), by a committee of the CCTA Pointe Noire Conference in 1958, and by a meeting of specialists set up by the Pointe Noire Conference, at N'dola in Northern Rhodesia in 1959. Among other things, this latter meeting listed 80 of the more important species of the African woodlands about which it considered that detailed information should be collected and on the silvics and silviculture of which research should be directed.
The list of species decided on by the N'dola Conference is as follows:
Acacia albida, A. campylacantha, A. nigrescens, A. sieberiana, A. tortilis
Adansonia digitata
Afrormosia angolensis
Afzelia quanzensis
Albizzia adianthifolia, A. antunensiana, A. versicolor
Anogeissus leiocarpus
Baikiaea plurijuga
Berlinia giorgii
Brachystegia allenii, B. boehmii, B. bussei, B. floribunda, B. gossweileri, B. longifolia, B. manga, B. microphylla, B. spiciformis, B. tamarindoides, B. taxifolia, B. utilis, B. wangermeeana.
Burkea africana
Canarium schweinfurthii
Chlorophora excelsa
Chrysophyllum bangweelense
Colophospermum mopane
Combretum laxiflorum, C. molle, C. mechovianum
Commiphora mollis
Cryptosepalum pseudotaxus
Daniella alsteeniana
Dialium englerianum
Entada abyssinica
Entandrophragma delevoyi
Erythrophleum africanum, E. guineense
Fourea saligna
Guibourtis coleosperma
Isoberlinia angolensis, I. dalzielli, I. doka, I. tormentosa
Julbernardia globiflora, J. paniculata
Kirkia acuminata
Lophira alata
Marquesia acuminata, M. macroura
Monotes kerstingii
Oldfieldia dactylophylla
Ostryoderris stuhlmannii
Parinari curatellifolia (mobola), P. excelsa, P. polyandra
Philippia bengualensis
Piliostigma thonningii
Pseudoberlinia baumii
Pseudolachnostylis maprounifolia
Pteleopsis anisoptera, P. myrtifolia
Pterocarpus angolensis, P. chysothrix, P. stevensonii
Ricinodendron rautanenii
Sclerocarya caffra
Sterculia quinqueloba
Syzygium guineense
Terininalia mollis, T. sericea
Uapaca nitida, U. kirkiana, U. somon
Zeyherella magalismontana
To this list should be added Prosopis africana (Africa's only indigenous Prosopis) which occurs generally in Senegal, Gambia, Guinea and countries north of the Equator but south of the Sahara, right across the Northern Belt and even into Uganda. Prosopis fischeri (the Pseudo-prosopis fischeri of the Tanganyika check list) might also be included. It is a miombo* species found in the Congo and Northern Rhodesia but it is probably not a Prosopis species at all.
* "Miombo" is a Tanganyika-Swahili word which originally was used for one species (Brachyategia boehmii) but it is now used for all the combinations of the Brachystegia-Isoberlinia-Julbernadia complex.
In the following sections some of the typical woodlands of Africa are described in some detail.
FIGURE 1. - Tropical and subtropical dry forests.
The Nigerian dry forests extend over some two thirds to three-quarters of the area and cover about 350,000 square miles (900,000 square kilometers), roughly 700 miles (1,100 kilometers) from east to west and some 500 miles (800 kilometers) from north to south. The rainfall varies from about 10 inches (250 millimeters) to 40 inches (1,000 millimeters) and there is, in general, a rainfall gradient from north to south.
Temperatures are not particularly severe (as is usual in similar dry forests further to the east), and it is rarely that they go over 105°F. (41°C.) or under about 45°F. (7°C.). The woodland formations are usually at altitudes from sea level to about 3,000 feet (900 meters) and are characterized by a long dry season and considerable fire hazard. Keay in several publications since 1949 has differentiated this general vegetation type into a number of subtypes. Going north from the wetter forests to the drier woodland he distinguishes the following zones: (1) Southern Guinea, (2) Northern Guinea, (3) Sudan and (4) Sahel. These general vegetation zones were described for the savanna regions of West Africa by the French botanist Professor Chevalier in 1900 and, apart from comparatively minor modifications, his differentiations are still recognized.
Southern Guinea zone
Typical species are Daniellia oliveri, Hymenocadia acida, Vitex doniana, Detarium senegalense and Afzelia africana. The typical vegetation is an open savanna woodland with trees up to 40 to 50 feet (12 to 15 meters) high with short boles and broad leaves. Heights of up to 100 feet (30 meters) are sometimes but rarely found. A characteristic of the type is that of fierce fires in the dry season. Most of the species have a thick bark capable of resisting grass fires and they possess remarkable powers of Bending up fresh sucker growth after the fires.
Northern Guinea zone
This zone is a northwestern extension of the miombo (Brachystegia-Isoberlinia) woodlands of East and Central Africa. The main species are Isoberlinia doka, I. tomentosa, Monotes kerstingii, and Uapaca somon while Swartzia, Combretum, Protea, Lannea and Paronari also occur. This vegetation is some 30 to 45 feet (9 to 14 meters) high and the tree crowns are almost touching. After felling there is usually profuse regeneration from coppice and suckers.
Sudan zone
This zone is generally covered by a drift of sand over gneisses and other igneous rocks. Typical species are, of the larger trees 25 to 50 feet (8 to 15 meters) high, Anogeissus, Sclerocarya, Balanites, Lannea and Prosopis africana, and of the smaller trees up to 20 feet (6 meters) high, Combretum, Strychnos, Acacia senegal and A. seyal.
Sahel zone
This zone occurs in the extreme, northeast of the group and is comparatively small in area. The rainfall is generally less than 20 inches (500 millimeters). The vegetation is up to about 30 feet (9 meters) high. Typical tree species are Acacia raddiana, A. senegal, A. seyal, A. laeta, and Commiphora africana with shrubs of Salvadora, Leptadenia and several species of Grewia.
These vegetation types are vital, to the local population for their fuel and charcoal, building poles, and fodder for their animals. They also have a great shelter and soil conservation value. Some minor products such as gum are harvested. They are essentially "local" woodlands.
The management of these dry areas is usually that of clear felling controlled by area and the regeneration is by coppice and suckers. Fire is the great danger and in the north is controlled by deliberate early burning.
In many parts of the savanna, artificial regeneration by plantations has been attempted, but this work must still be considered to he in its infancy. Species that have been successful so far are Azadirachta indica, Gmelina arborea, Tectona grandis, Cassia siamea, Dalbergia spp., Eucalyptus spp. The object of management is the production of fuel and poles on a short rotation of about 10 years.
In the natural savanna the rotation is usually from 20 to 30 years. Yields are generally low but very variable from 1.5 to 15 cords per acre (200 to 1,900 stacked cubic feet per acre or 2 to 22 stacked cubic meters per hectare).
The Sudan covers an area of nearly 1 million square miles (2.6 million square kilometers) and is included between latitude 4°N. and 22°N. and longitudes 22°E. and 39°E. Most of the woodland is in the southern two thirds of the country. The rainfall of this woodland area generally speaking increases as one goes south from about 8 inches (200 millimeters) to about 50 inches (1,400 millimeters). Most of the rainfall comes in the summer months of May to October. Temperatures are extreme and vary from about 120°F. (49°C.) in the hot weather to frost or near frost in the cold weather. The general elevation of these woodlands is from 1,000 to 3,000 feet (300 to 900 meters).
Going from north to south, we have in general clay plains in the east and sands in the west in the more northerly areas, and red ironstone soils in the south and southwesterly areas.
Smith (1949) divides these Acacia woodlands into a number of types:
1. Desert down to a rainfall as low as 2 inches (50 millimeters) with Acacia flava (syn. A. ehrenbergiana) as the type species: on favorable sites associate species are Capparis, Maerua and Leptadenia.
2. Acacia desert scrub. This formation stretches from 2 inches (50 millimeters) rainfall westwards to about 17 inches (400 millimeters) on the clays and heavy loams and about 10 inches (250 millimeters) on the sands. The type species is Acacia tortilis and with it often occur A. raddiana, A. flava, Maerua, Capparis and Boscia.
3. Acacia short grass country. In this formation A. mellifera is the type species on the clay soils and A. senegal (which yields the true gum arabic) on the sands. Other associated species are Albizzia, Commiphora and Combretum.
4. Acacia tall grass country. This formation usually occurs in rainfalls of about 18 to 35 inches (450 to 900 millimeters): the type species on the dry soils are A. mellifera, A. fistula, A. senegal, A. seyal and Balanites aegyptiaca.
5. Mixed deciduous fire-swept forest. In this type, a wetter type, the main acacia is A. campylacantha. Associate species are Tamarindus, Anogeissus, Piliostigma, Sclerocarpa, Albizzia. Prosopis africana, Isoberlinia doka and Uapaca spp. often occur in this zone.
Smith (1949) also lists the type species by rainfall and soils thus:
|
Rainfall (mm.) |
Type species in. |
|
All on clays |
|
|
4 (100) |
Acacia flava, A. orfota, A. tortilis, A. raddiana. |
|
8 (200) |
A. mellifera, A. fistula (syn. A. seyal var. fistula), A. senegal (syn. A. verek), A. seyal, A. drepanolobium, A. campylacantha, A. sieberiana (syn. A. verugera), A. albida, A. hebecladoides, A. seyal var. multijuga. |
|
47 (1,200) |
A. abyssinica. |
|
|
On red ironstone soils |
|
40 (1,050) |
A. hebecladoides, Isoberlinia doka, Vitex cuneata, Prosopis africana, Anogiessus schimperi, Strychnos spinosa. |
|
55 (1,400) |
Erythrophleum guineense. |
These woodlands are of extreme importance to the local peoples and their animals. They also have an important soil conservation value. They are worked for their fuel, charcoal, fodder, and minor products such as the gum arabic from A. senegal.
Regeneration is usually natural by wed, suckers or coppice. Two species that are reproduced artificially are Prosopis juliflora which does well in light soils in a rainfall as low as 4 inches (100 millimeters) and A. arabica (the babul of India) which likes river land which is subject to inundation. Both these species have many uses.
The main danger to these woodlands is fire, and they owe their purity to their superior fire resistance. A. seyal resists the heaviest fires by its fleshy bark. A. mellifera suppresses in its shade the grasses which the soils and rainfall are suited to growing. Erythrina tomentosa, Cussonia arborea, Diospyros mespiliformis and Balanites aegyptiaca resist fire by extreme formation of corky bark.
This area covers the Republic of Somalia and the Ogaden area of Ethiopia. Together they make up what is commonly called the Horn of Africa, and hence are considered together. Generally speaking the area is inhabited by pastoral nomads who are entirely dependent on the scrub for their day-to-day existence, and that of their animals. The former Somaliland Protectorate and Somalia are well described by Lawrie (1954) and Mooney (1958 and 1959). Little is on record of the Ogaden of Ethiopia and figures are not available.
The most northern area is approximately 70,000 sq. miles (110,000 sq. km.) and lies between latitudes 8°N. and longitudes 43°E. and 49°E. South is a coastal strip which runs down to Kenya about 200 miles (325 km.) wide and some 1,000 miles (1,600 km.) long. It lies between latitudes 12°N. and 2°S. and longitudes 41°E. and 51°E. The Ogaden of Ethiopia fills in the western boundary and is of about the same area as was Somaliland. The nomadic pastoralists know no political boundaries and wander from area to area.
Interesting figures of the population, human and animal are given below:
|
Former Somaliland Protectorate |
Former Somalia |
|
Area: 68,000 square miles (110,000 square kilometers) |
Area: 185,000 square miles (460,000 square kilometers) |
|
Population: 700,000 |
Population: 1,250,000 |
|
Camels: 1.2 to 2.5 million |
Camels: 1.4 million |
|
Cattle: 1.2 million |
Cattle: 850,000 |
|
Sheep: 2.4 million |
Sheep: 650,000 |
|
Goats: 1.6 million |
Goats: 3 million |
The annual rainfall is meager and erratic and in general about 10 inches (250 millimeters) even failing as low as 2 inches (50 millimeters). At higher elevations it may be as much as 25 inches (650 millimeters) and in favorable situations in the hills may even reach 40 inches (1,000 millimeters).
The altitude of the area is roughly from sea level to about 2,500 feet (760 meters). Temperatures are extreme and may be up to 110°F. (43°C.) in the hot weather with near frost temperatures in the cold weather. Very long, completely dry hot spells are the rule.
The species are in general Acacia and Commiphora very rarely over 10 feet (3 meters) high. In Somalia the most important acacias are probably Acacia bussei, A. tortilis, A. spirocarpa and A. etbaica. These are associated with Dobera, Salvadora, Boscia, Cadabra, Maerua, Balanites, Zizyphus and Dichrodachys species which often assume local importance. In Somaliland the important species are Acacia bussei, A. mellifera, A. senegal, A. misera, A. spirocarpa, A. etbaica, A. arabica, A. circummarginata and A. unispinosa.
Logan (1946), lists as occurring in or introduced into central and southern Ethiopia, Acacia abyssinica, A. albida, A. farnesiana, A. nilotica, A. seyal and A. stuhlmanii.
Little or nothing is known of the insect, fungal or virus pests and diseases of these areas nor of the growth and yield.
The main needs of forest development of these areas have been stated by Mooney (1959) as follows:
(a) the protection and management (including range management) of some of the better quality Acacia areas especially near towns to serve as shelterbelts and a permanent source of firewood and charcoal on a sustained yield basis (A. bussei, A. spirocarpa and A. tortilis are suggested);(b) improvement of pasture and browse outside the forest reserves by means of rotational grazing and the provision of water tanks at suitable widely separated points to disperse the livestock and spread out the incidence of grazing;
(c) the improvement of the frankincense, myrrh and gum arabic business;
(d) protection of the country against soil erosion;
(e) the conservation of a scanty and capricious rainfall.
This semidesert scrub woodland is of extreme importance for its soil conservation properties and its destruction would he catastrophic. The rapidly increasing population, both human and animal, make the danger of its destruction more serious and more imminent.
The woodland is used chiefly for pasture and browse and for the building of temporary huts, cattle pens, firewood and charcoal by the nomads. A detailed list of the uses of the many acacias in Somaliland is given by Lawrie (1954). In addition, there are some minor products from this area, the trade in which is thousands of years old. Frankincense from Boswellia species, myrrh from Commiphora species, and gum arabic from Acacia senegal are all mentioned in the year 500 B.C. by Herodotus who, talking of the Red Sea trade, said that it was in the hands of Egyptian and Phoenician traders. He also mentions cinnamon and cassia (senna pods) from South India.
FIGURE 2. - Deformed Brachystegia forest coppice on poor site.
Courtesy: Figures 2 to 5, C. R. Hursh, ICA
The northern frontier of Kenya occupies an area of 117,000 square miles (300,000 square kilometers) out of a total area of the territory of 220,000 square miles (560,000 square kilometers), i.e., it is some 53 percent of the territory (1948 Population Census published by the Kenya Government in 1953). Its northern boundary for some 500 miles (800 kilometers) is contiguous with the Ethiopian border and it extends south for about 200 to 300 miles (325 to 500 kilometers). It is generally between latitudes 1°N. and 4°N.
It lies at a general elevation of 1,000 to 2,500 feet (300 to 750 meters) but contains occasional high ground up to 8,000 feet (2,400 meters). The general rainfall is from 8 to 30 inches (200 to 750 millimeters) but most of the area is in the 8- to 15-inch belt (200 to 375 millimeters). This rainfall is very variable and there is a fluctuation of about 50 percent about the mean. Temperatures are extreme and vary from over 100°F. (38°C.) to near frost. The rain usually occurs in heavy thunderstorms and very long dry hot spells are normal.
The vegetation of the area may be described as a semidesert thorn scrub. It consists of Acacia spp. of which A. drepanolobium is frequent, Commiphora spp., Phyllanthus spp., Euphorbia spp., Salvadora persica, Dodonea viscosa and sagebrush (Atriplex spp.).
Silviculture is scarcely practiced in the area and the management is an attempted control of the nomadic pastoral population that is dependent on the vegetation (as in the Somali Republic already described).
Little or nothing is known of the insect and fungal pests (except locusts). Fire is not a hazard as the scrub is too open generally to permit continuous fires.
This scrub woodland is extremely important to the local nomads as they and their animals are entirely dependent on it for their day-to-day existence. It also has another very important effect on the economics of Kenya. The vegetation supports a large population of wildlife and game such as elephant, rhino, buffalo, pig, kudu and lion. This attracts tourists, both hunters and photographers, from all over the world and hence is a very important economic asset.
The need for the protection, conservation and management of this large area is shown by the fact that eland, zebra, oryx, buck and ostrich are rapidly disappearing. It is a question of the survival of the biggest, the roughest and the toughest both in fauna and flora.
These combinations of the Brachystegia-Isoberlinia-Julbernardia complex form a compact area of some three million square miles (7,750,000 hectares) in central and south central Africa - probably one of the largest compact areas of one forest type in the world. It occurs between latitudes 5°S. and 25°S. and hence is practically wholly in the southern tropics. From west to east at its greatest width it stretches almost from the Atlantic Ocean to the Indian Ocean. It occurs in Southern Tanganyika, Nyasaland, Northern Rhodesia, Southern Rhodesia, the Congo, Mozambique to the east, and Angola to the west. It also extends in a modified form into South Africa. It is typified in the woodlands of Northern Rhodesia which have a rainfall of some 45 inches annually (1,150 millimeters). This rainfall is very variable from 25 to 50 inches (625 to 1,270 millimeters). There is a long dry hot season of from six to eight months. Temperatures vary from 60°F. to 80°F. (16°C. to 27°C.) in the cold season although there is occasional frost, to 80°F. to 120°F. (27°C. to 49°C.) in the hot season.
The general range of altitude is from 3,000 to 5,000 feet (900 to 1,500 meters).
The soils in this area are generally the following:
(a) red earth - sandy and sandy clay soils;
(b) plateau soils (with laterite) which usually carry the miombo woodland;
(c) Kalahari sands which carry the Burkea, Baikiaea and Cryptocephalum woodlands;
(d) upper valley soils which usually support maize;
(e) lower valley soils with low rainfall and extremes of temperature which usually carry mopane (Colophospermum) Acacia and Combretum;
(f) the lake basin soils (Chipya) which are in general used as the most suitable for the introduction of exotics.
The main species of the type have already been listed but they are in general of the genera Brachystegia, Isoberlinia and Julbernardia. In addition, the bamboo Oxytenanthera abyssinica is common.
The silviculture of these woodlands is usually simple and the methods of working depend entirely on local conditions. For example, in the Copper Belt of Northern Rhodesia and the Congo the difficulty is to prevent the woodlands being destroyed, whereas in Tanganyika the difficulty is to find ways and means to work them. Yields are low and of the order of 50 cubic feet per acre (0.6 cubic meters per hectare) in forests that have been previously worked. The value of the produce is low also but it is absolutely essential if the mining of copper and uranium is to go on.
Little is known of the insect posts or of the mycological and virus diseases of these woodlands.
The general method of management is that of clear felling with natural regeneration by coppice and suckers. The rotation will probably be of the order of 100 years.
The clear felling is followed by strict fire protection. Complete fire protection is usually not possible and the system of "early burning" is generally followed. Intensive experiments on the effect of (a) complete fire protection, (b) early burning and (c) late burning, were started by C.E. Duff in 1933. These have been evaluated by Trapnell and written up in the Journal of Ecology in 1959, that is, after 26 years.
Utilization in these woodlands varies enormously. In the mining area of the Copper Belt of Northern Rhodesia and the Congo, as mentioned above, the difficulty is to stop massacre of the woodland whereas, for example, in Tanganyika the difficulty is to find utilization.
It must be emphasized that in copper mining very little timber is used as conventional pitprops. The Brachystegia is used as baulks often in the round, in sizes about 12 by 12 inches (0.3 by 0.3 meter) and some 12 to 16 feet (3.5 to 5 meters) long. Baikiaea is used in smaller sizes and usually for railway sleepers in the underground electric railway. It is often used green and untreated and has survived many years under appalling conditions of alternating wetness and dryness.
Another essential use of local timber in the copper mines is that of "smelter poles" (which must be freshly felled and green) for the de-oxygenization of the ore in its initial smelting. These are used about 22 feet (6.7 meters) long and of 12 to 22 inches (0.3 to 0.6 meter) diameter. They are often of "local" exotics such as Acrocarpus. In addition a large amount of fuel is needed in mining areas.
In Tanganyika, with no mining industry, these miombo forests have been developed for beekeeping (as is the case with the dry forests in southern Greece) and today Tanganyika is one of the largest exporters of honey and beeswax in the world. - Most of the export goes to the Far East, particularly to Japan.
The Tanganyika miombo woodlands differ from those of Northern Rhodesia in that although they have the same general rainfall range of from 25 to 50 inches (635 to 1,300 millimeters), their range of elevation is much wider, being from about 1,000 to 6,000 feet (300 to 1,800 meters).
Another big difference is that the Tanganyika miombo contains a comparatively valuable species Pterocarpus angolensis (locally called muninga). This is, however, sparsely scattered but, owing to its value, its distribution and silviculture is being intensively studied. Except in areas like the Copper Belt, the utilization of miombo timbers (except such valuable species as Pterocarpus angolensis) is difficult. The main problems are (a) the low stocking, (b) the low proportion of valuable species and (c) the refractory nature of most miombo species. (The correct type of saws, good maintenance, sufficiently heavy equipment and adequate power are essential for both milling and woodworking.) Seasoning and preservative treatments are both difficult but essential.
Much information should be collected on the botany, silviculture, management and utilization of this extensive forest type. Initial work should be concentrated on:
1. Information
Complete interchange of the information already in existence.
2. Silviculture
(a) Improvement of enumeration techniques.
(b) Effects of fire on regenerating exploited areas and on the quality of maturing timber.
(c) Effects of competition on the economically valuable species.
(d) Afforestation of miombo country with alternative species.
(e) Ecology of Pterocarpus angolensis with special reference to the possibility of obtaining a greater concentration.
(f) Artificial regeneration of P. angolensis.
3. Utilization
(a) Increased use of secondary species particularly Brachystegia spp. and Isoberlinia spp.
(b) Improved efficiency of haulage to counter the present low stocking of useful timber.
(c) Mobile milling to reduce the amount of waste material (sawdust and off cuts) hauled.
4. Beekeeping
This is being extensively studied in Tanganyika and the work should be extended.
In the previous subsection on the miombo woodlands, it has been stressed that the yield is low and that in mining areas such as the Copper Belt the demand for forest produce is very high. Many attempts at replacing the miombo by fast-growing exotics have been made, many exotic species have been tried and many failures have been experienced. This has resulted in an intensive study of soil conditions. So far, the most successful results have been obtained on the chipya areas. ("Chipya" is a Bantu word that means "heavily burned." It is either applied to a particular type of soil or to the vegetation that grows on it.)
The vegetation usually consists of various trees other than Brachystegia and Isoberlinia which grow mixed in very tall grass (hence the fires). The botanical evidence shows, from the evergreen nature of the relicts, that this-type of vegetation was once much more extensive. Such species as Syzygium, Steganthus, Chrysophyllum and even Entandrophragma occur. There are also scattered stands of Marquesia which give way to Brachystegia hockii woodland and fringes of Uapaca spp.
Beneath the surface, which is often quite different from the lower layers, is a deep gray-brown to chocolate-colored, or even blackish humic soil with a rather loose structure possessing a definitely sandy base, which is so richly humic as to be termed a loam. It is a definitely acid soil (Trapnell, 1953) with a pH value of 4.2 to 4.8 in the first two feet (0.6 meter).
The most promising exotics tried so far (after some 25 years' experience) are Pinus insularis from the Philippines, and P. michoacana from Mexico. But mention must also be made here of the introduction trials with eucalypts. The most successful species to date are Eucalyptus citriodora and E. saligna (this is, however, of South African origin and is therefore probably a hybrid of E. saligna and E. grandis with the latter greatly predominating). The eucalypts are raised by a technique locally called chitimene which corresponds to the rab method of India. It consists of a clear felling of indigenous bush followed by a collection and stacking of the slash in either groups or contour strips. This slash is then burned and the eucalypt seed is sown broadcast in the resulting ash accumulations. This method is often combined with the raising of field crops such as millet, ground nuts and beans. It is in effect a modified taungya technique. The local names are quoted as they are often found in literature without explanation.
These forests and woodlands have been ably described by Martin (1940), and by Watson (1950) in his working plan for the period 1951 to 1957. The species which belongs to the family Caesalpiniaceae is, of course, not to be confused with the true teak of Asia (Tectona grandis). It is confined more or less to the deep sands of Northern and Southern Rhodesia, northern Bechuanaland and the Kaprivi strip, and southeast Angola. This region is the northern continuation of the Kalahari region of South Africa and was part of a late tertiary desert that covered the greater part of southern Africa. In the southwest of Northern Rhodesia it occupies some 11,000 square miles (28,000 square kilometers) in Barotseland. These Barotseland woodlands lie approximately between latitudes 16°S. and 18°S. and longitudes 23°E. and 25°E.
The rainfall is approximately 20 inches (500 millimeters) but is very variable and can be from about 12 inches (300 millimeters) to 40 inches (1,000 millimeters). The temperature varies from mean maxima of 90°F. to 95°F. (32°C. to 35°C.) in winter. In summer, extremes of 110°F. to 120°F. (43°C. to 49°C.) have been recorded and in winter severe frosts may occur locally. The general altitude is from 3,000 to 4,000 feet (900 to 1,200 meters).
Martin (1939), as a result of many analyses, divided the soils into two types:
(a) upper Kalahari sands, coarse and relatively uneroded, having about 20 percent fine sand in the top foot, low water-retaining power, capillarity and water table, and therefore able to support only a relatively poor open savanna woodland;(b) lower Kalahari sands, eroded, with up to 50 percent fine sand in the top foot, high water-retaining power, capillarity and water table and therefore able to support a well-grown comparatively closed forest.
The feature of these forests and woodlands is the almost pure nature of the Baikiaea plurijuga stands whether as forest, woodland or savanna. In some of the better areas stands of up to 50 stems per acre (20 per hectare) occur with clean almost cylindrical boles up to 35 feet (11 meters) of the total height of some 60 to 65 feet (18 to 20 meters). There is a dense deciduous undergrowth of woody climbers and scramblers chiefly of Combreturm, Canthiums and Dalbergia glandulosa. Grass and herbaceous growth are almost absent. Pterocarpus stevensonii often occurs as a subdominant and sometimes as a codominant.
In this woodland type (as opposed to the forest type) Martin distinguishes (a) a mixed woodland where the Baikiaea has as associated species Copaifera coleosperma, Burkea africana, Pterocarpus angolensis Ricinodendron rautanenii and occasionally Brachydegia spiciformis, and (b) a pure woodland with few tree species other than the Baikiaea. This has a dense undergrowth but no climbers.
The exploitation of these woodlands has been through selection felling and the main effect of this has been the destruction of the fire-resisting qualities of the natural forest. The canopy is broken or removed, crown slash and branch wood is left on the ground and a heavy crop of grass comes in annually. This has necessarily led to a need for a fairly intensive fire protection by (a) patch burning (early burning), (b) boundary fire protection by external fire lines and (c) concentrated fire protection by internal fire breaks.
Full seed years occur at intervals of two to four years and abundant seed is produced. Trees in the open flower annually. The pods burst with a sharp report and the seed is flung to a distance of some 35 yards (32 meters). No other means of dispersal is known; the seed does not float and is destroyed if eaten by birds or animals. The tree starts to seed at about 30 years of age. The seeds are about 90 percent viable and germination is rapid when the rains break. Regeneration is often profuse but rodents eat the seed and young seedlings. This, combined with dry season casualties, often results in poor survivals after a year. The seedling sends down a long taproot which reaches a depth of six feet or more at the end of the first rains. At maturity this taproot may be 30 feet (9 meters) or more in depth. Growth above ground is slow and may be 5 inches (13 centimeters) in the first year and 12 inches (30 centimeters) in three years. Stumps under 12 inches (30 centimeters) in diameter coppice and sucker freely. Coppice and sucker regrowth is however usually inferior.
Little is known of the insect pests and the fungal and virus diseases of the species. Fire is the main danger and may cause a dieback of the seedlings year after year. Late fires do most damage. Early fires tend to die out at the forest edge.
Yields up to 450 cubic feet per acre (5 cubic meters per hectare) over quite large areas have been recorded but an average figure of some 60 to 100 cubic feet per acre (0.5 to 1 cubic meters per hectare) is probably more normal.
The wood of Baikiaea is hard, heavy, close-grained and durable, brown when first cut and turning red-brown on exposure. Its air-dry weight is 53 to 60 pounds per cubic foot (0.7 to 0.8 kilogram per cubic meter). It is moderately easy to work and takes a fine finish which together with its hard-wearing qualities makes it a valuable and durable flooring timber. Owing to the remoteness of the forests, Baikiaea can only be exploited on a large scale for railway sleepers. The heartwood which forms about 80 percent of the mature log is immune from borer and termite attacks and durable in contact with the soil. The sapwood takes preservative well, but the heartwood is very refractory. In Rhodesian Railways' tests Baikiaea sleepers have shown a minimum life of 15 years. Sleepers put in green and untreated have done extremely well underground in the copper mines.
The output of Baikiaea sawn timber by the Zambesi sawmills in 1938 (as an example) was:
|
|
cu. ft. |
m³ |
|
Railway sleepers |
900,000 |
(25,000) |
|
Flooring strips and blocks |
70,000 |
(2,000) |
|
Other uses |
250,000 |
(7,000) |
The working plan quotes annual yields from 1931 to 1948 varying from 1 to 1.5 million cubic feet (42,000 cubic meters) to over 3 million cubic feet (85,000 cubic meters).
As a final comment on these somewhat remote and comparatively unknown woodlands the following is quoted from the working plan:
"The dependence of the Barotse on their forests is very considerable not only to supply their needs but also to maintain the soil fertility of the sands on which they grow their food crops."
The position is fully discussed by Martin (1941) in his Report on forestry in Barotseland.
This species, Colophospermum mopane (formerly known as Copaifera mopane), sometimes grows to a height of 50 feet (15 meters) or more, and commonly to about 30 feet (9 meters).
In general, it is found in a peripheral belt of Southern Rhodesia but it extends into neighboring countries such as Northern Rhodesia, the southern province of Nyasaland, Bechuanaland and the northeastern Transvaal of South Africa. Its range is roughly from about latitude 150 S. to the Tropic of Capricorn. It is impossible to estimate the area of the type with reasonable accuracy.
It occurs in a rainfall of some 15 to 40 inches (375 to 1,000 millimeters). Temperatures are fairly extreme and vary from a few degrees of frost in the cold weather to over 100°F. (38°C.) in the hot weather. Long periods of drought are the rule rather than the exception and a dry period from April to November usually occurs. The altitude of the type varies from about 1,000 to 4,000 feet (300 to 1,200 meters).
The type is not usually found on the Kalahari sands, but on basalt outcrops in the general area of the sands. The general outward appearance of the mopane soils is that of infertility. The first few inches of soil show a pH of over 6 and from there down to some 6 feet (2 meters) it is extremely alkaline with a pH often over 8.
The forests are essentially local and there is no protection, silviculture or management of the type in Southern Rhodesia.
The type owes its survival to its great resistance to fire and its ability to reproduce itself by coppice and suckers. This reproduction is often slow to "get away". It flowers and fruits irregularly.
The species normally occurs reasonably pure due to its ability to withstand fire, extremely poor soil drainage and the high pH of the soil. When not growing pure, commonly associated species are: Adansonia digitata (baobab), Brachystegia boehmii, Kirkia acuminata, Acacia nigrescens and Commiphora species. Occasionally one finds Afzelia quanzensis, Albizzia anthelmintica and Combretum spp. as associates.
The wood is normally used for local fence posts and firewood. The heartwood is dark red to almost black, and is heavy, hard and durable. Air-dry it weighs about 75 pounds per cubic foot. It is suitable for railway sleepers and flooring blocks. It is extensively used for mining purposes (Pardy, 1953). The leaves have a browse value, and the tree is of great use for protection against soil erosion.
With the recent rapid increase in population and modern agricultural methods which have shown that maize and millets can be grown marginally on these lands, the type is likely to disappear and it is urgent that land use in these mopane areas be put on a sound basis.
The rate of growth of mopane is slow, probably even slower than that of Brachystegia or Baikiaea. A good average d.b.h. increment figure is about 0.1 inch (3 millimeters) per year under conditions of ordinary competition and a comparable volume increment figure is about 5 cubic feet per acre per year (0.06 cubic meter per hectare per year).
Little is known of the insect and fungal pests and diseases of this type, but a very high proportion of mopane trees in Southern Rhodesia are unsound. Hollow boles are common and obvious holes in the stems are even more common. This damage is commonly attributed to fire, but it is more likely to be caused by dieback of the branches followed by heart rot, the former being associated with progressive deterioration of soil conditions (Hodgson, unpublished, 1960).
The position is well described by Jurriaanse in an undated memorandum on fodder trees. In this paper he discussed the merits of other species such as Gleditsia spp., Zizyphus spp., Bauhinia spp., Copparis spp., Tarchonanthus spp., Salix spp., Grewia spp., Rhus spp., Acacia giraffae (Camel thorn), A. albida and various other acacia species, mopane (Colophospermum mopane).
A number of Prosopis species were introduced into South Africa from the drier southwestern areas of the United States and from Mexico to Chile where they grow under rather similar conditions to those of Acacia karoo in South Africa.
Prosopis species were introduced before 1900 and they soon adapted themselves in the drier areas such as the Karoo, Namaqualand and South West Africa. They spread to such an extent both artificially and naturally that today they are regarded as being indigenous. The records of the original introductions have been lost and the Prosopis has hybridized so that today it is almost impossible to distinguish species, let alone seed origins.
Under favorable conditions the Prosopis develops into fairly large trees reaching a height of some 50 feet (15 meters) with stem diameters of 3 feet (0.9 meter). In southern Africa Prosopis does not thrive in high rainfall areas but appears to like a rainfall of about 10 to 20 inches (250 to 500 millimeters) per year. Mesquite, the world accepted name for Prosopis species, prefers soils with a high mineral content. It also likes lime soils in a low rainfall.
In South and South West Africa the main object is the fodder value of the pods and provided that heavy bearing strains can be developed, mesquite, promises to be the ideal fodder tree for and regions as it can be propagated under conditions where practically no other fodder crop can be raised (Jurriaanse). Ceratonia saliqua (the carob bean of the Middle East) needs to be tested more adequately in the dry areas of Africa south of the Equator.
Little is known of how these fodder species should be raised with reference to the stock that they should benefit by their food and shelter. Should they be in irregularly arranged copses or should the arrangement be regular in lines or otherwise? Annual crop yields of fodder of mesquite, vary from about 200 to 500 pounds (90 to 225 kilograms) per tree.
Prosopis is well acclimatized in some parts of the low rainfall areas of South Africa and it is spreading naturally into the veld from undigested seed spread by stock so that it may become a pest, as has happened in other parts of the world, and it may have to be eradicated or at least controlled. This demonstrates the danger of indiscriminate introductions of possibly undesirable species and strains of the genus. The CCTA Phytosanitary Committee for Africa is well aware of this and is urging that all precautions practically possible be taken. It also emphasizes the need for careful consideration before introducing new species or new strains of the species into new areas. Many of the failures of Prosopis introductions have probably been due to using species or strains not suitable to the areas concerned.
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