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The status of the woody species of the Sudan and Sahel zones of West Africa
Forest Ecology and Management, 13 (1985) 247--255
247
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE STATUS OF THE WOODY SPECIES OF THE S U D A N A N D SAHEL ZONES OF WEST AFRICA
CHRIS GEERLING
Nature Conservation Department, Agricultural University, Wageningen (The Netherlands) (Accepted 21 May 1985)
ABSTRACT Geerling, C., 1985. The status of the woody species of the Sudan and Sahel zones of West Africa. For. Ecol. Manage., 13: 247--255. The 371 species of trees shrubs and w o o d y climbers of the West African Sudan and Sahel are, in general, widespread and c o m m o n ; only about 20 species are limited to the region. Given the seasonal climate, fire is the major natural ecological factor. It eliminates the fire-sensitive species from the savannas, and favours the grass layer. M a n has had far more influence on the overall vegetative structure than on the precise composition of the flora. Only one species Encephalartos barteri Carruth. can be considered to be threatened. Conservation of the few remaining intact parts of these ecosystems should have high priority.
INTRODUCTION
The vegetation of West Africa, between the Sahara to the north and the rain forest along the coast, has in recent years been the subject of a number of publications covering various ecological aspects (Le Houerou, 1980; Pelissier, 1980; Poupon, 1980; Menaut and Cdsar, 1982). This paper, together with the information in a previously published field guide (Geerling, 1982), deals with the conservation status of the 371 species o f trees, shrubs and woody climbers (attaining a height of at least 1.50 m) of the Sudan and Sahel zones of West Africa, from Senegal to Chad. ENVIRONMENT
The climate o f the region covered by the savanna and by the tropical steppe of the Sahel is characterized by a distinct dry season, which lasts for a few months in the south to up to 10 months in the Sahel. Annual rainfall varies from more than 1 4 0 0 m m in the south to less than 200 mm in the north. Temperatures are relatively low in the early dry season (average m a x i m u m about 34°C and minimum below 20°C) but increase towards
0378-1127/85/$03.30
© 1985 Elsevier Science Publishers B.V.
248 the end of the dry season when t h e y rise to a maximum of 40°C or more and a minimum of over 20°C. The desiccating north-eastern wind (harmattan), which blows during the dry season, is ecologically very important. The landscape consists for the greater part of rolling hill c o u n t r y , with higher elevations of importance only in Guinea (Fouta Djallon), Nigeria (Jos Plateau) and on the Nigeria--Cameroun border (Alantika and Mandara Mountains). The vegetation of b o t h the savanna and tropical steppe has a herb layer with or without an open layer of trees and shrubs. The herb layer -- predominantly grasses -- is continuous and n0n-sclerophilous in savanna, and discontinuous and sclerophilous in steppe (Schnell, 1 9 7 0 - - 1 9 7 7 ; Menaut, 1983 and Monnier, 1973). As the rainfall decreases, so the w o o d y trees and shrubs decrease in height rather than in their amount of ground cover, but towards the Sahara the cover also decreases. The vegetation patterns in these zones are closely related to the horizontal redistribution (run-off and run-on) of rainwater. PHYTOGEOGRAPHY The savanna and Sahel region is best characterized as t h e 'tropical intermediate zone' (Harris, 1980) b e t w e e n the forest and the desert. In West Africa the region is subject to major shifts in climatic zones, which can be related to comparable zones in most of Africa and tropical Asia. Phytogeographically the region can be divided into three distinct zones (White, 1983; Menaut, 1983): (1) the arid north, the Sahel, with a clear affinity with the arid regions of southern Asia. Here, there are 20 typical w o o d y species, o f which 7 also occur in Asia; (2) the 'true' savanna, the Sudan zone, with an affinity with the Miombo and Mopane savannas of southern Africa (Malaisse, 1978). There are here 325 typical w o o d y species, of which 140 are limited to northern tropical Africa, another 140 also occur in southern Africa, 35 are also f o u n d in Asia, 4 in tropical America and 7 are pantropical; (3) the transition zone b e t w e e n the Sudan savanna and the Guinean-Congolese forest with 25 species. None of these is a typical fire-resistant savanna species, b u t most of t h e m may be found in favourable conditions far into the savanna zone. This evidence of species distribution indicates that endemism is n o t an outstanding feature of the West Africa savanna flora. Further examination of the evidence indicates that: (1) none of the 370 species belongs to a family endemic to West Africa. All but 9 species belong to either pantropical or cosmopolitan families; (2) three o u t of 209 genera are limited to West Africa, b u t t w o of them, Bafodeya and Neocarya, have recently been separated from the pantropical
249
genus Parinari, of these genera, 55 are African and 54 (with 156 species) are pantropical; (3) 44 species are endemic to West Africa b u t the endemism is flawed: some have vicarian species elsewhere in the savanna; some m a y eventually be found elsewhere especially in the still poorly prospected Chad and the Central African Republic; some belong to taxonomically poorly defined groups. In all, out of the 44, it seems that a b o u t half are likely to be found to be limited to West Africa. Within the region on particular centres o f endemism occur, b u t firesensitive species found in rocky habitats are relatively well-represented. Brenan (1978) came to the same conclusion for the whole of the West African flora. VEGETATION AND MAN
The history of the vegetation of Africa, south of the Sahara in general, and of West Africa in particular, has been extensively treated by Aubr~ville (1949) and Schnell (1970--77) and will only be summarized here. The area n o w covered b y savanna and steppe in West Africa has been subjected to several shifts in the climatic zones and, consequently, in its vegetation. Evidence for these shifts has been found in fossile dunes, in what is n o w a savanna region in Nigeria, well south o f the desert, and is demonstrated by species, representative o f much more humid zones, which n o w survive in restricted localities within t h e arid zones. Such a shift appears to have occurred as late even as this c e n t u r y (Breman et al., 1984). The northern limits of the distribution of species like Acacia senegal have moved south for something like 200 km under the influence of the recent drought. These species have n o w returned to within the limits where t h e y were found in the 1930s, which is the period when the first reliable observations were made. In the much more humid period b e t w e e n the 1930s and the 1960s, these species became established further north. Savanna vegetation is for the greater part a fire-climax (West, 1965). Fire destroys the litter which suffocates the grasses, and it also kills young w o o d y plants. Thus, fire reduces shading and tips the balance, in the competition for light, in favour of the grasses. The grass-fires are n o w a d a y s practically all lit by man; 'natural' fires are rare. This, however, does not mean that fire is a recent p h e n o m e n o n in the savanna vegetation. Considering the stability, and the richness in species of even the humid Isoberlinia savanna woodlands, savannas must have had a long history of burning. Large herbivores have evolved which are typical of the savanna (McNaughton, 1979) and this provides another indication that savannas are n o t recent in northern tropical Africa. The frequency of burning has u n d o u b t e d l y increased with the increase in the human population, and the extent of the savannas in the more humid zones has also increased, b u t fire m a y have been a significant factor in
250
the past, at least in the drier zones. Aubrbville’s (1949) assertion that the original vegetation of the savanna zones was a dry, closed forest seems not likely to be correct. Especially in the drier regions, a closed but deciduous forest is tinder dry by the end of the season and is therefore highly vulnerable to fire. Even accidental fires, started by lightning, sparks from falling rocks, or the like, occurring decades apart, but then ranging over large areas of unbroken land, would be enough to wipe out the firesensitive species and allow the establishment of the grasses. The main effects of fire on the floristic composition have been the propagation and maintenance of a herb layer, mostly of grasses (see Brookman-Amissah et al., 1980), and the separation of fire-resistant species, covering vast tracts of land, from the fire-sensitive species, limited to moist habitats such as swamps and gallery forests, and to areas with a discontinuous grass layer such as rocky hills and scarps. In the past the influence of man, apart from burning, has been a factor of importance only in the drier zones which are free of the sleeping sickness-transmitting tsetse flies (Glossina spp). Agriculture will have changed the vegetation, at least locally, since cattle are, by their selective grazing, the major cause of the preponderance of thorny shrubs and trees in the drier zones. A certain stability had reigned in the savanna system of West Africa until the onslaught of modern technology. The main effects of modern times upon the vegetation have been a marked increase in the frequency of burning and an increase in human population density, with its accompanying farming, husbandry and firewood collection. This has led to the disappearance of the natural vegetation over large areas. The savanna and Sahel, though in general not particularly heavily populated, have local dense populations (Mossi Plateau in Upper Volta and Kane in Nigeria) and, as everywhere in the tropics, there is a high rate of population increase. The influence of mankind on the vegetation is manifold (Werger, 1983): agriculture, husbandry both semi-nomadic and sedentary (see Figs. 1 and 2), firewood collecting and burning. These developments are in general much more detrimental to the existing vegetation than to the long-term constitution of the flora (see Fig. l), as was indicated in Hedberg and Hedberg (1968). The impact of human activity varies within the three ecological zones: (1) the northern Sahel is ecologically well protected against changes induced by farming practises because of the lack of water in the dry season. (2) the southern Sahel and northern Sudan zone have the highest population densities and these areas have also suffered most from the droughts of the early 1970s. Over-grazing and cutting for firewood have led to the disappearance of the natural vegetation over large areas. (3) the southern Sudan zone and the Guinea savanna are still protected by the presence of diseases like sleeping sickness (transmitted by Glossina spp.) and river blindness (transmitted by Simulium spp.).
251
Fig. 1. Borassus aethiopum in farmland in northern Benin; note stumps of cut trees. ]
Fig. 2. Balanites aegyptiaca o n t h e Mossi p l a t e a u , B u r k i n a Faso, with branches c u t f o r fodder.
252 S T A T U S A N D T R E N D S IN C O N S E R V A T I O N
With regard to the ecological status of the region and the trends in the flora, the woody species can be divided into three categories, the fireresistant species of the savanna, species found in gallery forests and the fire-sensitive species of rocky habitats and of swamps in arid regions.
(1) The fire.resistant species of the savanna. These are in general widespread and more or less common. Only one species, Cassia arereh Del. can be considered as rare, but given its still vast distribution (Nigeria to Ethiopia) it cannot be considered endangered. A number of other species may be relatively rare in West Africa, but are widespread in the rest of Africa (e.g. Terminalia moUis Laws, Azanza garckeana (F. Hoffm.) Exell and Hillcoat). Fire-resistant species which are restricted to West Africa, but where they occur are not particularly rare, include the following: Acacia erythrocalyx Brenan; A. macrostachya Reichenb. ex Benth; A. gourmaensis A. Chev; Bafodeya henna (Sc. Elliot) Prance (syn.: Parinari benna Sc. Elliot); Cajanus kerstingii Harms; Cola cordifolia (Car.) N.Br.; Combretum lecardii Eng. and Diels; C. micranthum G. Don; C. nioroense Aubrev. ex Keay; Eriosema andohii Milne-Redhead; Euphorbia kamerunica Pax; E. paganorum A. Chev; E. sudanica A. Chev.; Fadogia erythrophloea (K. Schum. and K. Krause) Hutch. and Dalz.; Grewia barteri Burret; Grewia cissoides Hutch. and Dalz.; G. lasiodiscus Hutch. and Dalz.; Lannea egregia Engl. and K. Krause; L. microcarpa Engl. and K. Krause; L. velutina A. Rich; Neocarya macrophylla (Sabine) Prance (syn.: Parinari macrophylla Sabine); Pavetta cinereifolia Berhaut; Protea occidentalis Beard; Pteleopsis suberosa Engl. and Diels; Terminalia albida Sc. Elliot; Vangueriopsis spinosa (Schum.) Hepper. Capparis biloba Hutch. and Dalz., mentioned by Hall (in Hedberg and Hedberg, 1968) as being in danger, belongs to this group, but this species has since been re-classified and absorbed into Capparis tomentosa Lain., a widespread species. In the densely populated areas almost all species are exploited for such purposes as firewood and building material, but two species merit special attention in this respect: (a) Dalbergia melanoxylon Guill. and Pert. the dark heartwood of which is used for carvings; this has resulted in the practical disappearance of trees of any size, although the species as such is not necessarily endangered and (b) Borassus aethiopum Mart is about the only savanna species which produces straight poles, which, moreover, are termite-resistant. The leaves are used for basketwork and the fruits are also used, but in the southern savannas the biggest threat comes from the tapping of sap for the production of palm wine; this kills the palm. Borassus is mentioned by Oyewoke and Harris (in Hedberg and Hedberg, 1968) as being endangered by the excessive harvesting of its leaves for hat making, and by elephant browsing. The latter factor, how-
253 ever, is much more likely to be a useful one, given the elephant's role in disseminating the seeds.
(2) The species, found in gallery forests. These are w i t h o u t exception common, and in most cases widespread in the forest region. None is threatened or even rare. This category comprises the only subspecies (as considered in Geerling, 1982) endemic to West Africa, Andira inermis (Wright) DC ssp. grandiflora (Guill. and Perr.)Polhill. (3) The fire-sensitive species of rocky habitats and of swamps in arid regions. These species have, by the nature of their specialized habitat a localized distribution, b u t where t h e y occur, t h e y are usually c o m m o n . The lack of attractiveness of their habitat for the h u m a n population is still their best protection, b u t this group has the highest n u m b e r of vulnerable or threatened species. A species of rocky areas, endemic to the region b u t c o m m o n and widespread, is Gardenia sokotensis Hutch. Rare, and for that reason vulnerable are Fleurydora felicis A. Chev., Clerodendrum tomentellum Hutch. and Dalz. and Kleinia cliffordiana (Hutch.) C.D. Adams. Species which occur only in limited areas but are there not particularly rare, include Guibourtia copallifera Benn., Gilletiodendron glandulosa (Porteres) J. Leonard and Pteleopsis habeensis Aubrev. ex Keay (Geerling, 1984). Encephalartos barteri Garruth. which is known to exist in only a few localities in Ghana, Benin, Nigeria and the Central African Republic, is n o t restricted to West Africa, b u t it is certainly vulnerable, or even threatened. Amang the swamp-species only Raphia sudanica A. Chev. is endemic to the region. This species is exploited economically for palm wine and for building-materials to such an extent that it can certainly be considered to be under pressure. The other swamp-species, though n o t particularly c o m m o n in West Africa, are usually widespread and o f t e n more c o m m o n in other parts of Africa. CONCLUSION Most of the w o o d y species of the West African Sudan and Sahel are widespread and more or less c o m m o n , and they exhibit the following features: (1) none of the fire-resistant species is vulnerable or threatened, though a few are rare; (2) the species of the gallery forests and river banks are all widespread and relatively c o m m o n ; (3) the species of swamps, though n o t particularly common, are mostly widespread in Africa, only Raphia sudanica may suffer from over-exploitation;
254
(4) the species found in rocky areas are by the nature of their habitats, localized, but still c o m m o n . The lack of importance of their habitat to man is still an effective protection. Only Encephalartos barteri can be considered to be threatened. The main explanation for the stable balance in the conservation status of the woody species of the West African Savanna and Sahel is t h a t the environments are robust systems, with fire or aridity as dominating factors. These factors tend to override even such strong influences as man and cattle. The shifts in climatic zones may also have eliminated species which were not able to shift with them. The species which survive in such environments are likely to be the m o s t robust. This apparent stability, however, is the brighter part of the picture. The various developments in the region, b o t h climatic and human, have led to a situation where few complete and intact ecosystems still exist, let alone have a future. Conservation of the existing ecosystems -- the only effective way to protect the c o n s t i t u e n t species -- is urgent, not only from the viewpoint of conservation, but also more particularly because the present exploitation of the Sudan and Sahel zones is in no way stable or sustainable. Ecosystems are being d e s t r o y e d before their potential is known, and their over-exploitation is thus unavoidable. REFERENCES Aubr~ville, A., 1949. Climats, for~ts et d~sertification de l'Afrique tropicale. Soci~t~ d'Editions Geographiques, Maritimes et Coloniales, Paris, 344 pp. Breman, H., Geerling, C., Kessler, J.J. and Penning de Vries, F.W.T., 1984. Le rSle agrosylvo-pastoral de la strate ligneuse au Sahel. Colloque sur la d~sertification. Nouakchott. Club du Sahel, Paris, 25 pp. Brenan, J.P.M., 1978. Some aspects of the phytogeography of tropical Africa. Ann. Mo. Bot. Garden, 65: 437--478. Brookman-Amissah, J.B., Hall, J.B., Swaine, M.D. and Attakorah, I.Y., 1980. A reassessment of a fire-protection experiment in north-eastern Ghana savanna. J. Appl. Ecol., 17: 85--99. Geerling, C., 1982. Guide de terrain des ligneux sah~liens et soudano-guin4ens. Meded. Landbouwhogesch., Wageningen, 8 2 - 3 , 3 4 0 pp. Geerling, C., 1984. The Pteleopsis habeensis woodlands of Yankari Game Reserve, Nigeria. Biol. Conserv., 30: 359--362. Harris, D.R. (Editor), 1980. Human EcolQgy in Savanna Environments. Academic Press, London. Hedberg, I. and Hedberg, O., (Editors), 1968. Conservation of vegetation in Africa south of the Sahara. Proceedings 6th meeting AETFAT. Acta Phytogeogr. Suec., 54. Le Hou~rou, H.N., 1980. Browse in Africa. The current state of knowledge. International Livestock Centre for Africa, Addis Ababa, 491 pp. Malaisse, F., 1978. The Miombo ecosystem. In: Tropical Forest Ecosystems. Nat. Resour. Res., 14: 589--606. McNaughton, S.J., 1979. Grassland--herbivore dynamics. In: A.R.E. Sinclair and M. Norton-Griffiths (Editors), Serengeti an Analyses of an Ecosystem. University of Chicago Press, Chicago, IL,
255 Menaut, J.C. and C~sar, J., 1982. The structure and dynamics of a West African savanna. In: B.J. Huntley and B.H. Walker (Editors), Ecology of Tropical Savannas. Ecological Studies, Vol. 42. Springer, Berlin. Menaut, J.C., 1983. The vegetation of African savannas. In: F. Bourliere (Editor), Tropical Savannas. Elsevier,Amsterdam. Monnier, Y., 1973. La probl4matique des savanes en Afrique de l'Ouest. Ann. Univ. Abidjan, S~r. E, T o m e VI - 2. Pelissier, P. (Editor), 1980. L'arbre en Afrique tropicale. La fonction et le sigle. Cah. O R S T O M , Set Sci. Hum., 17, 320 pp. Poupon, H., 1980. Structure et dynamique de la strate ligneuse d'une steppe sah~lien au nord du S4n4gal. Tray. Doc. O R S T O M , 115,351 pp. Schnell, R., 1970--1977. Phytogeographie des Pays Tropicaux. Vols. 1--4. GauthiersVillars,Paris. Werger, M.J.A., 1983. Tropical grasslands, savannas, woodlands: natural and manmade. In: W. Holzner, M.J.A. Werger and I. Ikusima (Editors), Man's Impact on Vegetation. Junk, The Hague. West, O., 1965. Fire in vegetation and its use in pasture management. C o m m o n w e a l t h Agricultural Bureau, London, 53 pp. White, F., 1976. The vegetation map of Africa -- The history of a completed project. Boissiera, 24 : 659--666.
247
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE STATUS OF THE WOODY SPECIES OF THE S U D A N A N D SAHEL ZONES OF WEST AFRICA
CHRIS GEERLING
Nature Conservation Department, Agricultural University, Wageningen (The Netherlands) (Accepted 21 May 1985)
ABSTRACT Geerling, C., 1985. The status of the woody species of the Sudan and Sahel zones of West Africa. For. Ecol. Manage., 13: 247--255. The 371 species of trees shrubs and w o o d y climbers of the West African Sudan and Sahel are, in general, widespread and c o m m o n ; only about 20 species are limited to the region. Given the seasonal climate, fire is the major natural ecological factor. It eliminates the fire-sensitive species from the savannas, and favours the grass layer. M a n has had far more influence on the overall vegetative structure than on the precise composition of the flora. Only one species Encephalartos barteri Carruth. can be considered to be threatened. Conservation of the few remaining intact parts of these ecosystems should have high priority.
INTRODUCTION
The vegetation of West Africa, between the Sahara to the north and the rain forest along the coast, has in recent years been the subject of a number of publications covering various ecological aspects (Le Houerou, 1980; Pelissier, 1980; Poupon, 1980; Menaut and Cdsar, 1982). This paper, together with the information in a previously published field guide (Geerling, 1982), deals with the conservation status of the 371 species o f trees, shrubs and woody climbers (attaining a height of at least 1.50 m) of the Sudan and Sahel zones of West Africa, from Senegal to Chad. ENVIRONMENT
The climate o f the region covered by the savanna and by the tropical steppe of the Sahel is characterized by a distinct dry season, which lasts for a few months in the south to up to 10 months in the Sahel. Annual rainfall varies from more than 1 4 0 0 m m in the south to less than 200 mm in the north. Temperatures are relatively low in the early dry season (average m a x i m u m about 34°C and minimum below 20°C) but increase towards
0378-1127/85/$03.30
© 1985 Elsevier Science Publishers B.V.
248 the end of the dry season when t h e y rise to a maximum of 40°C or more and a minimum of over 20°C. The desiccating north-eastern wind (harmattan), which blows during the dry season, is ecologically very important. The landscape consists for the greater part of rolling hill c o u n t r y , with higher elevations of importance only in Guinea (Fouta Djallon), Nigeria (Jos Plateau) and on the Nigeria--Cameroun border (Alantika and Mandara Mountains). The vegetation of b o t h the savanna and tropical steppe has a herb layer with or without an open layer of trees and shrubs. The herb layer -- predominantly grasses -- is continuous and n0n-sclerophilous in savanna, and discontinuous and sclerophilous in steppe (Schnell, 1 9 7 0 - - 1 9 7 7 ; Menaut, 1983 and Monnier, 1973). As the rainfall decreases, so the w o o d y trees and shrubs decrease in height rather than in their amount of ground cover, but towards the Sahara the cover also decreases. The vegetation patterns in these zones are closely related to the horizontal redistribution (run-off and run-on) of rainwater. PHYTOGEOGRAPHY The savanna and Sahel region is best characterized as t h e 'tropical intermediate zone' (Harris, 1980) b e t w e e n the forest and the desert. In West Africa the region is subject to major shifts in climatic zones, which can be related to comparable zones in most of Africa and tropical Asia. Phytogeographically the region can be divided into three distinct zones (White, 1983; Menaut, 1983): (1) the arid north, the Sahel, with a clear affinity with the arid regions of southern Asia. Here, there are 20 typical w o o d y species, o f which 7 also occur in Asia; (2) the 'true' savanna, the Sudan zone, with an affinity with the Miombo and Mopane savannas of southern Africa (Malaisse, 1978). There are here 325 typical w o o d y species, of which 140 are limited to northern tropical Africa, another 140 also occur in southern Africa, 35 are also f o u n d in Asia, 4 in tropical America and 7 are pantropical; (3) the transition zone b e t w e e n the Sudan savanna and the Guinean-Congolese forest with 25 species. None of these is a typical fire-resistant savanna species, b u t most of t h e m may be found in favourable conditions far into the savanna zone. This evidence of species distribution indicates that endemism is n o t an outstanding feature of the West Africa savanna flora. Further examination of the evidence indicates that: (1) none of the 370 species belongs to a family endemic to West Africa. All but 9 species belong to either pantropical or cosmopolitan families; (2) three o u t of 209 genera are limited to West Africa, b u t t w o of them, Bafodeya and Neocarya, have recently been separated from the pantropical
249
genus Parinari, of these genera, 55 are African and 54 (with 156 species) are pantropical; (3) 44 species are endemic to West Africa b u t the endemism is flawed: some have vicarian species elsewhere in the savanna; some m a y eventually be found elsewhere especially in the still poorly prospected Chad and the Central African Republic; some belong to taxonomically poorly defined groups. In all, out of the 44, it seems that a b o u t half are likely to be found to be limited to West Africa. Within the region on particular centres o f endemism occur, b u t firesensitive species found in rocky habitats are relatively well-represented. Brenan (1978) came to the same conclusion for the whole of the West African flora. VEGETATION AND MAN
The history of the vegetation of Africa, south of the Sahara in general, and of West Africa in particular, has been extensively treated by Aubr~ville (1949) and Schnell (1970--77) and will only be summarized here. The area n o w covered b y savanna and steppe in West Africa has been subjected to several shifts in the climatic zones and, consequently, in its vegetation. Evidence for these shifts has been found in fossile dunes, in what is n o w a savanna region in Nigeria, well south o f the desert, and is demonstrated by species, representative o f much more humid zones, which n o w survive in restricted localities within t h e arid zones. Such a shift appears to have occurred as late even as this c e n t u r y (Breman et al., 1984). The northern limits of the distribution of species like Acacia senegal have moved south for something like 200 km under the influence of the recent drought. These species have n o w returned to within the limits where t h e y were found in the 1930s, which is the period when the first reliable observations were made. In the much more humid period b e t w e e n the 1930s and the 1960s, these species became established further north. Savanna vegetation is for the greater part a fire-climax (West, 1965). Fire destroys the litter which suffocates the grasses, and it also kills young w o o d y plants. Thus, fire reduces shading and tips the balance, in the competition for light, in favour of the grasses. The grass-fires are n o w a d a y s practically all lit by man; 'natural' fires are rare. This, however, does not mean that fire is a recent p h e n o m e n o n in the savanna vegetation. Considering the stability, and the richness in species of even the humid Isoberlinia savanna woodlands, savannas must have had a long history of burning. Large herbivores have evolved which are typical of the savanna (McNaughton, 1979) and this provides another indication that savannas are n o t recent in northern tropical Africa. The frequency of burning has u n d o u b t e d l y increased with the increase in the human population, and the extent of the savannas in the more humid zones has also increased, b u t fire m a y have been a significant factor in
250
the past, at least in the drier zones. Aubrbville’s (1949) assertion that the original vegetation of the savanna zones was a dry, closed forest seems not likely to be correct. Especially in the drier regions, a closed but deciduous forest is tinder dry by the end of the season and is therefore highly vulnerable to fire. Even accidental fires, started by lightning, sparks from falling rocks, or the like, occurring decades apart, but then ranging over large areas of unbroken land, would be enough to wipe out the firesensitive species and allow the establishment of the grasses. The main effects of fire on the floristic composition have been the propagation and maintenance of a herb layer, mostly of grasses (see Brookman-Amissah et al., 1980), and the separation of fire-resistant species, covering vast tracts of land, from the fire-sensitive species, limited to moist habitats such as swamps and gallery forests, and to areas with a discontinuous grass layer such as rocky hills and scarps. In the past the influence of man, apart from burning, has been a factor of importance only in the drier zones which are free of the sleeping sickness-transmitting tsetse flies (Glossina spp). Agriculture will have changed the vegetation, at least locally, since cattle are, by their selective grazing, the major cause of the preponderance of thorny shrubs and trees in the drier zones. A certain stability had reigned in the savanna system of West Africa until the onslaught of modern technology. The main effects of modern times upon the vegetation have been a marked increase in the frequency of burning and an increase in human population density, with its accompanying farming, husbandry and firewood collection. This has led to the disappearance of the natural vegetation over large areas. The savanna and Sahel, though in general not particularly heavily populated, have local dense populations (Mossi Plateau in Upper Volta and Kane in Nigeria) and, as everywhere in the tropics, there is a high rate of population increase. The influence of mankind on the vegetation is manifold (Werger, 1983): agriculture, husbandry both semi-nomadic and sedentary (see Figs. 1 and 2), firewood collecting and burning. These developments are in general much more detrimental to the existing vegetation than to the long-term constitution of the flora (see Fig. l), as was indicated in Hedberg and Hedberg (1968). The impact of human activity varies within the three ecological zones: (1) the northern Sahel is ecologically well protected against changes induced by farming practises because of the lack of water in the dry season. (2) the southern Sahel and northern Sudan zone have the highest population densities and these areas have also suffered most from the droughts of the early 1970s. Over-grazing and cutting for firewood have led to the disappearance of the natural vegetation over large areas. (3) the southern Sudan zone and the Guinea savanna are still protected by the presence of diseases like sleeping sickness (transmitted by Glossina spp.) and river blindness (transmitted by Simulium spp.).
251
Fig. 1. Borassus aethiopum in farmland in northern Benin; note stumps of cut trees. ]
Fig. 2. Balanites aegyptiaca o n t h e Mossi p l a t e a u , B u r k i n a Faso, with branches c u t f o r fodder.
252 S T A T U S A N D T R E N D S IN C O N S E R V A T I O N
With regard to the ecological status of the region and the trends in the flora, the woody species can be divided into three categories, the fireresistant species of the savanna, species found in gallery forests and the fire-sensitive species of rocky habitats and of swamps in arid regions.
(1) The fire.resistant species of the savanna. These are in general widespread and more or less common. Only one species, Cassia arereh Del. can be considered as rare, but given its still vast distribution (Nigeria to Ethiopia) it cannot be considered endangered. A number of other species may be relatively rare in West Africa, but are widespread in the rest of Africa (e.g. Terminalia moUis Laws, Azanza garckeana (F. Hoffm.) Exell and Hillcoat). Fire-resistant species which are restricted to West Africa, but where they occur are not particularly rare, include the following: Acacia erythrocalyx Brenan; A. macrostachya Reichenb. ex Benth; A. gourmaensis A. Chev; Bafodeya henna (Sc. Elliot) Prance (syn.: Parinari benna Sc. Elliot); Cajanus kerstingii Harms; Cola cordifolia (Car.) N.Br.; Combretum lecardii Eng. and Diels; C. micranthum G. Don; C. nioroense Aubrev. ex Keay; Eriosema andohii Milne-Redhead; Euphorbia kamerunica Pax; E. paganorum A. Chev; E. sudanica A. Chev.; Fadogia erythrophloea (K. Schum. and K. Krause) Hutch. and Dalz.; Grewia barteri Burret; Grewia cissoides Hutch. and Dalz.; G. lasiodiscus Hutch. and Dalz.; Lannea egregia Engl. and K. Krause; L. microcarpa Engl. and K. Krause; L. velutina A. Rich; Neocarya macrophylla (Sabine) Prance (syn.: Parinari macrophylla Sabine); Pavetta cinereifolia Berhaut; Protea occidentalis Beard; Pteleopsis suberosa Engl. and Diels; Terminalia albida Sc. Elliot; Vangueriopsis spinosa (Schum.) Hepper. Capparis biloba Hutch. and Dalz., mentioned by Hall (in Hedberg and Hedberg, 1968) as being in danger, belongs to this group, but this species has since been re-classified and absorbed into Capparis tomentosa Lain., a widespread species. In the densely populated areas almost all species are exploited for such purposes as firewood and building material, but two species merit special attention in this respect: (a) Dalbergia melanoxylon Guill. and Pert. the dark heartwood of which is used for carvings; this has resulted in the practical disappearance of trees of any size, although the species as such is not necessarily endangered and (b) Borassus aethiopum Mart is about the only savanna species which produces straight poles, which, moreover, are termite-resistant. The leaves are used for basketwork and the fruits are also used, but in the southern savannas the biggest threat comes from the tapping of sap for the production of palm wine; this kills the palm. Borassus is mentioned by Oyewoke and Harris (in Hedberg and Hedberg, 1968) as being endangered by the excessive harvesting of its leaves for hat making, and by elephant browsing. The latter factor, how-
253 ever, is much more likely to be a useful one, given the elephant's role in disseminating the seeds.
(2) The species, found in gallery forests. These are w i t h o u t exception common, and in most cases widespread in the forest region. None is threatened or even rare. This category comprises the only subspecies (as considered in Geerling, 1982) endemic to West Africa, Andira inermis (Wright) DC ssp. grandiflora (Guill. and Perr.)Polhill. (3) The fire-sensitive species of rocky habitats and of swamps in arid regions. These species have, by the nature of their specialized habitat a localized distribution, b u t where t h e y occur, t h e y are usually c o m m o n . The lack of attractiveness of their habitat for the h u m a n population is still their best protection, b u t this group has the highest n u m b e r of vulnerable or threatened species. A species of rocky areas, endemic to the region b u t c o m m o n and widespread, is Gardenia sokotensis Hutch. Rare, and for that reason vulnerable are Fleurydora felicis A. Chev., Clerodendrum tomentellum Hutch. and Dalz. and Kleinia cliffordiana (Hutch.) C.D. Adams. Species which occur only in limited areas but are there not particularly rare, include Guibourtia copallifera Benn., Gilletiodendron glandulosa (Porteres) J. Leonard and Pteleopsis habeensis Aubrev. ex Keay (Geerling, 1984). Encephalartos barteri Garruth. which is known to exist in only a few localities in Ghana, Benin, Nigeria and the Central African Republic, is n o t restricted to West Africa, b u t it is certainly vulnerable, or even threatened. Amang the swamp-species only Raphia sudanica A. Chev. is endemic to the region. This species is exploited economically for palm wine and for building-materials to such an extent that it can certainly be considered to be under pressure. The other swamp-species, though n o t particularly c o m m o n in West Africa, are usually widespread and o f t e n more c o m m o n in other parts of Africa. CONCLUSION Most of the w o o d y species of the West African Sudan and Sahel are widespread and more or less c o m m o n , and they exhibit the following features: (1) none of the fire-resistant species is vulnerable or threatened, though a few are rare; (2) the species of the gallery forests and river banks are all widespread and relatively c o m m o n ; (3) the species of swamps, though n o t particularly common, are mostly widespread in Africa, only Raphia sudanica may suffer from over-exploitation;
254
(4) the species found in rocky areas are by the nature of their habitats, localized, but still c o m m o n . The lack of importance of their habitat to man is still an effective protection. Only Encephalartos barteri can be considered to be threatened. The main explanation for the stable balance in the conservation status of the woody species of the West African Savanna and Sahel is t h a t the environments are robust systems, with fire or aridity as dominating factors. These factors tend to override even such strong influences as man and cattle. The shifts in climatic zones may also have eliminated species which were not able to shift with them. The species which survive in such environments are likely to be the m o s t robust. This apparent stability, however, is the brighter part of the picture. The various developments in the region, b o t h climatic and human, have led to a situation where few complete and intact ecosystems still exist, let alone have a future. Conservation of the existing ecosystems -- the only effective way to protect the c o n s t i t u e n t species -- is urgent, not only from the viewpoint of conservation, but also more particularly because the present exploitation of the Sudan and Sahel zones is in no way stable or sustainable. Ecosystems are being d e s t r o y e d before their potential is known, and their over-exploitation is thus unavoidable. REFERENCES Aubr~ville, A., 1949. Climats, for~ts et d~sertification de l'Afrique tropicale. Soci~t~ d'Editions Geographiques, Maritimes et Coloniales, Paris, 344 pp. Breman, H., Geerling, C., Kessler, J.J. and Penning de Vries, F.W.T., 1984. Le rSle agrosylvo-pastoral de la strate ligneuse au Sahel. Colloque sur la d~sertification. Nouakchott. Club du Sahel, Paris, 25 pp. Brenan, J.P.M., 1978. Some aspects of the phytogeography of tropical Africa. Ann. Mo. Bot. Garden, 65: 437--478. Brookman-Amissah, J.B., Hall, J.B., Swaine, M.D. and Attakorah, I.Y., 1980. A reassessment of a fire-protection experiment in north-eastern Ghana savanna. J. Appl. Ecol., 17: 85--99. Geerling, C., 1982. Guide de terrain des ligneux sah~liens et soudano-guin4ens. Meded. Landbouwhogesch., Wageningen, 8 2 - 3 , 3 4 0 pp. Geerling, C., 1984. The Pteleopsis habeensis woodlands of Yankari Game Reserve, Nigeria. Biol. Conserv., 30: 359--362. Harris, D.R. (Editor), 1980. Human EcolQgy in Savanna Environments. Academic Press, London. Hedberg, I. and Hedberg, O., (Editors), 1968. Conservation of vegetation in Africa south of the Sahara. Proceedings 6th meeting AETFAT. Acta Phytogeogr. Suec., 54. Le Hou~rou, H.N., 1980. Browse in Africa. The current state of knowledge. International Livestock Centre for Africa, Addis Ababa, 491 pp. Malaisse, F., 1978. The Miombo ecosystem. In: Tropical Forest Ecosystems. Nat. Resour. Res., 14: 589--606. McNaughton, S.J., 1979. Grassland--herbivore dynamics. In: A.R.E. Sinclair and M. Norton-Griffiths (Editors), Serengeti an Analyses of an Ecosystem. University of Chicago Press, Chicago, IL,
255 Menaut, J.C. and C~sar, J., 1982. The structure and dynamics of a West African savanna. In: B.J. Huntley and B.H. Walker (Editors), Ecology of Tropical Savannas. Ecological Studies, Vol. 42. Springer, Berlin. Menaut, J.C., 1983. The vegetation of African savannas. In: F. Bourliere (Editor), Tropical Savannas. Elsevier,Amsterdam. Monnier, Y., 1973. La probl4matique des savanes en Afrique de l'Ouest. Ann. Univ. Abidjan, S~r. E, T o m e VI - 2. Pelissier, P. (Editor), 1980. L'arbre en Afrique tropicale. La fonction et le sigle. Cah. O R S T O M , Set Sci. Hum., 17, 320 pp. Poupon, H., 1980. Structure et dynamique de la strate ligneuse d'une steppe sah~lien au nord du S4n4gal. Tray. Doc. O R S T O M , 115,351 pp. Schnell, R., 1970--1977. Phytogeographie des Pays Tropicaux. Vols. 1--4. GauthiersVillars,Paris. Werger, M.J.A., 1983. Tropical grasslands, savannas, woodlands: natural and manmade. In: W. Holzner, M.J.A. Werger and I. Ikusima (Editors), Man's Impact on Vegetation. Junk, The Hague. West, O., 1965. Fire in vegetation and its use in pasture management. C o m m o n w e a l t h Agricultural Bureau, London, 53 pp. White, F., 1976. The vegetation map of Africa -- The history of a completed project. Boissiera, 24 : 659--666.