A phytogeographic survey of Southern Benin

Southern Benin has a dry subequatoriai climate with a rainfall gradient from 850 mm in the west to 1 500 mm in the east, the geomorphology is varied and the vegetation has been subjected to strong human influence. There are numerous plant formations, namely: 1, forest islands which are probably relics of the primitive vegetation and include (a) dense semi-deciduous forests of several types, (b) swamp forests of two types, (c) periodically flooded forest of two types, (d) Lophira lanceolata (Hutchinson & Dalziel, 1954-72) woodlands and (e) mangrove swamps; 2, formations which are probably derived and include (a) thickets of several types, (b) tree savannas and shrub savannas, (c) grassy savannas and prairies varying according to soil characteristics and (d) halophytic grasslands; and 3, floating vegetation on fresh-water lakes.


L 'action humaine actuelle tend á détruire les derniers ilots forestiers et á favoriser les fourrés et la végétation herbeuse.
Since Adjanohoun's report on conservation of vegetation in Le Dahomey (1968), several investiga tions have been carried out on the vegetation of Southern Benin (see References).This article endeavours to take stock of the phytogeographic knowledge of that region.altitude, with red sandy argillaceous soil and lateritic crusts.The southern plateaux, called Bopa, Allada, Sakété, rise to 20 m in the south and 90 m in the north; they consist of argillaceous, red to ochre sand, pleistocene, called 'terre de barre.' Circular depressions and small flat-bottomed valleys cut through them.The north-south valleys correspond to small rivers the Ouémé (and the So), the Kouffo (and Lake Ahémé and the Aho), the Mono (and the Sazué).These valleys have a rather varied topo graphy viz.sandy buttes which are never covered by flood waters, low argillaceous and silty clay banks which are flooded every year, constantly hydromorphic peaty parts and open expanses of water (Lakes Aziri, Sélé, Hounhoun, Ahémé).
A coastal complex of variable width (3-5 km) lies a short distance from the ocean, consisting of old offshore bars, recent bars, depressions correspond ing to former lagoons, and present-day lakes and lagoons (lagoon of Porto-Novo, Lake Nokoué, coastal lagoon).Fig. 1A shows the geology of the southern part of Benin.

Climate
Southern Benin has a dry subequatorial climate with two rainy seasons and two dry seasons of unequal length: the long rainy season from April to July, the short rainy season in October, the long dry season from December to February and the short dry season in August.Two gradients characterize the region: -a light south-north gradient with a tropical tendency with decrease in the length of the short dry season and increase in the length of the long dry season; -a strong east-west gradient of decreasing annual rainfall (1 350 mm at Porto-Novo, 1 100 mm at Ouidah, 850 mm at Grand-Popo).Another charac teristic is the great irregularity of annual rainfall from one year to the next.
The mean monthly temperatures vary little, for example Cotonou is 28°C in March, 25°C in August, with a mean temperature of 26,5°C.The maximum temperatures are close to 34°C on the coast and a little higher in the interior.The largest temperature variations occur during the days of the Harmattan (from December to February).
The dominant winds come from the south-west except during the Harmattan.

Hydrology
Rivers, lakes and lagoons are at their lowest depth from November to May.Most years, they are highest in September and October.Thus, in the valleys, there is an annual alternation of submersion and emersion which, with the topography, is one of the determining factors of phytocoenoses.
In the dry season, salt water from the ocean can enter the Mono River and Lake Nokoué through inlets: this explains the presence of halophytic phytocoenoses very far inland (Fig. IB).

Human influence
Lower Benin appears to have been inhabited for a long time.The density of villages is high.The crops (maize, cassava, taro), the plantations (palm-oil from the last century onwards, coconut), the techniques of salt extraction (by boiling) and fishing (by placing a great number of branches in the water) and the gathering of fire wood, have brought about the destruction of forests (only a few forest islands remain) and their replacement by thickets and grassy phytocoenoses.
Man probably brought the baobab to the south-west part and very likely modified the distribution of palm trees (Borassus sp., Phoenix reclinata, Raphia spp.).
Finally, the opening of the channel of Cotonou (1885), and the construction of the Port of Cotonou (1960) have had a great impact on the environment.
As evidence of past human involvement, we were able to date with C14 some charcoal associated with neolithic quartz at Godomey* and with pottery at The greatest number of species is found on the Sakété Plateau, with a very sharp decrease in number of species from east to west.An example of a typical forest island is that of Ké (northwest of Porto-Novo; area: 4 hectares) with the following strata and main species: -lianas (Alafia barteri, Dioscorea minutiflora, Leptoderris brachyptera, Ritchiea reflexa and Smilax kraussiana . ..).
In the villages various trees which are typical of these forests can be seen.It has sometimes been assumed that all the southern plateaux were covered by a dense semi-deciduous forest (Mondjannagni, 1969), but that does not seem to be the case for the southern part of the Bopa and Allada Plateaux (Paradis et al., 1978): there, the forest-savanna mosaic appears to be of long standing.Around Porto-Novo, throughout the centuries, an Elaeis plantation replaced the dense forest through human activity (Brasseur-Marion & Brasseur. 1953).
Here and there in the forest, can be found isolated stands of Anogeissus leiocarpus -evidence of former land-clearing, and which must not be interpreted as relics of a southward extension of dry plant formations.
It seems likely that the clay of the entire transversal depression between the northern and southern plateaux was formerly occupied by a forest of this type.The planting of crops (maize) destroyed it almost everywhere, and today, the subsisting island, despite being theoretically a reserve, is in the process of being destroyed.

Swampy fresh-water forests
These are of two types.
( In such an environment, a Ficus congensis forest can be found in the old course of the Kouffo to the north of Lake Ahémé. Man has had an influence on the swampy forests, by cutting down tall trees with straight trunks (Alstonia, Cleistopholis, Mitragyna and Symphonia), by planting Raphia palms to be used in various ways, by penetrating into the clearings to harvest species of forbs (Thalia welwitschii, Cyrtosperma senegalense), the leaves of which are used to wrap food sold in the markets.In the Ouémé Valley, taro is grown at the site of a swampy forest which was almost entirely cleared away for that purpose.Now, in that valley, rice-growing, through the extension of draining, is modifying the environment.The final result of human influence is an abundance of thickets and marshy prairies with a few scattered, tall trees.
(a) on the low banks of the Ouémé and the So.To the north of Adjohoun, along the edge of these two rivers, made up of silty argillaceous soil, a riverbank forest lies on the low banks, forming a veritable gallery forest.It is inundated during the flood season for one to two months a year.Its composition is as follows: -a stratum of trees ( 8 -a herbaceous stratum with Crinum jagus, Haemanthus rupestris, Amorphophallus atrovirens, Herderia truncata and Hypolytrum poecilolepis. (b) On the sand along the edge o f the depressions o f the coastal complex.Here, the level of phreatic water fluctuates throughout the year, rising to one metre above ground from June to November, and falling to one metre below during the dry season.Such an environment is occupied by forest in which Symphonia globulifera is abundant, along with such species as Cleistopholis patens, Alstonia congensis, Memecylon blakeoides, Anthocleista vogelii, Berlinia grandiflora, Syzygium guineese, Chrysobalanus el lipticus and Ancistrophyllum secundiflorum.
Man has, in many places, tranformed this forest into Elaeis groves and grass savanna with Loudetia phragmitoides and Panicum congoense (Paradis, 1975a).
(c) On the sandy islets o f the Ouémé Valley.Here there are occasionally-flooded forests, comprised of a mixture of riverbank forest species such as Cynometra vogelii, Cola laurifolia, Lonchocarpus spp., Manilkara obovata, Ostryocarpus riparius and Pterocarpus santalinoides and semi-deciduous forest species such as Ceiba pentandra, Cola gigantea and Cola millenii.

Woodlands with Lophira lanceolata
In the Ahozon Reserve, on yellow sand, L. lanceolata forms a monospecific, 6-10 m-high, closed community with a single stratum.The herbaceous plants, well-developed in October, include a stratum, 1 -1,3 m high, with Ctenium canescens, C. newtonii, Setaria pallide-fuscae and Schizachyrium sanguineum and a lower stratum with Cyperaceae like Bulbostylis aphyllantoides, Kyllinga erecta and Cyperus margaritaceus.Almost every where, these woodlands have been tranformed into tree and shrub savannas invaded by small thickets (see below).(Paradis & Adjanohoun, 1974;Paradis, 1979Paradis, , 1980Paradis, , 1981) ) At some points (north-west of Lake Ahémé, east of the coastal lagoon, between the Aho and Grand-Popo) there remain some relics of a mangrove swamp with Avicennia africana and Rhizophora racemosa, formerly more extensive.Between Ouidah and the estuary, on the areas degraded for the extraction of salty soil, there are some small populations of Laguncularia racemosa.

Mangrove swamps
2.2 Thickets, savannas, prairies, grasslands and floating vegetation 2.2.1 Thickets (a) Littoral thicket (Paradis, 1976;Souza, 1979).In rare places on the coast there remain patches of this thicket with shrubs (Manilkara obovata, Fagara zanthoxyloides, Diospyros tricolor, Syzygium guineense and Chrysobalanus orbicularis), liana (Sarcostemma viminale) and a geophyte (Sansevieria liberica).In addition, there is the occasional Adenia lobata, Agelaea obliqua, Carissa edulis, Chassalia kolly, Flacourtia flavescens, Ritchiea reflexa, Trema guineensis, and Phoenix reclinata.It seems likely that this coastal thicket constituted the edge of a littoral forest with Manilkara obovata, destroyed for the extraction of salt by boiling sea water.(b) Thicket with Dalbergia ecastaphyllum (Paradis, 1979).This species is abundant in readily-flooded parts on the banks of the coastal lagoon, where it is often associated with Drepanocarpus lunatus, in the clearings of Rhizophora mangrove swamp, and along depressions with Typha australis and Kyllinga peruviana of the coastal sand.This thicket, 2-4 m in height, is not easily penetrated, and constitutes a phytocoenose of long duration.At the edge of the mangrove swamp, it impedes the reconquest of the environment by Rhizophora and Avicennia.
(c) Thicket on yellow sand and 'terre de barre.' On yellow sand, scattered among the Lophira lanceolata formations, thickets 6-12 m in height and 10-100 m in diameter are plentiful.Their flora, which is quite abundant, includes Barteria nigritiana, Bridelia ferruginea, Dialium guineense, Fagara zantho xyloides, Annona senegalensis, Byrsocarpus coccineus, Carissa edulis, Cnestis ferruginea and Vitex doniana.The majority of these species are zoochorological (ornithochore and cheiropterochore).These thickets are a stage in the progression of Lophira savanna to forest: one can often observe rotting trunks of Lophira in the thickets.
On 'terre de barre,' likewise, there is a thicket formed by numerous discontinuous copses more or less extensive, 4-8 m in height.The tallest trees are Bridelia ferruginea, Fagara zanthoxyloides, Malacantha alnifolia, Phyllanthus discoideus and Vitex doniana.They tower over a number of small shrubs.Here, these thickets are undoubtedly a stage in the progression of old savannas to forest (Paradis et al., 1978).(d) Wooded thicket at the edge o f the Lama Forest.This thicket clearly results from the invasion of savanna by forest.Indeed, it is comprised of: -large savanna trees, from 5-12 m in height, widely spaced (Anogeissus leiocarpus, Combretum molle, Fagara zanthoxyloides, Lannea kerstingii, Piliostigma thonningii, Pterocarpus erinaceus and Terminalia glaucescens)\ -a closed stratum, from 0-5 m in height, with the same floristic composition as that of the Lama Forest with much Dialium guineense, Diospyros mespilifor mis, Mimusops adongensis and Celtis brownii.The diameter of these forest species is small.This wooded thicket on the forest periphery appeared after the area was made a reserve, which put a stop to annual fires.As long as fires occurred each year, the savanna species grew without competition but, as soon as they stopped, the forest species, better adapted to the present environment, were able to occupy the former burnt sites.
2.2.2 Tree savannas and shrub savannas (a) With Terminalia glaucescens o f the Lama depression and north o f Lake Ahémé.These are Guinean savannas with numerous species, but without Lophira lanceolata or the Shea butter tree.It is possible that these savannas have arisen through the destruction of woodlands by man.(b) With Lophira lanceolata.These result from deterioration of the Lophira woodlands.Their floristic composition is the same as that of the woodlands.(c) With Mitragyna inermis and Phoenix reclinata (in the valleys).In the Ouémé valley, this savanna is characterized by a tree stratum (with Mitragyna inermis and some Pterocarpus santalinoides), a shrub stratum (with Antidesma venosum, Drepanocarpus lunatus, small M. inermis, Mimosa pigra and Vitex doniana) and an herbaceous stratum (with Andropogon gayanus var.squamulatus, Axonopus flexuosus, Cyperus articulatus, Eriosema glomeratum, Hibiscus asper and Hygrophila auriculata).In some scattered spots there are copses with Phoenix reclinata.In the valleys of the Kouffo and the Mono, these palm-trees are abundant, and in the south-west there is also Borassus aethiopum.Near Cotonou and in the south-east there are some dispersed stands of M. inermis.These M. inermis savannas occur on soil which is seasonally flooded and composed of clay or silty-argillaceous sand.They appear to have been derived under the influence of fire, from riverside forests.

Grass savannas and varied prairies
These low phytocoenoses, with superficial roots, can be classified according to the characteristics of the substratum.
(a) On sand never or rarely flooded: -on 'terre de barre' grassy savanna with Andropogon tectorum and lmperata cylindrica, characteristic of exhausted soils, which become very sandy; -on yellow sand grassy savanna with Ctenium newtonii and Schizachyrium sanguineum, resulting from the great deterioration of Lophira shrub savannas.
(b) On sand periodically flooded: -littoral zone: the highest areas are occupied by a prairie with Vetiveria nigritana, the lowest points are covered by a prairie with Panicum congoense (Guinko, 1974), the places, topographically very low, by a prairie with Loudetia phragmitoides; -low Ouémé valley: the bulge of the steep river bank, silty and sandy, is occupied by a low Cynodon dactylon prairie, used very much for grazing, with numerous ruderal species.In flood season, this bulge is inundated almost every year and invaded by floating vegetation; (c) On argillaceous and silty clay soil, periodically flooded: -littoral zone (in the depressions parallel to former bars): medium prairie with Fuirena umbellata; -low Ouémé valley: medium prairie with Echinochloa pyramidalis and Brachiaria mutica (and during the dry season Aeschynomene indica, Digitaria horizontalis, Hyptis lanceolata, Paspalum orbiculare and Pentodon pentandrus).During the flood season, most of these species become floating.In addition, there are some Cyperaceae (Cyperus haspan.Fuirena umbellata and Rhynchospora corymbosa), and various strictly floating species.
(d) On substrata saturated with fresh water all year long: -high prairie with Typha australis (with very few species: Paspalum vaginatum, where the salinity increases during the dry season, Polygonum pulchrum, Cyperus papyrus, Cyclosorus striatus and Cyrtosperma senegalense where the water remains fresh all the time).Such a prairie is located around the lagoon of Porto-Novo (Paradis & Rabier, 1979), between Cotonou and the eastern limit of the mangrove swamp, in the abandoned meanders of the Kouffo (north of Lake Ahémé), and in the depression between the Allada plateau and the yellow sand (from Pahou to Godomey); -a community of Cyperus papyrus, which is not very extensive (behind the preceding community where the water is acid) with Phragmites australis and Echinochloa pyramidalis; -a community with Nymphaea lotus in the numerous expanses of open water; -a community with Cyclosorus striatus, very extensive on the periphery of Lakes Aziri and Hounhoun, and in the gaps of swamp forests.This fern floats during floods.The death of the fern, without decomposition of its rhizome, produces floating peat on which seeds from trees of swamp forests can germinate; -a community with Pycreus mundii, localized in one spot (Guinko, 1974).
(e) On hydromorphic substrata, slightly brackish at certain times o f the year.On these substrata there occur an Eleocharis spp.prairie behind the mangrove swamps (from Ouidah to Togbin), and a community behind the offshore bar (between it and the mangrove swamps) with Kyllinga peruviana, Phyla nodiflora, Fimbristylis obtusifolia, Mariscus ligularis, Pycreus polystachyos, Hydrocotyle bonariensis and Philoxerus vermicularis.
(f) On a salty substratum (Paradis, 1980): -grassland with Sesuvium portulacastrum and Philoxerus vermicularis on the periphery of areas where salty earth is extracted (south-western region).
-littoral grassland with two features (in the low areas, on prograding, sand, pioneer vegetation with Ipomoea pes-caprae, Remirea maritima and Cyperus maritimus; in the high areas of the offshore bar, grassland with Sporobolus virginicus, Schizachyrium pulchellum, Ipomoea stolonifera and Diodia vagin alis, with small dunes having scattered Scaevola plumieri).(Paradis & Rabier, 1979;Texier et al., 1980) Floating vegetation begins to multiply rapidly when floods occur in the low valleys.On the lagoon of Porto-Novo, they constitute floating islands with:
In the lagoon of Porto-Novo the floating vegetations spreads from January onwards following an increase in salinity.It begins to take over the lagoon in June-July.

CONCLUSIONS
This phytogeographic overview of Southern Benin shows the great variety of phytocoenoses caused, on the one hand, by climatic, topographic and edaphic factors and, on the other, by human influence on the environment.The result is a mosaic of plant formations in which thickets and savannas of several types predominate.
The natural historical factors, especially paleoclimate, have not yet been elucidated.Future research should focus on detailed carto graphy, to a scale of 1: 50 000 (already begun), on the determination of kinds of charcoal (in collabora tion with archaeologists) and on the palynological study of soil samples.
solely of sedimentary rocks from the Upper Cretaceous to the Quaternary.Its geomorphology is dominated by two series of plateaux separated by an east-west depression and by north-south valleys.The east-west depression, called Lama, is chiefly argillaceous and flooded for part of the year (August-September).The northern plateaux, called Aplahoué, Abomey, Kétou, reach 100-150 m in * Ecole Normale Supérieure, 08 BP 10 Abidjan -08, Ivory Coast.
semi-deciduous forestsOnly a few rare stands of secondary forest remain on the southern plateaux-on their slopes and in the Lama depression.(a)On the southern plateaux.
Such zones exist in the coastal complex and correspond to the courses of former rivers, quite evident from Cotonou to Nigeria.For example, near Krake, the forest is dominated by Ficus congensis with Anthocleista vogelii, Syzygium owariense, Chrysobalanus ellipticus.Only during the flood season (August to November) does the water flow freely.The rest of the year it is stagnant.