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Patterns of semi-deciduous forest are investigated in Benin by means of phytosociological relevés and multivariate analyses. Species and family importance values are assessed for each forest type. The classifications and DCA ordination of 176 semi-deciduous forest relevés result in six forest types, patterned along climatic and edaphic gradients. The described forest types and their floristic composition (both at species and family levels) are comparable to those recognised in other parts of West Africa where the semi-deciduous forest is more continuous. The leading dominant families were Malvaceae, Fabaceae, Cannabaceae, Putranjivaceae, Ebenaceae and Moraceae. The Drypetes aframensis-Nesogordonia papaverifera forest type appears to be that in which the Cannabaceae, Malvaceae, Putranjivaceae and Violaceae are best represented. The record of many Upper Guinean endemic species in Benin's forest islands provids strong evidence for past floristic connections of the Dahomey Gap with the West African rain forest zone.
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Acta Botanica Gallica
ISSN: 1253-8078 (Print) 2166-3408 (Online) Journal homepage: https://www.tandfonline.com/loi/tabg20
Semi-deciduous forest remnants in Benin:
patterns and floristic characterisation
Aristide C. Adomou , Akpovi Akoègninou , Brice Sinsin , Bruno de Foucault &
Laurentius J.G. van der Maesen
To cite this article: Aristide C. Adomou , Akpovi Akoègninou , Brice Sinsin , Bruno de
Foucault & Laurentius J.G. van der Maesen (2009) Semi-deciduous forest remnants in
Benin: patterns and floristic characterisation, Acta Botanica Gallica, 156:2, 159-171, DOI:
10.1080/12538078.2009.10516148
To link to this article: https://doi.org/10.1080/12538078.2009.10516148
Published online: 26 Apr 2013.
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Acta Bot. Gallica, 156 (2), 159-171, 2009.
Semi-deciduous forest remnants in Benin: patterns and floristic cha-
racterisation
by Aristide C. Adomou(1), Akpovi Akoègninou(1), Brice Sinsin(2), Bruno de Foucault(3)
and Laurentius J.G. van der Maesen(4)
(1) Faculté des sciences et techniques, Université d’Abomey-Calavi, 01 B.P. 4521, Cotonou, Bénin;
adomouaristide@yahoo.fr
(2) Faculté des sciences agronomiques, Université d’Abomey-Calavi, B.P. 526, Cotonou, Bénin
(3) Département de botanique, Faculté de pharmacie, BP 83, F-59006 Lille Cedex
(4) Biosystematics Group, Wageningen University, Gen. Foulkesweg 37, NL-6703 BL Wageningen
Abstract.- Patterns of semi-deciduous forest are investigated in Benin by
means of phytosociological relevés and multivariate analyses. Species and fami-
ly importance values are assessed for each forest type. The classifications and
DCA ordination of 176 semi-deciduous forest relevés result in six forest types,
patterned along climatic and edaphic gradients. The described forest types and
their floristic composition (both at species and family levels) are comparable to
those recognised in other parts of West Africa where the semi-deciduous forest
is more continuous. The leading dominant families were Malvaceae, Fabaceae,
Cannabaceae, Putranjivaceae, Ebenaceae and Moraceae. The Drypetes afra-
mensis–Nesogordonia papaverifera forest type appears to be that in which the
Cannabaceae, Malvaceae, Putranjivaceae and Violaceae are best represented.
The record of many Upper Guinean endemic species in Benin’s forest islands
provids strong evidence for past floristic connections of the Dahomey Gap with
the West African rain forest zone.
Key words : semi-deciduous forest - patterns - ordination - importance value
index - Benin.
Résumé.- Les groupements de forêt dense humide semi-décidue du Bénin sont
étudiés au moyen de relevés phytosociologiques et d’analyses multivariées. Les
valeurs d’importance des espèces et familles sont calculées pour chaque type
de forêt. La classification et l’ordination de 176 relevés ont permis la discrimina-
tion de six groupements s’ordonnant le long d’un gradient climatique et éda-
phique. Les groupements identifiés et leur composition floristique sont similaires
à ceux décrits dans les autres régions ouest-africaines où la forêt dense humide
semi-décidue est plus continue. Les familles écologiquement importantes sont :
Malvaceae, Fabaceae, Cannabaceae, Putranjivaceae, Ebenaceae and
Moraceae. Le groupement à Drypetes aframensis et Nesogordonia papaverife-
ra est celui dans lequel les familles comme Cannabaceae, Malvaceae,
Putranjivaceae et Violaceae sont mieux représentées. La présence d’espèces
endémiques au bloc forestier occidental dans les îlots forestiers du Bénin est le
fait d’une irradiation de la forêt dense ouest-africaine dans le Dahomey Gap.
Key words : forêt semi-décidue - patterns - ordination - indice de valeur d’im-
portance - Bénin.
received January 5, 2008, accepted April 21, 2008
I. INTRODUCTION
The Dahomey Gap (hereafter DG) is the arid corridor in the West African rain forest belt,
where savannahs extend southwards to the coast between eastern Ghana, through Togo and
Benin, to the western border of Nigeria. It is considered to be an important ecogeographi-
cal barrier to both forest animals and plants and, as such, is of a crucial significance for
their distribution patterns in West Africa (van Bruggen, 1989; Martin, 1991; Jenik, 1994).
It remains one of the major imprints of the Pleistocene and Holocene arid periods. It is eco-
logically characterised by a decline in annual rainfall from more than 2000 mm in the
Upper and Lower Guineas to 900-1300 mm in the dry corridor. Consequently, the DG is
almost completely devoid of evergreen lowland rain forest and its associated plant and ani-
mal species. It should be emphasised that the African lowland rain forest consists of two
floristically and climatically different components: i) the evergreen forest belt with annual
rainfall > 2000 mm/year, dry season period < 3 months, tree layer rich in evergreen spe-
cies belonging to the Fabaceae and ii) the semi-evergreen or semi-deciduous forest belt
with annual rainfall < 1600 mm/year, dry season period > 3 months, tree layer rich in deci-
duous species belonging to the Cannabaceae, Malvaceae and Moraceae (White, 1983;
Vooren & Sayers, 1992).
Although the DG is somewhat arid, there are intermittent islands of semi-deciduous
forest in a forest-savannah mosaic pattern, lowland evergreen forest being completely
absent. It can be hypothesized that these forest islands were part of the West African semi-
deciduous forest belt. Thus, the species composition and the community structure of these
forest remnants are expected to be similar to the semi-deciduous forest situated in Upper
and Lower Guineas.
Important contributions to the phytogeography of the DG include vegetation studies in
southern Togo and southern Benin, which constitute the core area of the DG (Aubréville,
1937; Paradis, 1983; Akoègninou, 2004; Ern, 1988; Akpagana, 1989; Jenik, 1994; Sokpon,
1995; Dupont & Weinelt, 1996; Kokou, 1998; Kokou et al., 1999; Tossou, 2002; Kokou &
Sokpon, 2006). It results from these works that the climatic climax of the DG is the semi-
deciduous forest. Yet, the types of semi-deciduous forest based on species composition
remain to be documented. None of the previous studies have attempted to explicitly and
objectively assess the patterns of semi-deciduous forest in Benin. This paper analyses the
floristic composition of semi-deciduous forest remnants in Benin and compares the forest
types to those described in other parts of West Africa; Ghana being the West African coun-
try where forest floristic composition is more fully described (Hall & Swaine, 1976, 1981;
Swaine, 1996). The insight into the current forest pattern is a prerequisite knowledge for
palynological research that has been carrying out in Benin (Tossou, 2002; Salzmann &
Hoelzmann, 2005). Thus, this paper is a contribution to the West African phytogeography
and ecology.
II. STUDY AREA
The study area involved the semi-deciduous forest islands of Benin (Fig. 1), most of which
are located in southern and centre-western parts of the country. Southern Benin falls in the
core area of the Dahomey Gap.
Southern Benin has a Guinean climate with a bimodal rainfall regime i.e. two rainy sea-
sons and two dry seasons (Adjanohoun et al., 1989; Akoègninou, 2004; Fig. 2a). The area
160
161
Fig. 1.- Vegetation map of West Africa showing the rain forest zone and Benin indicating the
semi-deciduous forest sites of data collection. The Dahomey Gap is the light grey area
between the Lower and Upper Guinea forest blocks.
Fig. 1.- Carte de végétation de l’Afrique de l’Ouest montrant la zone de forêt dense humi-
de et le Bénin et indiquant les sites de collecte de données. Le Dahomey Gap corres-
pond à la zone gris-clair entre les blocs forestiers oriental et occidental.
Fig. 2.- Ombrothermograms showing the climate of the area where semi-deciduous forest
islands occur in Benin. (a) Pobè in southern Benin with bimodal rainfall regime, (b)
Bassila in centre-western Benin with unimodal rainfall regime.
Fig. 2.- Diagrammes ombrothermiques montrant le climat de la zone d’occurrence de forêt
dense humide semi-décidue au Bénin. a) Pobè au sud du Bénin avec un régime pluvio-
métrique bimodal, (b) Bassila au centre-ouest du Bénin avec un régime pluviométrique
unimodal.
162
is characterised by two rainfall gradients (Paradis, 1983): a strong east-west gradient of
decreasing annual rainfall (1200-1300 mm in the east and 900 mm in the west) and a weak
south-north gradient with a trend towards a unimodal rainfall regime i.e. north of latitude
7° North the two rainfall peaks tend to merge into one peak. The coastal area is characte-
rised by high air humidity (70-80%). The mean annual temperature is 27 °C. Much of the
area is dominated by a plateau (with ferrallitic soil) bisected by the depression of Lama
(with vertisol). The dominant soil is deep-ferrallitic on sedimentary rock. Forest sites such
as Pobè, Dangbo and Niaouli are dissected by streams, around which the soil moisture is
higher.
The centre-western part of the country has a tropical climate with a unimodal rainfall
regime (Fig. 2b). The annual rainfall ranges from 1100 to 1300 mm. The mean annual tem-
perature is 26-28 °C. The dominant soil is shallow-ferrallitic with concretions on crystal-
line rock.
The prevailing wind throughout the year is a breeze from the south-west, but from
January to February the Harmattan, a northerly dry wind from the Sahara, reaches the
coast.
III. MATERIALS AND METHOD
A. Data collecting
The existing semi-deciduous forest remnants were gazetted with the help of locally
experienced botanists and ecologists. Site selection was based on the size and the conser-
vation state of forest reserves (legally protected forest, sacred forest and communal forest).
The forest size ranges from 0.5 to 4777 ha. At each site, we carried out phytosociological
relevés within quadrates of 30 x 30 m2according to Braun-Blanquet’s (1932) approach.
We visually estimated the cover of each species using the following cover/abundance scale
(Westhoff & van der Maarel, 1978): + rare or less than 1% cover, 1 1-5% cover, 2 5-25%
cover, 3 25-50% cover, 4 50-75% cover and 5 75-100% cover. The vegetation structure
(number of layers, their cover and height) was visually described. Moreover, all trees and
lianas of a diameter at breast height (dbh) ≥ 10 cm were tagged, their number counted and
dbh measured. General information related to soil conditions (texture and flooding) and
topography (plateau, slope and valley) was noted. A total of 176 vegetation relevés were
performed with a record of 559 plant species. Botanical nomenclature for genus and spe-
cies followed Lebrun & Stork (1991-1997) and Akoègninou et al. (2006); for families, it
follows the modern systematics (Angiosperm Phylogeny Group, 2003).
B. Data processing
Semi-deciduous forest patterns
We constructed a data matrix of 176 semi-deciduous forest relevés and 559 plant spe-
cies using the abundance/cover class data. The 176 forest relevés were classified into clus-
ters (groups of relevés) using two different cluster analysis techniques: the Two Way
Indicator Species Analysis or TWINSPAN with the default options (Hill, 1979a) and the
Average Linkage technique using Euclidean distance as similarity measure (Dunn &
Everitt, 1982). We drew a consensus pattern by confronting the outputs from the two clus-
tering methods. We used the Detrended Correspondence Analysis or DCA (Hill, 1979b) to
ordinate the forest relevés and to detect the underlying ecological gradients/factors within
the data structure. The delineation of forest types in the ordination space was facilitated by
163
the clusters generated by the classifications. Forest types were characterised using the most
differential species with high ecological importance value.
Species and family importance values (IV)
We used importance values (IV) to characterise the different forest types and reveal
dominance. They were calculated both for species and families using the following for-
mulas:
1) The plant species importance value (SIV) is defined as the sum of its relative density,
relative dominance and relative frequency (Cottam & Curtis, 1956).
relative density = 100x(number of stems of a species)/(total number of stems)
relative dominance = 100x(total basal area of a species)/(total basal area of all species)
relative frequency = 100x(frequency of a species)/(sum frequency of all species)
basal area = (dbh/2)2x π, where dbh is the diameter at breast height
2) The plant family importance value (FIV) is defined as the sum of its relative density,
relative dominance and relative diversity (Mori et al., 1983).
relative diversity = 100x(number of species in a family)/(total number of species)
The relative density and relative dominance of a family were obtained by summing up the
values of itsspecies components.
The values of SIV and FIV range from 0, indicating an absence of dominance, to 300,
indicating a monodominance. They allow identifying the leading dominant entities, i.e.
those species and families having the highest ecological value. Reitsma (1988) considered
as ecological dominant species or families those having SIV or FIV ≥ 10.
Similarity among semi-deciduous forest types
The species lists of the described forest types were pairwise compared using the
Jaccard’s (1901) similarity index Pj, which is given by the formula:
Pj= 100xc/(a+ b - c)
where ais the number of species present in the community A, bis the number of species
in the community B and cis the number of species shared by A and B. The values of Pj
range from 0, for an absence of similarity, to 100, for a complete similarity.
IV. RESULTS
A. Semi-deciduous forest patterns in Benin
Cluster analyses of the 176 semi-deciduous forest relevés and 559 plant species result
in six main clusters, which correspond to six forest types. The DCA ordination of the same
matrix shows that these forest types are patterned along two ecological gradients (Fig. 3,
Table I).
The axis 1 can be interpreted as an edaphic gradient with the Chrysobalanus ellipti-
cus–Barteria nigritana community on coastal sandy soil (Ahozon forest) to the Drypetes
aframensis–Nesogordonia papaverifera community on ferrallitic soil (Ewè forest) through
the Mimusops andongensis–Cynometra megalophylla community on vertisol (Lama’s
forest).
The axis 2 can be interpreted as a climatic gradient with the Strombosia pustulata-
Piptadeniastrum africanum and Triplochiton scleroxylon-Celtis zenkeri communities in
the Guineo-Congolian region with a bimodal rainfall regime and the Khaya grandifoliola-
Aubrevillea kerstingii community in the Guineo-Sudanian transition zone with a unimodal
rainfall regime.
The Strombosia pustulata-Piptadeniastrum africanum community - the most humid
type in Benin - always occurs under a bimodal rainfall regime of 1100-1300 mm/year on
the lower slope, a few strides from the stream crossing the forest site. Reported from the
forest islands of Pobè, Dangbo and Niaouli (Fig. 1), it is characterised by the exclusive pre-
sence of species such as Piptadeniastrum africanum, Parkia bicolor, Pentaclethra macro-
phylla, Canarium schweinfurthii, Distemonanthus benthamianus and Strombosia
pustulata. The Triplochiton scleroxylon-Celtis zenkeri community generally occurs on pla-
teau in southern Benin, under a bimodal rainfall regime of 900-1300 mm/year, but is also
reported in the centre-western Benin in the tract Doumè-Bantè under a unimodal rainfall
regime with an annual rainfall of 1200 mm; it can be considered as a relatively dry and flo-
ristically-poor type of the Strombosia pustulata-Piptadeniastrum africanum community.
Similarly, the Khaya grandifoliola-Aubrevillea kerstingii community can be regarded as a
climatically dry type of the Triplochiton scleroxylon-Celtis zenkeri forest type. The semi-
deciduous forest species that reach the latitude 9° 40’ North in the locality of Djougou
(Fig. 1) include Celtis zenkeri, Holoptelea grandis, Cola gigantea and Trilepisium mada-
gascariense. The establishment and maintenance of semi-deciduous forest at this latitude
164
Fig. 3.- DCA ordination of 176 semi-deciduous forest samples and 559 plant species in
Benin, showing six forest types. I: Drypetes aframensis-Nesogordonia papaverifera com-
munity, II: Mimusops andongensis-Cynometra megalophylla community, III:
Chrysobalanus ellipticus-Barteria nigritana community, IV: Strombosia pustulata-
Piptadeniastrum africanum community, V: Triplochiton scleroxylon-Celtis zenkeri commu-
nity and VI: Khaya grandifoliola-Aubrevillea kerstingii community. Axis 1: eigenvalue =
0.46, gradient length = 4 SD-units. Axis 2: eigenvalue: 0.42, gradient length = 3.5 SD-
units. SD = standard deviation.
Fig. 3.- Ordination utilisant la DCA de 176 relevés de forêt dense humide semi-décidue et
559 espèces au Bénin, montrant six communautés végétales.
165
Table I.- Ecological and floristic characteristics the semi-deciduous forest types described
in Benin.
Tableau I.- Caractéristiques écologiques et floristiques des communautés de forêt dense
humide semi-décidue décrites au Bénin.
Plant community Differential species Rainfall Soil
Strombosia pustulata- Strombosia pustulata, Piptadeniastrum Bimodal rainfall Ferrallitic soils on
Piptadeniastrum africanum (4 layers, africanum, Parkia bicolor, regime, sedimentary rock
tree layer is 15-25 m tall) Terminalia superba, Distemonanthus with 1100- (deep with temporary
(40 relevés) benthamianus 1300 mm/year flooding)
Triplochiton scleroxylon-Celtis Triplochiton scleroxylon, - Bimodal rainfall Mainly on
zenkeri (4 layers, tree layer is 15- Celtis zenkeri regime, with 900- ferrallitic soils
25 m tall) (30 relevés) 1300 mm/year on sedimentary
- Unimodal rainfall regime or crystalline
with 1200 mm/year rock
Khaya grandifoliola-Aubrevillea Khaya grandifoliola, Unimodal rainfall Ferrallitic soils on
kerstingii (3 layers, tree layer is 10- Aubrevillea kerstingii regime with 1200- crystalline rock
20 m tall) (40 relevés) 1300 mm/year (shallow)
Chrysobalanus ellipticus-Barteria Chrysobalanus ellipticus, Bimodal rainfall regime, Coastal sand
nigritana (3 layers, tree layer is 10- Barteria nigritana, with 900-1200 mm/year
20 m tall) Syzygium guineense var. littorale,
(29 relevés) Trichocypha oba,
Tricalysia faranahensis,
Spatandra blackeoides
Mimusops andongensis-Cynometra Mimusops andongensis, Bimodal rainfall regime Vertisols
megalophylla (3 layers, tree layer is 10- Cynometra megalophylla, with 1100 mm/year (temporary flooding)
20 m tall) (12 relevés) Erythroxylum emarginatum
Drypetes aframensis-Nesogordonia Drypetes aframensis, Bimodal rainfall regime Ferrallitic soils
papaverifera (4 layers, tree layer is 15- D. gilgiana, Rinorea ilicifolia, with on sedimentary
25 m tall) (25 relevés) R. kibbiensis, Nesogordonia 1100-1200 mm/year rock (deep)
papaverifera, Octobolus spectabilis,
Mansonia altissima,
Pterygota macrocarpa
seem to be favoured by the rainfall amount of 1300 mm/year and the soil which is fairly
ferrallitic.
Tables II and III summarise the importance values of ecological dominant families and
species for each forest type. The leading dominant species include Dialium guineense,
Cola gigantea, Trilepisium madagascariensis, Celtis zenkeri, Triplochiton scleroxylon,
Diospyros mespiliformis, Drypetes aframensis, etc. The most dominant families, in decrea-
sing order, include Malvaceae, Fabaceae, Cannabaceae, Putranjivaceae, Ebenaceae and
Moraceae.
The plant families such as Malvaceae, Cannabaceae and Putranjivaceae are best repre-
sented in the Drypetes aframensis-Nesogordonia papaverifera community which is res-
tricted to the forest remnant of Ewè (Fig. 1). This forest type corresponds to the richest in
Rinorea with four species recorded: R. brachypetala, R. kibbiensis, R. dentata and R. ili-
cifolia, which dominate the underwood in several stands with a cover sometimes reaching
80%.
166
Table II.- Species importance value index (SIV) computed for the six described semi-deci-
duous forest types. Only values of SIV ≥ 10 (ecological dominant species) are displayed;
values of SIV ≥ 20 are in bold. Semi-deciduous forest types: Chr-Bar: Chrysobalanus
ellipticus-Barteria nigritana community, St-Pi: Strombosia pustulata-Piptadeniastrum afri-
canum community, Tr-Ce: Triplochiton scleroxylon-Celtis zenkeri community, Mi-Cy:
Mimusops andongensis-Cynometra megalophylla community, Dr-Ne: Drypetes aframen-
sis-Nesogordonia papaverifera community, Kh-Au: Khaya grandifoliola-Aubrevillea kers-
tingii community.
Tableau II.- Indice de valeur d’importance des espèces (SIV) calculé pour les six types de
forêt dense humide semi-décidue. Seules les valeurs SIV ≥ 10 (espèces écologiquement
dominantes) sont affichées; valeurs de SIV ≥ 20 en gras.
Plant species Family Chr-Bar Mi-Cy St-Pi Tr-Ce Dr-Ne Kh-Au
Afzelia africana Fabaceae - 19.18 -- - -
Albizia ferruginea Fabaceae ---- -10
Anogeissus leiocarpa Combretaceae - 18.52 -- -21.80
Antiaris toxicaria Moraceae --16 15.46 18.15 15.46
Barteria nigritana Passifloraceae 11.49 --- - -
Ceiba pentandra Malvaceae - 18.80 11 31 32 11
Celtis integrifolia Cannabaceae ---- -10
Celtis mildbraedii Cannabaceae --11.60 - 13.64 -
Celtis philippensis Cannabaceae 10.30 23.71 - 12.24 - 10
Celtis zenkeri Cannabaceae --15.21 49.18 16.10 13.58
Chrysobalanus icaco Chrysobalanaceae 14.55 -- - - -
Chrysophyllum albidum Sapotaceae --10 ---
Cleistopholis patens Annonaceae 13.30 --21.26 --
Cola gigantea Malvaceae ---42.42 - 30.30
Cola millenii Malvaceae 10.25 - 13.71 --10
Cynometra megalophylla Fabaceae - 13.12 -- - -
Dialium guineense Fabaceae 84.65 60.85 -- - -
Diospyros mespiliformis Ebenaceae - 43.71 -- -15.84
Drypetes aframensis Putranjivaceae ----41.15 -
Drypetes floribunda Putranjivaceae - 12.51 -- 12 -
Holoptelea grandis Ulmaceae ---- -19
Khaya grandifoliola Meliaceae ---- -10
Lannea nigritana Anacardiaceae ---13 - 11.75
Lecaniodiscus cupanioides Sapindaceae 17.14 -- - - -
Macaranga barteri Euphorbiaceae 10.35 --- - -
Maranthes robusta Chrysobalanaceae 10.25 --- - -
Mimusops andongensis Sapotaceae - 30.14 -- - -
Nesogordonia papaverifera Malvaceae ----12.30 -
Olax subscorpioidea Olacaceae 10.18 --- - -
Parkia bicolor Fabaceae --11.60 ---
Piptadeniastrum africanum Fabaceae --10.50 ---
Pouteria alnifolia Sapotaceae ---- -21.80
Rothmannia urcelliformis Rubiaceae ---11.19 --
Strombosia pustulata Oleaceae --30.50 ---
Tabernaemontana pachysiphon Apocynaceae --18.20 15.30 --
Terminalia superba Combretaceae --12.24 ---
Trilepisium madagascariensis Moraceae --39 17 --
Triplochiton scleroxylon Malvaceae --11.80 35 24.38 -
Zahna golungensis Sapindaceae ---- -13
Others - 107.54 59.46 88.64 36.95 130.28 76.47
167
The Chrysobalanus ellipticu-Barteria nigritana community on coastal sand (Ahozon’s
forest) is largely dominated by Fabaceae and Chrysobalanaceae while Fabaceae,
Sapotaceae and Ebenaceae are the leading families of the Mimusops andongensis-
Cynometra megalophylla forest type on vertisol (Lama’s forest).
B. Floristic similarity among semi-deciduous forest types
Table IV summarises the floristic relationships among the six forest types described.
The highest similarity value is between the Strombosia pustulata-Piptadeniastrum africa-
num community on lower slope and
Triplochiton scleroxylon-Celtis zenkeri commu-
nity on plateau, which share 64% of their spe-
cies. We keep them as separate communities,
since they are ecologically distinct and the lea-
ding dominant species and families appear to be
different (Tables II, III). The similarity values
among all other pairs of communities are low
and range from 28 to 38; this is due to the fact
that the forest types share a common floristic
background, which includes forest plant species
with large ecological amplitude such as Antiaris
toxicaria, Milicia excelsa, Ceiba pentandra,
Dialium guineense, Celtis philippensis,
Oxyanthus racemosus and Rothmannia longi-
flora.
Table III.- Family importance value index (FIV) computed for the six described semi-deci-
duous forest types. Only values of FIV ≥ 10 (ecological dominant families) are displayed;
values of FIV ≥ 20 are in bold. Semi-deciduous forest types as in Table II.
Tableau III.- Indice de valeur d’importance des familles (FIV) calculé pour les six types de
forêt dense humide semi-décidue. Seules les valeurs FIV ≥ 10 (familles écologiquement
dominantes); Valeurs FIV ≥ 20 en gras.
Family Chr-Bar St-Pi Tr-Ce Mi-Cy Dr-Ne Kh-Au
Chrysobalanaceae 23.34 -----
Passifloraceae 12.22 -----
Euphorbiaceae 12.20 10.05 ----
Olacaceae - 26 ----
Anacardiaceae --11.50 --15
Annonaceace 15 14 27.67 ---
Sapotaceae ---25 - 24.33
Putranjivaceae ---11 51.52 -
Ulmaceae -----11.51
Phyllanthaceae -----13
Ebenaceae ---38 - 13.16
Cannabaceae - 18.35 52.70 17.20 33.40 37
Combretaceae - 15.32 - 20 - 24
Fabaceae 105.14 50.1 22.26 100.65 18 49
Malvaceae 10.24 59.22 108.13 15.55 78.54 37
Moraceae 11.52 31.58 25 11.11 19 30
Rubiaceae - 21.91 --14.47 -
Sapindaceae 12.63 - 10 15 - 16
Others 97.71 53.47 42.74 46.49 85.07 30
Table IV.- Matrix of similarity between
semi-deciduous forest types based on
the distribution of 559 plant species
using Jaccard’s similarity index. Semi-
deciduous forest types: as defined in
table II.
Tableau IV.- Matrice de similarité entre
les types de forêt dense humide semi-
décidue basée sur la distribution de
559 espèces utilisant l’indice de simi-
larité de Jaccard.
Forest types Chr-Bar St-Pi Tr-Ce Mi-Cy Dr-Ne
St-Pi 34
Tr-Ce 34 64
Mi-Cy 33 28 33
Dr-Ne 31 32 34 36
Kh-Au 30 33 38 33 33
168
V. DISCUSSION
We described six major semi-deciduous forest types in Benin. These can be considered as
part of the West African semi-deciduous foret belt. Thus, the following comparisons can
be made.
The Strombosia pustulata-Piptadeniastrum africanum is part of the West African moist
semi-deciduous forest (Lebrun & Gilbert, 1954; Letouzey, 1968; Hall & Swaine, 1981;
Vooren & Sayers, 1992). It corresponds to the Triplochiton scleroxylon-Strombosia pustu-
lata community of Sokpon & Lejoly (1994), which was described from the forest of Pobè
in south-east Benin. This plant community is fundamentally typified by the presence of
moist tree species such as Piptadeniastrum africanum, Parkia bicolor, Pentaclethra
macrophylla, Canarium schweinfurthii, Strombosia pustulata, Distemonanthus bentha-
mianus, Celtis mildbraedii, Terminalia superba and Anthonotha macrophylla (Lebrun &
Gilbert, 1954; Bongers et al., 1999). The Guineo-Congolian endemic genera such as
Amphimas, Anthonotha, Distemonanthus, Anthrocaryon, Coelocaryon and
Discoglypremna (White, 1983; Swaine, 1996) are restricted to this forest type in Benin. In
Ghana, this forest type occurs is characteristic of wet and infertile site (Swaine, 1996),
while in Benin it is associated to down slope fringing wet-swampy areas.
The Drypetes aframensis-Nesogordonia papaverifera community, which is restricted to
the Ewè’s forest relic (Fig. 1), is floristically comparable to the moist semi-deciduous
forest of Richards (1939) in Nigeria and corresponds to the Celtis spp.-Mansonia altissi-
ma community of Guillaumet & Adjanohoun (1971) in Ivory Coast. These authors consi-
dered this forest type as the climatic climax of the West African semi-deciduous forest
because of the dominance of Cannabaceae and Malvaceae. Forest species distribution stu-
dies in Ghana (Swaine, 1996) showed that this forest type is characteristic of dry-fertile
sites with Mansonia altissima-Pterygota macrocarpa as species association. We found that
this semi-deciduous forest type is characterised by the best representation of species belon-
ging to the families of Cannabaceae such as Celtis spp., Malvaceae such as Nesogordonia
papaverifera and Mansonia altissima and Putranjivaceae such as Drypetes spp. (Vooren &
Sayers, 1992; Swaine, 1996). Furthermore, its richness in Rinorea species (with R. bra-
chypetala, R. dentata, R. kibbiensis and R. ilicifolia) and their abundance in the forest floor
are unique in the country and good indicators of its climax state (Achoundong et al., 2000).
R. ilicifolia, characteristic of semi-deciduous forest in Cameroon (Achoundong, 1996),
appears to be restricted to this forest type in Benin.
The Triplochiton scleroxylon-Celtis zenkeri community is floristically similar to the
Triplochiton scleroxylon-Afzelia africana community of Schnell (1952) in West Africa, the
Celtis-Triplochiton community of Guillaumet & Adjanohoun (1971) in Ivory Coast, the
dry semi-deciduous forest of Hall & Swaine (1981) in Ghana and the Triplochiton scle-
roxylon-Dialium guineense community of Sokpon (1995) in south-eastern Benin.
The Chrysobalanus ellipticus-Barteria nigritana community, the coastal semi-deci-
duous forest type, is similar to the south-east outlier forest type in Ghana where the most
important species is Dialium guineense (Hall & Swaine, 1981). In Benin, this species also
appears to be the leading element in this forest type. The frequent occurrence of species
such as Baphia nitida, Xylopia aethiopica, Barteria nigritana, Pycnanthus angolensis,
Alstonia congensis and Symphonia globulifera suggests that this forest type can be regar-
ded as the dry type of the West African coastal evergreen forest (Letouzey, 1968; Tchouto,
2004).
169
The Mimusops andongensis-Cynometra megalophylla community corresponds to the
Diospyros mespiliformis community of Kokou (1998) in South Togo and seems to repre-
sent the pedo-climax semi-deciduous forest on vertisol. The abundance of Dialium gui-
neense, Diospyros mespiliformis, Cynometra megalophylla, Afzelia africana and
Erythroxylum emarginatum illustrates its floristic link to the dry semi-deciduous forest in
Upper Guinea (Hall & Swaine, 1981; Hovestadt et al., 1998).
The Khaya grandifoliola-Aubrevillea kerstingii community is floristically similar to the
forest dominated by Malvaceae and Sapotaceae of Akpagana (1989) in Togo and the dry
semi-deciduous forest « fire zone subtype » of Hall & Swaine (1981) in Ghana.
Recognised as such in Ivory Coast by Guillaumet & Adjanohoun (1971), these authors
regarded it as the northernmost type of semi-deciduous forest characteristic of the fire-
prone forest-savannah boundary. The two characteristic species Aubrevillea kerstingii and
Khaya grandifoliola, which are Guineo-Congolian and Upper Guinean endemics respecti-
vely (White, 1983; Bongers et al., 2004), are found to be restricted to centre-western
Benin. In Ivory Coast the northern limit of the semi-deciduous forest is marked by the dis-
tribution limit of Aubrevillea kerstingii (Bongers et al., 2004).
To summarise, the semi-deciduous forest of Benin exhibits strong affinities to the West
African semi-deciduous forest in terms of overall species composition and forest types. It
corresponds to the dry peripheral semi-evergreen rain forest of the regional Guineo-
Congolian phytochorion (White, 1983). The occurrence of the Dahomey Gap seems to
have little influence on the plant community structure.
The ecological dominant species varied from one forest type to another (Table II). The
most important included Triplochiton scleroxylon, Cola gigantea, Celtis spp., Drypetes
spp., Trilepisium madagascariensis, Dialium guineense, Ceiba pentandra, etc. Higher
figures would have been observed for some species such as Mansonia altissima (pole used
for roofing and ceiling) and Khaya grandifoliola (timber) if they had not been overexploi-
ted.
The leading dominant families of forest types described in Benin are similar to those
characteristics of West African semi-deciduous forest belt (White, 1983; Vooren & Sayers,
1992; Bakayoko et al., 2001); it includes Malvaceae, Cannabaceae, Fabaceae, Moraceae,
Ebenaceae, Rubiaceae, Putranjivaceae and Euphorbiaceae.
The effect of climate on the distribution of forest types in Benin seems to be both in
terms of annual rainfall amount and rainfall regime, i.e. the distribution of rainfall over the
year (Hall & Swaine, 1976, 1981; Swaine, 1996; Bongers et al., 1999; Adomou, 2005;
Adomou et al., 2006). This was illustrated by the fact that dry types of semi-deciduous
forest colonise the centre-western Benin where the rainfall regime is unimodal with an
annual rainfall of 1100-1300 mm while wet types are restricted to southern Benin where
the rainfall regime is bimodal with an annual rainfall of 1100-1300 mm (Fig. 2). Thus, the
length and severity of the dry season seem to be strong determinants of forest community
patterns in Benin (Veenendaal et al., 1996; Wieringa & Poorter, 2004). On the other hand,
soil type seems to vary along with forest type in a way that it would be difficult to separa-
te its influence from that of climate (Swaine, 1996; Kouamé et al., 2004).
Records of Upper Guinean endemics including Zanthoxylum gilletii, Turraea hetero-
phylla, Tricalysia reticulata, T. faranahensis, Urera obovata, Dennetia tripetala,
Pierreodendron kerstingii and Khaya grandifoliola (Poorter et al., 2004) in Benin forest
relics strongly support the past floristic connection of southern and centre-western Benin
with the Upper Guinean region.
170
VI. CONCLUSION
Six major semi-deciduous forest types were recognised in Benin. The combined effect of
climate and soil seem to be the major driving forces of the variation in species composi-
tion. They are floristically comparable to those recognised in other parts of West Africa.
Thus, they can be considered as part of the West African semi-deciduous forest belt.
Acknowledgements - This study, in the framework of the Project Flora of Benin, was funded by the Netherlands
government grant under the project contract number Bj 003407. We are grateful to Dr. P. Ketner for helpful comments
and suggestions and Dr. J.J. Wieringa for the GIS mapping.
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