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Latitudinal variation in the woody species diversity of Afzelia africana Sm. habitats in West Africa

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Abstract
This study assessed the woody flora composition of Afzelia africana Sm. habitats along a latitudinal gradient, from the northern limit of the species distribution to the Guinean littoral forest. Data were collected from 201 sample units located in different vegetation types that span four bioclimatic zones: Guinean, Sudano-Guinean, Sudanian and Sahelo-Sudanian zones. The woody flora diversity was described by computing the estimated species richness and the Shannon diversity index within EstimateS 9.1, based on the observed species richness. A sample-based randomization procedure with 95 % confidence intervals was used to compare the patterns of plant richness between vegetation stands. A Non-Metric Multidimensional Scaling was performed on presence-absence data matrix to explore the patterns of woody species composition in natural stands. A Canonical Correspondence Analysis was further applied to correlate the patterns of habitat differentiation with climatic variables (temperature, precipitation) and altitude. A total of 165 woody species were recorded, with the highest species richness in Sahelo-Sudanian zone. There was no significant difference in richness between samples from Guinean, Sudano-Guinean and Sudanian zones. Plots in the Sudanian and Sudano-Guinean zones were similar but distinct from those of Guinean and Sahelo-Sudanian zones, a pattern that is supported by precipitation and temperature distributions. Results also suggest important co-occurring species characteristic of each habitat as inferred from the Important Value Index (IVI). It is recommended that habitats of A. africana in Sudanian and Sudano-Guinean zones receive similar management and conservation plans while the Guinean and the Sahelo-Sudanian zones can be treated separately.
Tropical Ecology 57(4): 717-726, 2016 ISSN 0564-3295
© International Society for Tropical Ecology
www.tropecol.com
Latitudinal variation in the woody species diversity of Afzelia africana
Sm. habitats in West Africa
SYLVANUS MENSAH1,3*, THIERRY DÈHOUÉGNON HOUÉHANOU1,2, ACHILLE EPHREM
ASSOGBADJO1,2, KENNETH AGBESI ANYOMI4, AMADÉ OUEDRAOGO5 & ROMAIN GLÈLÈ KAKAÏ1
1Laboratory of Biomathematics and Forest Estimations, Faculty of Agronomic Sciences,
University of Abomey-Calavi, 03 BP 2819, Cotonou, Bénin
2Laboratory of Applied Ecology, Faculty of Agronomic Sciences, University of Abomey-Calavi, 03
BP 2819, Cotonou, Bénin
3Department of Forest and Wood Science, Stellenbosch University, South Africa
4Centre d’Étude de la Forêt, Faculté de foresterie, de géographie et de géomatique, Université
Laval, 2405 rue de la Terrasse, Québec, QC, G1V 0A6 Canada
5Département de Biologie et Physiologie Végétales, Université de Ouagadougou, 03 BP 7021
Ouagadougou, Burkina Faso
Abstract: This study assessed the woody flora composition of Afzelia africana Sm. habitats
along a latitudinal gradient, from the northern limit of the species distribution to the Guinean
littoral forest. Data were collected from 201 sample units located in different vegetation types
that span four bioclimatic zones: Guinean, Sudano-Guinean, Sudanian and Sahelo-Sudanian
zones. The woody flora diversity was described by computing the estimated species richness and
the Shannon diversity index within EstimateS 9.1, based on the observed species richness. A
sample-based randomization procedure with 95 % confidence intervals was used to compare the
patterns of plant richness between vegetation stands. A Non Metric Multidimensional Scaling
was performed on presence-absence data matrix to explore the patterns of woody species
composition in natural stands. A Canonical Correspondence Analysis was further applied to
correlate the patterns of habitat differentiation with climatic variables (temperature,
precipitation) and altitude.
A total of 165 woody species were recorded, with the highest species richness in Sahelo-
Sudanian zone. There was no significant difference in richness between samples from Guinean,
Sudano-Guinean and Sudanian zones. Plots in the Sudanian and Sudano-Guinean zones were
similar but distinct from those of Guinean and Sahelo-Sudanian zones, a pattern that is
supported by precipitation and temperature distributions. Results also suggest important co-
occurring species characteristic of each habitat as inferred from the Important Value Index
(IVI). It is recommended that habitats of A. africana in Sudanian and Sudano-Guinean zones
receive similar management and conservation plans while the Guinean and the Sahelo-
Sudanian zones can be treated separately.
Key words: Climatic gradient, conservation ecology, floristic composition, importance
value index, multidimensional scaling.
Handling Editor: Witness Mojeremane
*Corresponding Author; e-mail: sylvanus.m89@gmail.com
718 AFZELIA AFRICANA HABITATS IN WEST AFRICA
Introduction
There is consensus to the fact that large scale
variations in environmental factors drive natural
forest structures, plant distribution and diversity
(Higgins et al. 2012; Ouédraogo et al. 2013;
Stohlgren et al. 1999; Thuiller et al. 2008; Walther
et al. 2002). Yet, the influence of climate on the
patterns of associated plants remained a source of
concern and a topic for continuous research
(Hijmans & Graham 2006; Imbach et al. 2013;
Richerson & Lum 1980). Accordingly, previous
studies have attempted to explain species distri-
bution patterns along varied gradients (Austin &
Smith 1989; Grace 1999; Grime 2001; Meier et al.
2011; Whittaker 1973; Willig et al. 2003), that may
be regulatory (e.g. air temperature, soil pH),
ecological (e.g. productivity gradients, disturbance,
succession), or spatial (e.g. elevational, lati-
tudinal). The latitudinal gradient is associated
(positively or negatively) with many other causal
factors such as average temperature and preci-
pitation, range of temperature and precipitation,
net primary productivity, soil texture and
topography that could affect plant species diversity
and therefore plant species distribution.
Knowledge on species distribution patterns
and habitat diversity is of interest to both
ecologists and conservationists, especially because
co-occurrence or assemblage of species is relevant
for habitat integrity and ecosystem functioning.
Co-occurrence or co-existence of species is strongly
controlled by competition between species for
resources such as soil nutrients, moisture, light
and space (Begon et al. 2006). Competition may
vary spatially, as a result of spatial variation in
environmental conditions (Meier et al. 2011; Smith
et al. 1971).
West Africa (e.g. Benin, Burkina Faso and
Niger) is characterized by environmental and
climatic conditions that vary from the Sahel to the
Guinean littoral forest. The Sahel is characterized
by recurrent droughts and lower precipitation (100
mm and 600 mm), and is much drier than the
Guinean zone which benefits from an important
rainfall regime (1400 mm to 2700 mm) (Hijmans et
al. 2005a; Ouédraogo et al. 2013). Because of the
strong influence of this gradient on the sequence of
vegetation, plant communities differ in the
patterns of species distribution. Therefore, taking
into account the effects of these environmental
changes on the patterns of associated plants could
improve insight into tree species composition for
the maintenance of habitat integrity and the
conservation of targeted species. There are many
studies on plant community assembly, but only
those oriented in a way that makes it possible for
decision-makers to take into account an entire
habitat and not only the individual species have
the potential of saving species from becoming
extinct.
Afzelia africana Sm. (Fabaceae-Caesal-
pinioideae) is a widespread tree species found
along the latitudinal gradient, from the southern
Sahel to the Guinean littoral forest. Previous
studies showed that populations of A. africana Sm.
faced strong anthropogenic pressures (Houehanou
et al. 2013; Nacoulma et al. 2011; Mensah et al.
2014). The species is harvested for timber mainly
by indigenous communities and its foliage serves
as forage for livestock. A. africana is found with
many other species across its habitat (Akoègninou
et al. 2006) and such co-occurrence is likely
governed by climate-driven mechanisms. We
believe that, an analysis of woody floristic
composition of A. africana dominated vegetation
types along a latitudinal gradient could reveal the
important species assemblages and provide useful
information to support conservation strategies.
In this paper, we explored the woody flora
composition of A. africana Sm. habitats along a
latitudinal gradient, from the Northern limit of the
species distribution to the Guinean littoral forest
in West Africa. We addressed three research
questions: (1) Do habitats of A. africana signi-
ficantly vary in woody flora composition? (2) What
climatic variables are the major drivers of A.
africana habitats species composition? (3) What
are the most ecologically important species
associated with A. africana across its distribution
range?
Material and methods
Studied systems
This study was conducted between July, 2012
and June, 2013. Vegetation types were sampled
from four bioclimatic systems: the Guinean, the
Sudano-Guinean, the Sudanian and the Sahelo-
Sudanian zones. In each zone, sampling sites were
selected according to the availability of natural
populations of A. africana. The Lama Forest
reserve (Guinean zone), the Wari Maro forest
reserve (Sudano-Guinean zone), the Pendjari
biosphere reserve (Sudanian zone) in Benin and
the Sudanian phytogeographical areas of eastern
Burkina Faso (Sahelo-Sudanian zone) were selected
MENSAH et al. 719
Table 1. Characteristics of the study area.
Characteristics Lama Forest
Reserve (LFR)
Wari Maro Forest
Reserve (WMFR)
Pendjari Biosphere
Reserve (PBR)
Sudanian
phytogeographical areas
Geographical location 6° 55’ - 7° 00’ N
2° 04’ - 2° 12’ E
8° 80’ - 9° 10’ N
1° 55’ - 2° 25’ E
10° 30’ - 11° 30’ N
0° 50’ - 2° 00’ E
12° 35’ - 11° 14’ N
0° 10’ - 2° 30’ E
Area (ha) 1900 120686 266040 4669400
Climatic zone Guinean Sudano-Guinean Sudanian Sahelo-Sudanian
Dominant vegetation Dense forest and
fallows
Woodland Savannah and
woodland
Tree and shrub-
savannah
Rainfall regime Bimodal Unimodal Unimodal Unimodal
Annual rainfall range
(mm)
1000 - 1400 1100 - 1300 800 - 1100 600 - 900
Temperature range
(°C)
25 - 29 25 - 29 24 - 31 25 - 35
for data collection. Some characteristics of these
sites are presented in Table 1.
Forest inventories were done by means of
stratified random sampling scheme. One hundred
and thirty five 1 ha-plots were established,
respectively, in the Guinean and Sudano-Guinean
zones, whereas 35 and 31 plots of 0.09 ha were,
respectively, set up in the Sudanian and Sahelo-
Sudanian zones. The differences in plot sizes and
plot densities are due to the change in the type of
vegetation (Sudanian and Sahelo-Sudanian zones
are essentially savannahs, while woodland and
dense forest characterized the vegetation of
Sudano-Guinean and Guinean zones). The use of
0.09 ha as plot size in the Sudanian and Sahelo-
Sudanian zones was justified by the fact that
previous studies successfully used it in these
climatic zones (Houehanou et al. 2013; Nacoulma
et al. 2011). Within each plot, presence or absence
of A. africana was noted, only plots with A.
africana recorded were considered for further
analysis. Species names as well as diameter at
breast height (dbh) were recorded from all tree
species having dbh 10 cm. Data were also
collected on the altitude of each plot.
Climate data
Climate is expected to affect the ecology of
plant species (Algar et al. 2009; Braunisch et al.
2013; Record et al. 2013; Thuiller et al. 2004).
Climatic variables were extracted from WorldClim
data base (1950 - 2000) using DIVA-GIS 7.5
(Hijmans et al. 2005b). Variables included annual
mean temperature, mean temperature of wettest
quarter, mean temperature of the driest quarter,
mean temperature of warmest quarter, mean
temperature of coldest quarter, total annual preci-
pitation, precipitation of wettest quarter, preci-
pitation of driest quarter, precipitation of warmest
quarter, precipitation of coldest quarter and
altitude. Quarter refers to four months.
Data analysis
Discrimination of A. africana habitats along the
latitudinal gradient
The woody flora diversity was described by
calculating the estimated species richness and the
Shannon diversity index within the program
EstimateS 9.1 (Colwell 2011), based on the
observed species richness. The estimated richness
was calculated by using the abundance based
coverage estimator (Colwell 2011). A sample-based
randomization procedure with 95 % confidence
intervals was used to compare the patterns of
plant richness between vegetation stands. The
patterns of habitats were investigated by per-
forming a Non Metric Multidimensional Scaling
(NMDS) on presence-absence data matrix. Plots
were mapped onto two axes to enable each one to
be assigned to a cluster. The nearest plots were
considered as the most similar in species com-
position (Podani 2005) and were grouped in the
same cluster. The NMDS makes it possible to
detect an explanatory gradient of the variation of
woody flora composition. To explain the patterns of
habitat, a Canonical Correspondence Analysis
(CCA) was performed on the presence-absence
data in combination with climatic variables.
Monte-Carlo permutations test was further
performed to show which variables strongly affect
woody species composition.
720 AFZELIA AFRICANA HABITATS IN WEST AFRICA
Characterization of species composition of A.
africana habitats
To characterize the habitat species
composition, we assessed the relative availability
of woody species (tree and shrub) in each habitat,
by computing the Importance Value Index (IVI)
(Curtis and Macintosh 1951). IVI was computed
for each species as sum of its relative frequency,
density and dominance (basal area) in each
climatic zone:
Where ni, fi and ci are, respectively, the
density, frequency and basal area of species i. IVI
ranges from 0 to 300. Species with IVI > 10 were
retained to be ecologically important in each
climatic zone (Reitsma 1988). Statistical analyses
were carried out using the R 2.15.3 statistical
software package (R Development Core Team,
http://www.Rproject.org).
Results
Woody floristic composition of A. africana
habitats along the latitudinal gradient
A total of 165 woody species were observed.
The species accumulation curves (Fig. 1) showed
higher species richness in Sahelo-Sudanian
vegetation stands, and no significant differences in
richness between samples from Guinean, Sudano-
Guinean and Sudanian zones. Values of the
Shannon diversity index were 2.90 ± 0.02, N=100;
3.29 ± 0.035, N = 35; 2.66 ± 0.048, N = 35 and 4.18
± 0.032, N = 31 in the Guinean, Sudano-Guinean,
Sudanian and Sahelo-Sudanian zones, respecti-
vely.
According to the NMDS results, plots from the
same climatic zone were clustered (Fig. 2). The low
stress value (0.103) obtained from the NMDS
revealed an excellent representation into reduced
dimensions. Three clusters were discriminated: the
first one was made up of only samples from
Guinean zone, the second dominated by plots from
the Sudanian and Sudano-Guinean zones and the
third cluster constituted plots from the Sahelo-
Sudanian zone. These three groups were
designated to constitute the three habitats
patterns of A. africana along the latitudinal
gradient.
Fig. 1. Species accumulation curves for vegetation
stands in Sahelo-Sudanian, Guinean, Sudano-
Guinean and Sudanian zones. Grey dashed lines are
the lower and upper bounds of the 95 % confidence
interval for Sahelo-Sudanian zone. Plant richness of
Guinean, Sudano-Guinean and Sudanian fell outside
the 95 % confidence interval of the Sahelo-Sudanian
habitat.
Fig. 2. Non Metric Multidimensional scaling of plots
from Guinean, Sudano-Guinean, Sudanian and
Sahelo-Sudanian zones; P stands for plot.
Climatic variables influencing the floristic
composition of A. africana habitats
The results of CCA indicated that the first two
axes accounted for 57 % (38 % for the first axis and
19 % for the second one) of the total variance. The
eigen values of the first and the second axis were
0.83 and 0.40, respectively. Most of the climatic
variables showed high correlations (0.62 to 0.98)
with the two axes. The projection of these climatic
variables onto the two CCA axes (Fig. 3) showed
MENSAH et al. 721
Table 2. Results from permutations test showing the
most influencing variables.
Climatic variables ChiSquare
2)
F Pr
(> F)
Altitude 0.109 1.707 0.016
Annual mean temperature 0.106 1.665 0.025
Mean temperature of
wettest quarter
0.093 1.459 0.072
Mean temperature of driest
quarter
0.062 0.973 0.701
Mean temperature of
warmest quarter
0.100 1.577 0.043
Mean temperature of
coldest quarter
0.110 1.727 0.016
Annual precipitation 0.124 1.947 0.006
Precipitation of wettest
quarter
0.128 2.010 0.002
Precipitation of driest
quarter
0.100 1.576 0.028
Precipitation of warmest
quarter
0.123 1.930 0.054
Precipitation of coldest
quarter
0.119 1.874 0.030
gradients of altitude and precipitation on axis 1
while the axis 2 revealed temperature and
precipitation gradients. The habitat of A. africana
in the Guinean zone is discriminated by preci-
pitation of warmest quarter (PWmQ), total annual
precipitation (AnP) and precipitation and tempe-
rature of driest quarter (PDrQ, MnTDrQ). The
discrimination of A. africana habitat in the Sahelo-
Sudanian zone is explained by the effects of mean
temperatures of the coldest and warmest quarters
(MnTWmQ, MnTCoQ) and annual mean tempe-
rature (AnMT). The habitat in Sudanian and
Sudano-Guinean zone is discriminated by total
annual precipitation (AnP) and precipitation of
coldest quarter (PCoQ). The most influencing
variables were AnP, PWtQ, Altitude, MnTCoQ and
AnMT (Table 2).
Most ecologically important species
associated with A. africana along the
latitudinal gradient
The most dominant species in A. africana
habitats based on their IVI, are presented in
Table 3. A. africana was present in the four cli-
matic zones, with the highest IVI (65.2 %) recorded
in the Sudanian zone. Apart from A. africana, only
one species, Anogeissus leiocarpa, was common
in the four climatic zones. Also, apart from the
Fig. 3. Loading of sample units from Guinean,
Sudanian, Sudano-Guinean and Sahelo-Sudanian
zones in combination with environmental variables; P
stands for plot. Alt = Altitude, AnMT = Annual Mean
Temperature, MnTWmQ = Mean Temperature of
Warmest Quarter, MnTCoQ = Mean Temperature of
Coldest Quarter, MnTWtQ = Mean Temperature of
Wettest Quarter, MnTDrQ = Mean Temperature of
Driest Quarter, AnP = Annual Precipitation, PWmQ
= Precipitation of Warmest Quarter, PWtQ = Preci-
pitation of Wettest Quarter, PDrQ = Precipitation of
Driest Quarter, PCoQ= Precipitation of Coldest
Quarter.
Guinean zone, four important species (A. africana,
Pterocarpus erinaceus, Burkea africana and A.
leiocarpa) were all found in the Sudano-Guinean,
the Sudanian and the Sahelo-Sudanian zones.
Moreover, the Sudano-Guinean and Sudanian
zones shared many species. The most ecologically
important species in the Sudanian zone were A.
africana, P. erinaceus, Vitellaria paradoxa, Lannea
acida, Crossopteryx febrifuga, Daniellia oliveri and
B. africana whereas the most important ones in
the Sudano-Guinean zone were A. africana, P.
erinaceus, V. paradoxa, L. acida, B. africana,
Isoberlinia doka, Isoberlinia tomentosa, Monotes
kerstingii, and A. leiocarpa. All these species were
associated with A. africana in both Sudanian and
Sudano-Guinean zones. In addition to P. erinaceus,
B. africana and A. leiocarpa that were common,
other characteristic species associated with A.
africana in the Sahelo-Sudanian zone were
Bombax costatum, Sterculia setigera and Boswelia
dalzielii. In the Guinean zone, the important
species were Dialium guineense, Diospyros
mespiliformis, Drypetes floribunda, Mimusops
andongensis, Ceiba pentandra and Celtis brownii,
and seemed to be restricted to this habitat.
722 AFZELIA AFRICANA HABITATS IN WEST AFRICA
Table 3. Most ecologically important species in A. africana habitats in the four climatic zones with their IVI;
only species that recorded an IVI greater than 10 % in a climatic zone are shown.
Guinean Sudano-Guinean Sudanian Sahelo-Sudanian
Afzelia africana Sm. 16.7 16.4 65.2 40.2
Pterocarpus erinaceus Poir. - 14.2 52.6 50.1
Burkea africana Hook. - 13.5 14.1 34.7
Anogeissus leiocarpa (DC.) Guill. & Perr. 6.3 19.5 1.0 58.7
Vitellaria paradoxa C.F. Gaertn. - 27.7 43.7 -
Lannea acida A. Rich. - 15.4 18.8 -
Isoberlinia doka Craib & Stapf - 27.2 - -
Isoberlinia tomentosa (Harms) Craib & Stapf - 22.3 - -
Monotes kerstingii Gilg. - 21.2 - -
Crossopteryx febrifuga Benth. - 5.7 17.0 -
Daniellia oliveri (Rolfe) Hutch. & Dalziel - 7.4 15.3 -
Bombax costatum Pellegr. & Vuillet - 3.0 - 37.6
Sterculia setigera Del. - - - 33.3
Boswellia dalzielii Hutch. - - - 45.2
Dialium guineense Willd. 68.9 - - -
Diospyros mespiliformis Hochst. 49.2 - - -
Drypetes floribunda (Müll.Arg.) Hutch. 32.7 - - -
Mimusops andongensis Hiern 19.1 - - -
Ceiba pentandra (L.) Gaertn. 18.5 - - -
Celtis brownii Rendle. 17.4 - - -
Discussion
Patterns of diversity in A. africana habitats
along latitudinal gradient
In this study, a variation in species richness of
A. africana habitat was observed from the Sahelo-
Sudanian zone to other bioclimatic zones. The
Guinean zone seemed to have similar species
richness to the other regions, except for the
Sahelo-Sudanian zone where richness and
diversity were the highest. Higher species richness
at drier site shows that, climate does not seem to
substantially affect the trend in species richness.
This result contradicts the well-known global
patterns of species diversity, especially the high
tree species diversity of tropical rain forests
(Huston 1994). But such global pattern is more
likely evident at scales larger than the scale
covered in this study. The higher diversity in the
drier portion of the study area could be explained
by Huston’s model on recurrent disturbance.
Indeed, species diversity could become higher at
relatively low productivity (drier) sites, as a result
of a coupled effect of degree of disturbance and
growth rate (Huston 1979). This result also
accords with the intermediate disturbance
hypothesis (Connell 1978) in that, there would be
some intermediate levels of disturbance allowing a
community of plants to be maintained, with a
greater number of species recovering from that
disturbance (Catford et al. 2012; Huston 1979).
Moreover, if heterogeneity is expected to occur
where resources are abundant, it is possible that
heterogeneity be induced by recurrent disturbance
so that different patches can be observed. The
heterogeneity of the environments in Sudanian
and Sahelian areas (as results of disturbance
through grazing and pruning activities) might
have enabled more niches that support more
species. This seems to be true for the habitat of
A. africana in light of the current human pressure
that the species is facing.
Changes in woody flora composition of A.
africana habitats along latitudinal gradient
From past studies (Borchert 1998; Eamus et al.
2001; Meier et al. 2011; Thuiller et al. 2004), it is
evident that co-occurrence or assemblage life is the
outcome of functional strategies (e.g. competive-
MENSAH et al. 723
ness reduction, depth of soil exploited) developed
as responses to natural selection and climatic
specificity. In fact, climate creates environmental
conditions enable some species to achieve regular
growth. Moreover, because different species have
different physiological requirements, species co-
occurrence is realized through facilitation and
complementary effects. Our results revealed a
discrimination in habitat woody species diversity
along a latitudinal gradient. Indeed, the Sahelo-
Sudanian, the Sudanian and the Sudano-Guinean
zones are known for their marked long dry seasons
and irregular precipitations (Ouédraogo et al.
2013). These zones are drier than the Guinean
zone that displays a bimodal rainfall regime with
high and regular precipitations (Mensah et al.
2014). The habitats of semi-arid zones might have
enabled some functional strategies (Thuiller et al.
2004), so that many species can be found from
Sudano-Guinean to Sudanian zone. This may
explain why plots from the Sudanian and the
Sudano-Guinean zones were grouped together into
the same cluster (Fig. 2), indicating a habitat quite
distinct from Guinean and Sahelo-Sudanian zones.
The distinction that we experienced in the
spatially closer habitats (e.g. Guinean and Sudano-
Guinean zones on the first hand and Sudanian and
Sahelo Sudanian zones on the second hand),
demonstrates that small scale environmental
variations are also important for the coexistence of
species with A. africana. Large scale environ-
mental gradient however, may strongly influence
coexistence because climatic variables can limit
access to others habitats. This is true for
characteristic species for whose distribution
seemed to be hindered by climatic effects.
The high discrimination between the Guinean
and the Sahelo-Sudanian zones reflects the limits
of the distribution range of A. africana. Indeed,
there is presumably a high number of uncommon
species in each zone. The South-Sahel (Sahelo-
Sudanian zone) is the northern most limit of the
species distribution in the semi-arid savannas
(Terrible 1984). The species’ natural populations in
this zone are in jeopardy due to the combined
effects of climate pejoration and anthropogenic
pressure (Ouédraogo & Thiombiano 2012). On the
contrary, the species seems to have the most
favourable growing conditions in the Guinean zone
characterized by relatively low human pressure
(Bonou et al. 2009). A similar pattern was observed
for the widespread species, Anogeissus leiocarpa,
across its distribution range in semi-arid areas
(Ouédraogo et al. 2005; Ouédraogo et al. 2013).
The results of CCA support patterns observed
previously and confirmed the influential role of
climate in governing species assemblages. These
findings are in line with those reported by Pyke et
al. (2001) in a Neo-tropical lowland forest of
Panama canal, where both precipitation and
geology were shown as useful in predicting species-
level floristic variations at broader scales.
Moreover, whether precipitation and temperature
of the warmest quarter were discriminative of
habitat of A. africana, appears congruent with the
findings of Gwitira et al. (2014) which reported
that, precipitation and temperature of the
warmest period were important to understand the
effect of climate change on plant species diversity
in Southern African savannah. In finding
influences of temperature and precipitation on
plant diversity and distribution, our results
confirmed the truism that plants respond to
warming and precipitation, and changes in climate
may directly affect plant species vital rates (Adler
et al. 2012; Holbrook et al. 1995) and may govern
pattern of species occurrence. Climate has always
been a reasonable predictor of the distribution of
individual species, because each species has a
realized niche which respects climatic limits.
Moreover, variation in climate may directly alter
the abiotic environment (e.g. soil moisture, nutrient
cycling, or resources availability) and thus
influence the local patterns of functional groups
and habitats (Hobbie et al. 1993). But in reality,
the change in habitat diversity cannot be
explained only by considering climatic factors. The
variation in habitat species composition may be
adequately evident, when considering additional
factors such as resources availability, disturbance
regime and soil properties. For example, our study
revealed, inter alia, that the Sahelo-Sudanian zone
(characterized by higher temperature) sheltered
many species. Indeed, in dry climate, increased
temperatures have the potential to decrease soil
moisture, but the effect of drought stress may be
counter-balanced by increased water use efficiency
(Hobbie et al.1993), thus promoting occurrence and
coexistence of limited species. Soil properties and
disturbance regime could potentially interact with
climate to influence the variability in habitat
composition (Huston 1979; Yelemou et al. 2015).
The characteristic species were the ones
having an IVI greater than 10. According to
Reitsma (1988), species with IVI > 10 are often
considered as ecologically important in their
habitat. The importance value index of species in
each climatic zone revealed that the Sudanian and
724 AFZELIA AFRICANA HABITATS IN WEST AFRICA
Sudano-Guinean zones shared many important
species that are different from the ones observed in
the Guinean zone. Likewise four different impor-
tant species were also common to the Sudanian,
Sudano-Guinean and Sahelo-Sudanian zones.
These observations may seem self-evident. Indeed,
the Guinean zone shared only two important
species (A. africana and A. leiocarpa) with the
remaining zones (Sudanian, Sudano-Guinean and
Sahelo-Sudanian). The most important species of
these habitats are not likely to maintain a wide
distribution range as A. africana probably because
the abilities to reach a community differ greatly
among individual species (Pimm 1993). Moreover,
important species of Guinean habitats are
restricted to their geographical distribution
because of climate-related limits to their regene-
ration that does not enable them to colonize a
wider distribution range of A. africana (Ouédraogo
& Thiombiano 2012). Alternatively, the density of
vegetation cover in the habitat of Guinean zone
could induce a high degree of competition that
might not be favorable for non-competitive species
(Baumberger et al. 2012).
Knowledge on ecologically important species
co-occurring in a habitat is essential for two main
reasons. First, it provides evidence in validating
ecological assumptions of biotic interactions such
as mutualism, neutralism and facilitation. It also
supports the possibility that these ecologically
important species likely mitigate the immigration
of invasive species, thus maintaining habitat
integrity. This can aid in conservation processes
that seek to promote conservation of the habitat
for native species by restricting the chance of
invasion. Conservation biologists will also be
concerned about important species because
conservation actions would be of advantage for
these associated tree species. The habitat integrity
could likely contribute to mitigate undesirable
effects of changing climate conditions. This is more
important for conserving widespread species like
A. africana in the face of eminent threat from
harsh environmental conditions and human
pressure (Mensah et al. 2014; Ouédraogo &
Thiombiano 2012).
Conclusions and implications for conservation
and management
This study assessed the variation in woody
floristic composition of A. africana habitats along a
latitudinal gradient in West Africa. The observed
patterns were explained by the effects of
precipitation and temperature. Furthermore, the
study revealed that A. africana co-occurs with
some important species in three different habitats
across the four bioclimatic zones. The assemblage
patterns of these co-occurring species are impor-
tant for maintaining the integrity of the habitat
and for conservation of targeted species. Thus, the
variation in habitat composition should be taken
into account when promoting the species conser-
vation. More specifically, distinction in the habitat
may suggest different considerations while pro-
posing guidelines for management and conser-
vation of A. africana within its habitats; the
Sudanian and Sudano-Guinean zones could use
similar management schemes for the species
habitat while the Guinean and the Sahelo-
Sudanian zones should be treated as distinct
habitats. In the Sudanian and Sudano-Guinean
zones, enrichment planting program could be
embarked upon for A. africana with native species
like P. erinaceus, V. paradoxa, L. acida, C.
febrifuga, D. oliveri, B. africana and A. leiocarpa.
Species such as Pterocarpus erinaceus, B. africana
and A. leiocarpa should be considered in the
Sahelo-Sudanian zone, but with other important
species such as B. dalzielii, B. costatum, S.
setigera, Pericopsis laxiflora, Combretum molle,
Pavetta crassipes, Stereospermun kunthianum and
Ziziphus abyssinica. However, in Guinean zone,
species such as D. guineense, D. mespiliformis, D.
floribunda, M. andongensis, C. pentandra and C.
brownii should be considered together with A.
africana. Also, actions should be taken to maintain
the interspecific relationships and habitat
integrity. As disturbance facilitates biological
invasion of alien plants, these actions should aim
at preventing any form of disturbance around A.
africana dominated ecosystems. More specifically,
such actions should concern reinforcement
conservation of these important species in
protected areas of each bioclimatic zone, pre-
vention of urbanization close to these protected
areas and management of surrounding botanical
gardens (von der Lippe & Kowarik 2008; Hulme
2011).
Acknowledgements
This work was financially supported by the
International Tropical Timber Organization
(ITTO) through a research grant [Ref. 142/12A]
provided to Dr. Achille E. Assogbadjo. We are also
grateful to the two anonymous reviewers who
helped us improve the manuscript.
MENSAH et al. 725
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