Dendrometric characteristics as indicators
of pressure of Afzelia africana Sm. dynamic changes
in trees found in different climatic zones of Benin
*, O. EYOG MATIG
, A.E. ASSOGBADJO
and T. SINADOUWIROU
Laboratoire d’Ecologie Applique
´des Sciences Agronomiques, Universite
International Plant Genetic Resources Institute (IPGRI); *Author for correspondence (e-mail:
firstname.lastname@example.org; fax: þ229-30-30-84)
Received 5 November 2002; accepted in revised form 2 June 2003
Key words: Afzelia africana, Benin, Regeneration, Threats, Tree size
Abstract. Afzelia africana is a forest species used by local inhabitants for various purposes, especially
as forage to feed cattle, as medicinal plant and its wood is used to make furniture or for cooking. Its
utilisation in its current form constitutes a threat to this species. However, the lack of data on this species
is a hindrance towards drawing up an efﬁcient program for its sustainable management. In order to ﬁll in
some gaps in the knowledge of A. africana tree populations, dendrometric characteristics of this species
were studied within different climatic zones where it occurs in Benin. Data collected on each of them
included height and diameter, and with regard to the levels of pressure, ﬁve categories were deﬁned
namely: null, weak, moderate, severe and very severe. As far as diameter is concerned every size was
taken into consideration in all the climatic zones. However, average diameter and height of the A.
africana individuals varied signiﬁcantly according to climatic zones. Anthropogenic pressure increased
while moving from the humid zone towards the drier zone. Moreover, there was a noticeable signiﬁcant
change in the level and quality of pressure between trees found in the different climatic zones in the
sense that the lower the height of the trees, the more severe the level of pressure exerted. Such a
relationship was not signiﬁcant when one considers tree diameter in accordance with the climatic zones
in the country.
Located in the low rainfall dry corridor named ‘Dahomey Gap’, Benin does not
have as much forest zone as its neighbouring countries of the coastal zone of
West Africa such as Co
ˆte d’Ivoire, Ghana and Nigeria. Its natural forest covers
2 538 000 ha (FAO 2001), which represents 23% of the total surface area of the
country. This low forest surface of the country is due to increased demographic
pressure with the subsequent high exploitation of Afzelia africana, a savannah
tree species, by the local population for various purposes, speciﬁcally for
feeding cattle (Sinsin 1993; Onana 1998), for traditional medicine (Kerharo and
Adam 1974; Adjanohoun et al. 1989; Ahouangonou and Bris 1997) and for
timber (Ahouangonou and Bris 1997; Bayer and Waters-Bayer 1999). Un-
controlled use of the species leads to degradation and reduction of its habitat
#2004 Kluwer Academic Publishers. Printed in the Netherlands.
Biodiversity and Conservation 1555–1570, 2004.
and its population in the country. It is therefore urgent to gather data on
endangered species, such as A. africana, with a view to assess its conservation
status and also to develop effective conservation strategies.
Size class distributions have been used to understand the tree population dy-
namics (Cunningham 2001). They are considered to be a very useful predictive tool
(Geldenhuys 1992). This tool could be used to evaluate the impact of man’s ac-
tivities on trees’population (Cunningham 2001). However, several other factors
could affect the shape and the size of categories of distribution of a species (Van
Wyk et al. 1996; Sokpon and Biaou 2002).
The major aim of this study is to determine the level of pressure on A.
africana populations as a result of harvesting and utilisation of its leaves and
small branches for fodder purposes. Dendrometric parameters were therefore
collected, processed and analysed. This paper highlights the salient results of the
Study sites and studied species
The study was conducted in the three bioclimatic zones of the Republic of Benin
(112 622 km
), located between 68200and 128250N and 18and 38400E in West
Africa. These zones are: the sub-humid Guineo-Congolean afﬁnity zone (from 6825
to 78300N), the Sudano-Guinean zone (from 78300to 98300N) and the Sudanian
zone (from 98300to 1280N) (White 1983). The stations surveyed in each of these
climatic zones are indicated in Figure 1.
The rainfall regime is bimodal in the Guineo-Congolean zone (from April to
June and from September to November) with a mean annual rainfall of 1200 mm.
Beyond 88N and towards the north, rainfall distribution becomes unimodal (May–
October) with a mean annual rainfall above 900 mm.
The Guineo-Congolean zone has ferrallitic soils, deep and hardly fertile
(700 000 ha), alluvial soils and heavy clay soils (360 000 ha) localised in the valleys
of the Mono, Niger, Couffo and Oueme rivers, and in the Lama depression. These
soils are rich in clay, humus and inorganic elements. The quality of the country’s
vegetation has suffered severe impoverishment as a result of various intense eco-
nomic and human activities, speciﬁcally in the southern region, where human
population density is high. In the southern zone, vegetation is composed of fallows
and small forest patches of less than 5 ha. The original vegetation at its early stages
was made up of semi-deciduous dense forests and Guinean savannah (Adjanohoun
et al. 1989).
In the Sudanian and the Sudano-Guinean zones, there are infertile mineral soils
(1 500 000 ha) and ferruginous soils (8 600 000 ha). The Sudano-Guinean transition
zone is characterised by mosaics of woodlands, dry dense forests, strewn with tree
and shrub savannahs and gallery forests. The vegetation of the Sudanian
zone consists of savannas and gallery forests with small trees and shrubs slightly
covering the ground. The main activities of local communities are extensive agri-
culture, animal husbandry, and reckless exploitation of woodlands and gallery
Figure 1. Stations surveyed.
A. africana is a timber species with high potentials for fodder and medicine. In the
agro-pastoral zone of the country, the leaves are considered as important forage for
cattle during the dry season. The branches are pruned by the Fulanis to feed their
animals. The powdered bark of A. africana mixed with salt was reported to have
improved cattle intake. A. africana is used for curing several diseases such as
oedemas, intercostal neuralgias, convulsions, staturo-ponderal backwardness, and
so forth. The powdered bark is used as a febrifuge and gastro-intestinal stimulant.
When this powder is mixed with Morinda lucida, it is used as an antiseptic for
wounds. The decoction of the roots of A. africana is used to cure blennorrhoea,
stomach ache and hernias.
The timber of this species is one of the best sold in the open markets all over the
country. Because of various usages, A. africana is currently under severe human
Six stations were selected in the Guineo-Congolean zone, nine stations in the
Sudano-Guinean zone and 12 stations in the Sudanian zone. Observations and
measurements were carried out in these stations, both in protected areas (national
parks and reserve forests) and state ranch and village zones (free zones). In each
station, one or two rectangular sample plots of 1000 m
were established and
considered as a centre of the plot, the ﬁrst individual of A. africana randomly
selected. A total of 669 individuals of A. africana were measured: 160 individuals
in the Guineo-Congolean zone, 198 individuals in the Sudano-Guinean zone and
311 individuals in the Sudanian zone. The parameters recorded in each plot were as
.The diameter and height of all individuals, with dbh (diameter at breast height)
above 10 cm;
.The proportion of branches cut or mutilated and the total number of branches for
each individual of A. africana, were considered and analysed in accordance with
the following ﬁve levels of pressure:
P0 ¼null: individuals without damage (without branch neither cut nor mutilated
P1 ¼weak: individuals with 0–25% of their crown pruned,
P2 ¼average: individuals with 25–50% of their crown pruned,
P3 ¼severe: individuals with 50–75% of their crown pruned,
P4 ¼very severe: individuals with 75–100% of their crown pruned.
.The causes of mutilations on trees were identiﬁed.
Data analysis consisted of analysing variations in the diameters and heights of
individuals according to climatic zones. The Newman and Keuls test of comparison
was used to compare tree diameter and height among the different climatic zones.
In addition, factorial analysis was carried out to identify the localities with similar
levels of pressure. While taking into consideration data adjustment models such as
the test of normality, of homogeneity and the independence of regression residues,
the transformation of variables was utilised to measure the height and diameter
distributions of individual trees. For each transformation process, three tests were
therefore undertaken, namely: the test of normality, the test of independence of
residual variance and the decline residual independence test of Breush-Pagan.
In addition, the impact of climatic and anthropic pressure on tree height within
each climatic zone was evaluated by studying the trees’height-class distribution.
The distribution was undertaken according to height size categories among in-
dividuals without pressure (P0) and those with moderate pressure (P1 and P2), to
the ones with severe pressures (P3 and P4) as well.
Tree diameter (dbh) variations according to climatic zones
Diameter class-size distribution
Diameter class-size distribution for A. africana was not signiﬁcantly different ac-
cording to climatic zones (Figures 2–4). A bell-shaped distribution type was ob-
served for each climatic zone. The two extreme class-size individuals are absent in
the three zones. The slight differences in the distribution noticed are the logarith-
mic function in the Guineo-Congolean zone (log(y)¼0.0103x
1.1379x0.2584) while it is a polynomial function in the Sudano-Guinean
individuals with a diameter between 20 and 60 cm are most predominantly
Figure 2. Diameter size-class distribution for A. africana in the Guineo-Congolean zone.
represented in the stands. The small diameters are less represented at the sites.
However, the diameter distribution showed a second peak with low amplitude in the
upper diameter categories (Figure 3).
In the Sudanian zone (Figure 4), the diameter size-class distribution matches a
logarithmic function (log(y)¼0.008x
þ0.8848xþ0.4876). This pat-
tern reveals constant reduction in the number of A. africana individuals from the
lower diameter categories to the upper diameter categories.
The mean diameter
The mean diameter for A. africana trees varies signiﬁcantly from 42 cm in the
Sudanian zone to 48 cm in the Sudano-Guinean zone (p<0.05). The test of
Newman–Keuls classiﬁes the different climatic zones into two homogeneous
Figure 3. Diameter size-class distribution for A. africana in the Sudano-Guinean zone.
Figure 4. Diameter size-class distribution for A. africana in the Sudanian zone.
groups when considering the mean diameter (Table 1). The Guineo-Congolean and
Sudano-Guinean groups are separated from the Sudanian one.
Tree height variations according to climatic zones
The height class distribution
The height class distribution for A. africana varied in accordance with speciﬁed climatic
zones (Figures 5–7). It was roughly a bell-shaped curve in the Guineo- Congolean zone
and ﬁtted to an increasing polynomial function (y¼5x
120.2). The median height class size varies from 12.5 to 17.5 m. In the Sudano-
Guinean zone, the distribution was truncated in the low height categories and ﬁtted a
logarithmic function (log(y)¼0.4289x
¼1). The height
size-class distribution in the Sudanian zone was different from the other zones and
ﬁtted a reverse function (1/y¼0.0351x
But in the three zones, individual trees with up to 30 m height were observed.
The mean height
This study shows that in Benin, the mean height of A. africana varies from 9 m in
the Sudanian zone to 13 m in the Guineo-Congolean zone. With regard to the
Table 1. Bioclimatic zones classiﬁcations based on diameter of A. africana trees.
Climatic zone Mean diameter
(cm) Number of individuals measured
Sudano-Guinean zone 48 A 198
Guineo-Congolean zone 47 A 160
Sudanian zone 42 B 311
There is no signiﬁcant difference between mean diameters followed by the same letter.
Figure 5. Height size-class distribution for A. africana in the Guineo-Congolean zone.
stations, the lowest mean height is observed in Penessoulou (4 m) located in the
Sudanian zone while the highest is observed in the reserve forest of Lama (17 m)
located in the Guineo-Congolean zone.
Analysis of variations of the mean heights of A. africana trees (Table 2) showed a
signiﬁcant difference from one climatic zone to another (p<0.05). The mean
height decreases progressively from the humid (Guineo-Congolean) to the driest
Figure 6. Height size-class distribution for A. africana in the Sudano-Guinean zone.
Figure 7. Height size-class distribution for A. africana in the Sudanian zone.
Table 2. Mean heights comparison for A. africana between the climatic zones.
Climatic zone Mean heights
(m) Number of individuals measured
Guineo-Congolean zone 13 A 160
Sudano-Guinean zone 10 B 198
Sudanian zone 9 C 311
The letters A, B and C indicate signiﬁcant differences between the three zones
with respect to height (p<0.05).
zones (Sudanian). In fact, the tallest individual trees were found in the Guineo-
Congolean zone (e.g. in the Lama Forest Reserve (17 m) and in Pobe
While in Penessoulou, Sudano-Guinean zone and in Segbana, Sudanian zone, in-
dividual trees observed measured 4.20 and 5.25 m, respectively.
Levels of pressure on A. africana in the climatic zones
The levels of pressure varied according to climatic zones (Figure 8). Pressure was
low in the Guineo-Congolean zone and gradually increased when moving towards
the Sudanian zone. The highest frequencies of the pressure levels P3 (severe
pressure) and P4 (very severe) were recorded in the Sudano-Guinean and Sudanian
zones. On the contrary, in the Guineo-Congolean zone, the highest frequencies
were recorded for the levels of pressure null (P0) and low (P1). The Sudano-
Guinean zone presented an intermediate situation. Indeed, in the Guineo-Congolean
zone, more than 60% of the trees measured were neither pruned nor debarked.
Almost all the trees of A. africana recorded in the Sudanian and Sudano-Guinean
zones were mutilated. The Sudano-Guinean zone has the highest frequencies of
severe pressure on trees (55% against 52% in the Sudanian zone).
Inﬂuence of pressure on tree heights
Figures 9 and 10 compare the tree heights with and without pressures in the
Guineo-Congolean zone. The impact of the pressure is both on the number of
individuals and on the height, where the height size class 20–30 m disappears with
very severe pressures. For the Sudano-Guinean zone the 10–15 m height size class
no longer exists under very severe pressure conditions (Figures 11 and 12). In the
Figure 8. Frequencies of level of pressure in the different climatic zones. The ﬁgures P0–P4 indicate
the different levels of pressure (P0¼null, P1 ¼weak, P2 ¼average, P3 ¼severe, P4 ¼very severe).
Figure 9. Height size-class distribution for A. africana in the Guineo-Congolian zone without pressure
Figure 10. Height size-class distribution for A. africana in the Guineo-Congolian zone with pressure
Figure 11. Height size-class distribution for A. africana under weak and average pressure in the
Sudano-Guinean zone (P1 and P2).
Sudanian zone, with minimum pressure (P0–P2), a good distribution of heights
from 5 to 30 m is observed but there are no more individuals of height size class
20–25 m under severe pressure (Figures 13 and 14). The number of individuals per
height size class is relatively low. Within the parameters ranging from severe to
very severe pressures, the maximum heights observed are 15 m. But the number of
individuals is higher than under low pressure conditions.
In conclusion, in virtually all the climatic zones, various pressures inﬂuence the
growth of A. africana.
The factorial analysis (Figure 15) of the matrix crossing the 25 surveyed stations,
coupled with the different levels of pressure (P0–P4), identiﬁed three homogeneous
groups of stations according to their levels of pressure.
Figure 12. Height size-class distribution for A. africana under severe pressure in the Sudano-Guinean
zone (P3 and P4).
Figure 13. Height size-class distribution for A. africana under null and average pressure (P0, P1 and
P2) in the Sudanian zone.
Group I (GI), including the reserve forests of Lama, Ketou and Pobe, the ranch of
Samiondji and forest patches at Lonkly and Azove
`in Kouffo province, is mainly
composed of the stations located in the Guineo-Congolean zone. It is well corre-
Figure 15. Factorial analysis for the level of pressure and the localities surveyed. The ﬁgures P0–P4
indicate the different levels of pressure (P0¼null, P1¼weak, P2 ¼average, P3 ¼severe, P4 ¼very
Figure 14. Height size-class distribution for A. africana under severe pressure (P3 and P4) in the
lated with the lowest levels of pressure (P0 and P1) (Figure 8). It regroups stations
where A. africana trees were undergoing relatively low pressure. Indeed, in these
stations, 66% of the individuals recorded were intact and less than 10% of in-
dividuals were subjected to severe or very severe pressures.
Group II (GII) is composed of the following stations: Tasso, Sakabansi, Kere-
mou, Okpara, Beterou, Segbana, Bense
´kou, Agbassa, Dogo, Toui and Bessassi. In
these clustered stations important pressures occurred with regard to mutilations on
the branches of A. africana. This group was correlated with the average (P2) and
severe (P3) levels of pressure (Figure 8). Intact individuals in these stations were
absent or rare.
Group III (GIII) comprises the stations Bassila, Penessoulou, Perma, Bembereke,
Firou, Gbeba, Pikire, Birni and Toucountouna. In these stations individual trees
were under very severe level of pressure (P4), and intact trees (P0) were rare.
The stations in groups II and III are generally characterized by a high mutilation
pressure on A. africana and are located in the Sudanian or Sudano-Guinean zones.
Group II encompasses not only several stations of the Sudano-Guinean zone such as
Okpara, Toui, Beterou and Agbassa, but also stations from the Sudanian zone,
including Keremou, Segbana, Sakabansi and Bensekou.
The tree diameter and height size-class variations according to climatic zones
It was noticed in general that most of the diameter shape categories are represented
within each climatic zone. This could be explained by the presence of protected
areas within each surveyed zone. In protected areas, the pressure on A. africana is
low. In all climatic zones, there was a high concentration of individuals in the
diameter classes ranging from 20 to 50 cm. This low amplitude reveals the existing
pressure on the individuals of the other diameter classes. These pressures are
mainly due to the commercial logging of individuals with diameters above 50 cm,
followed by the high foliage pruning and the de-barking of trees for traditional
medicine purposes. These pressures were accentuated in the Sudanian zone, which
is the main transhumance zone at country level, and where illegal and uncontrolled
logging activities of high diameter trees were observed.
In general, trees are shorter in the Sudano-Guinean zone where most of the trees
are within a narrow range of class height (5–20 m). In the Guineo-Congolean zone,
greater tree heights such as 17–28 m in Lama reserve forest (68550N), and 15–20 m
`station (78N) were recorded. This fast growth could be explained inter
alias, on one hand, by the protected area status of these stations where Afzelia trees
are protected from pruning; and on the other hand by the more favourable rainfall
regime occurring in these areas situated in the Guineo-Congolean climatic zone
where bush ﬁre is rare in forest patches.
Several authors (Paradis and Houngnon 1997; Cunningham 2001; Sokpon and
Biaou 2002) have used the diameter size-class distribution as a ﬁeld method to
assess the impact of harvest practices on the regeneration of the species. It is also a
valid tool for assessing the pressure undergone by a tree species population.
However, it is important to take into consideration the species temperament and the
development stage of the population when analysing its diameter or height size-
It was observed in this study that in the three climatic zones, the species has a
bell-shaped curve for its diameter size-class distribution. The same distribution was
observed by Paradis and Houngnon (1997) in Lama Reserve, Benin. This dis-
tribution was also found by Sokpon and Biaou (2002) in another forest reserve in
Benin (Bassila reserve forest). According to Cunningham (2001), the bell-shaped
curve indicates either light required or competition-intolerant species or low
numbers of seeds due to an unusual reproductive strategy. But A. africana is known
as a shade-intolerant species. According to Sokpon and Biaou (2002), its diameter
size-class distribution indicates the absence of young trees with less than 20 cm
diameter and also difﬁculty for its recruitment in a heavy clayed-soil forest such as
the Lama Reserve. The absence of individuals with larger diameters mainly in the
Sudanian zone could be explained by logging activities where the trees are facing
very severe pressure, which can lead to the extinction of the population of the
species. Indeed, the big trees are the best seed-bearers of the population, which
ensures the production of seeds and thus supports the regeneration of the species.
The scarcity in the stands of an optimum density of sexually mature individuals
(sexually mature diameter trees) will accordingly be a weakness for the re-
generation of the population.
Impact of pressure on the tree height and diameter within climatic zones
The comparison made for the tree heights in the same climatic zone, such as the
Guineo-Congolean zone without pressure (Figure 9) and under pressure (Figure
10), shows that the height class size 20–30 m is no longer represented within the
areas under pressure. The species height varies from 4.20 m (Penessoulou) in the
Sudanian zone to 17.20 m (Lama Reserve) in the Guineo-Congolean zones. This
trend is in accordance with the data collected (8–10 m) in savannah by Ahouan-
gonou (1997) on the same species. Kerharo and Adam (1974) indicated that the
species could grow up to 25–30 m under favourable conditions while it is short and
stocky in Senegal (dry zone). This means that climatic conditions inﬂuence the
species growth (the height of the species). Climatic conditions are not the single
factor that inﬂuences the height of the species, as the study shows individual trees
with up to 30 m height within the three climatic zones. This study also demon-
strated the reduction in the same climatic zone of tree population heights in severe
and very severe pressure conditions. Pressure (pruning, debarking, etc.) inﬂuences,
amongst others, the growth of A. africana.
Repeated tree crown pruning observed between 88N and 128N, in a region after
bush ﬁres during the dry season, is the main cause of the pressures on A. africana.
This pruning practice reduces the growth of the tree. Bayer and Water-Bayer (1999)
note that pruning and coppicing strongly inﬂuence the quantity of leaves propor-
tionate to the branches and in accordance with the growth of trees. The species is
also used in traditional medicine and as timber species. Onana (1998) observed in
the northern part of Cameroon that this practice of regular pruning and coppicing
performed by herdsmen, the frequent bush ﬁres and the regular browsing of the
seedlings, already jeopardise many populations of A. africana in the Sudano-Sa-
helian zone. This is the reason why FAO has also selected this species for a
conservation programme in Cameroon (Palmberg 1987). In Benin, no relevant
conservation strategies have been put in place for this tree species that is under
increasing severe pressures. Its conservation becomes more than ever urgent.
The size-class distribution and the average values of A. africana tree height and
diameter allowed the assessment of the level of pressure affecting the populations
of this species within the different climatic zones of Benin. The height and the
diameter of the species varied signiﬁcantly from one climatic zone to another. The
lowest diameter was observed within the latitudes ranging from 88Nto128N. This
area includes the localities where A. africana is facing very strong pressure. On the
contrary, the big individuals of A. africana were recorded in the Guineo-Congolean
zone (extending from 68250Nto78300N) with recorded lowest pressures. In certain
ecosystems such as the reserve forest of Lama, which are well-protected areas, are
found very good individuals of A. africana because of the absence of the various
forms of human pressure.
In situ conservation measures of the species should be taken in areas where the
species is facing low pressures. In addition, measures focussing on the enrichment
of the impoverished forests with the species should be one of the major concerns of
Benin forest managers. Silvicultural techniques successfully experimented on the
species in other African countries could be adapted to Benin conditions, as was the
case for Tectona grandis (teak).
This work was completed with the ﬁnancial support of the United Nations En-
vironment Programme (UNEP) provided by the Sub Saharan African Forest Ge-
netic Resources Programme (SAFORGEN) of the International Plant Genetic
Resources Institute (IPGRI). We would like to thank these institutions.
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