Structure of Anogeissus leiocarpa Guill., Perr. Natural stands in relation to anthropogenic pressure within Wari-Maro Forest Reserve in Benin

Abstract

The present study focused on the analysis of the structure of the Anogeissus leiocarpa dominated natural stands in the Wari-Maro forest reserve which are under high and minimal anthropogenic pressures. These stands were considered for forest inventories after carrying out a random sampling scheme of 40 sample units of 30 m × 50 m. In each level pressure stand, the dbh and tree-height of identified tree-species were measured in each plot. Data analyses were based on the computation of structural parameters, establishment of diameter and height distributions and the floristic composition of the two types of stands. Results obtained showed higher values for the overall basal area (9.78 m2 ha-1), mean height (22.37 m) and mean diameter (36.92 cm) for A. leiocarpa in low-pressure stands. In high-pressure stands, some species like Afzelia africana had lower Importance Value Index and the frequency of A. leiocarpa trees in the successive diameter classes dropped rapidly and the value of the logarithmic slope of the height-diameter relationship was lower (9.77) indicating a lanky shape. Results obtained suggest that effective conservation is needed for A. leiocarpa stands under high pressure by limiting human interference and developing appropriate strategy for restoration purposes. © 2009 The Authors. Journal compilation

Full text

Structure of Anogeissus leiocarpa Guill., Perr. natural
stands in relation to anthropogenic pressure within
Wari-Maro Forest Reserve in Benin
Achille Ephrem Assogbadjo
1
, Romain Lucas Glele Kakaı
¨
1
*, Brice Sinsin
1
and
Dieter Pelz
2
1
Laboratoire d’Ecologie Applique
´e, Faculte
´des Sciences Agronomiques, Universite
´d’Abomey-Calavi, 01 BP 526, Cotonou, Be
´nin and
2
Department
of Forest Biometry, University of Freiburg, Tennenbacherstr. 4, D-79085, Freiburg, Germany
Abstract
The present study focused on the analysis of the structure of
the Anogeissus leiocarpa dominated natural stands in the
Wari-Maro forest reserve which are under high and minimal
anthropogenic pressures. These stands were considered for
forest inventories after carrying out a random sampling
scheme of 40 sample units of 30 m ·50 m. In each level
pressure stand, the dbh and tree-height of identified tree-
species were measured in each plot. Data analyses were
based on the computation of structural parameters, estab-
lishment of diameter and height distributions and the flo-
ristic composition of the two types of stands. Results
obtained showed higher values for the overall basal area
(9.78 m
2
ha
)1
), mean height (22.37 m) and mean diameter
(36.92 cm) for A. leiocarpa in low-pressure stands. In high-
pressure stands, some species like Afzelia africana had lower
Importance Value Index and the frequency of A. leiocarpa
trees in the successive diameter classes dropped rapidly and
the value of the logarithmic slope of the height–diameter
relationship was lower (9.77) indicating a lanky shape.
Results obtained suggest that effective conservation is nee-
ded for A. leiocarpa stands under high pressure by limiting
human interference and developing appropriate strategy for
restoration purposes.
Key words: Anogeissus leiocarpa, deforestation, structure,
vegetation communities, Wari-Maro
Re
´sume
´
Cette e
´tude s’est focalise
´e sur l’analyse de la structure de
peuplements naturels a
`dominance de Anogeissus leiocarpa,
dans la fore
ˆt classe
´e de Wari-Maro, qui subissent a
`certains
endroits, des pressions anthropiques fortes et a
`d’autres
endroits des pressions anthropiques minimes. Ces peuple-
ments ont e
´te
´inventorie
´s en conside
´rant un e
´chantillon-
nage ale
´atoire de 40 placeaux de 30 m ·50 m. Pour chaque
niveau de pression, on a mesure
´dans chaque placeau le
diame
`tre a
`1,3 m et la hauteur totale des arbres d’espe
`ces
identifie
´es. L’analyse des donne
´es s’est base
´e sur le calcul des
parame
`tres structuraux, sur l’e
´tablissement de la distribu-
tion en diame
`tre et en hauteur et sur la composition floris-
tique des peuplements des deux types de formation. Les
re
´sultats obtenus indiquent les plus grandes valeurs pour la
surface terrie
`re globale (9,78 m
2
ha
)1
), la hauteur moyenne
(22,37 m) et le diame
`tre moyen (36,92 cm) chez A. leiocarpa
dans les peuplements soumis a
`une faible pression. Dans les
peuplements subissant une forte pression, certaines espe
`ces
comme Afzelia africana avaient les plus faibles Indices
d’importance, la fre
´quence de A. leiocarpa dans les classes de
hauteurs successives diminuait rapidement et la valeur de la
pente logarithmique de la relation hauteur ⁄diame
`tre e
´tait
plus faible (9,77), ce qui indique une forme e
´lance
´e. Les
re
´sultats obtenus sugge
`rent que les peuplements de
A. leiocarpa sous forte pressions anthropiques requie
`rent
une conservation efficace, en limitant les pertubations
humaines et en de
´veloppant une strate
´gie approprie
´e en vue
de leur restauration.
Introduction
Forests have always supported human lives economically,
by provision of firewood, sawlog, barks, foods, shelters,
hunting ground, recreation sites and temple for religious
*Correspondence: E-mail: gleleromain@yahoo.fr
644 2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

rites (Bary-Lenger et al., 1988). African forests constitute
an immense reserve of biological diversity and play a
fundamental role in the satisfaction of many needs for the
local populations.
Degradation of the forest ecosystems in Africa occurs at
a very high rate and is accompanied by a growing
imbalance between the availability of the natural resources
and demands in forest products as a result of continuous
growing population (FAO, 2001). The intensity of forest
degradation varies in African countries. Benin is a country
of low forest cover with only 23% of its total area of
2,538,000 hectares under forest. Forest degradation cau-
ses high anthropogenic pressure on the natural resources.
Among the important natural forest resources available in
Benin is the multipurpose tree species, Anogeissus leiocarpa
Guill., Perr.
A. leiocarpa is a tree species with height ranging
between 15–18 m and diameter measures up to 1 m
(Arbonnier, 2002). Fruits of the species contains about
40 wind-dispersed seeds of 10 mg each (Hovestadt et al.,
1999). It has exceptional ecological amplitude that
allows it to survive well in the Sahara borders and the
equatorial forest. It is the main species from the old dry
Sudano-Guinean forest that is found after the tropical
rainy forest (Wickens, 1976; White, 1983). The distri-
bution of A. leiocarpa spreads from the northern Guinea
zone up to the Sahelian zone in savanna, dry forests and
gallery forests (Couteron & Kokou, 1997; Mu
¨ller &
Wittig, 2002).
The results of forest inventories done in most of the
forests in the Central and Northern Benin reaveled the
existence of A. leiocarpa dominated stands (Fonton 1997;
Adoko 2005). Two types of A. leiocarpa stands were dis-
tinguished in the Wari-Maro forest reserve (Centre-Benin):
stands under high anthropogenic pressure and those
under no or minimal pressure. In this paper, the possible
impacts of anthropogenic pressure on the A. leiocarpa
dominated stands were discussed by comparing the
structure of the two types of stands in the Wari-Maro forest
reserve in Benin.
Materials and methods
Study Site
The inventory of A. leiocarpa natural stands was done in
the Wari-Maro forest reserve, which is located at the
centre of Benin Republic (112,622 km
2
), between 880–
910N and 155–225E (Fig. 1). This forest covers an
area of about 120,686 ha. It is located in the Guineo-
Sudanian transition zone defined by White (1983) as
‘Sudanian woodland mainly composed of Isoberlinia’ and
is characterized by a Sudano–Guinean climate with two
seasons: a dry season from November–March and a
rainy season from April–October. Annual rainfall rang-
ing from 1964–1997 fluctuated between 1000 and
1100 mm with a mean of 1052 mm (Orthmann, 2005).
A. leiocarpa dominated stands are spread out in the
Wari-Maro forest.
From 2002–2007, the forest was managed by the Pro-
ject for Management of Agoua, Monts-Kouffe
´and Wari-
Maro forest reserves (PAMF). With the participation of the
local populations, the Wari-Maro forest was divided into
‘protecte’ and ‘free’ stands.
The protected areas are subject to sustainable manage-
ment organized by the project with the help of local
committes of the bordering villages. In these stands, log-
ging or pasture are prohibited and no or minmal anthro-
pogenic pressure was noticed.
On the contrary, the ‘free stands’ are under high
anthropogenic pressure with no banning and were char-
acterized by a frequent harvest of A. leiocarpa leaves,
branches, bark and wood for various purposes and also the
use of these stands as pasture for cattle (Adoko, 2005). In
these stands, high presence of poaceaes was noticed and a
scarcity of lianas and traces of bush-fires.
Forest inventory
Forest inventory was carried out in Woodlands through a
random sampling scheme with sample units having rect-
angular shape of 30 m ·50 m. In each level pressure
stands (stands under low human pressure and stands
under high pressure), 20 sample units were considered. In
each sample unit, all trees of diameter at breast height
more than 10 cm were counted and measured in height
with the relascope and in diameter with a calliper. Species
were identified by the botanist of the team. The number of
trees per species was also recorded.
Data analysis
For each sample unit or plot, the following parameters
were considered.
The tree-density of the stands (N), i.e. the average
number of trees per plot expressed as trees ha
)1
:
Description of A. leiocarpa dominated stands 645
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

N¼n
s;ð1Þ
nis the overall number of trees in the plot and sthe area
(s= 0.15ha).
The basal area of the stand (G), i.e. the sum of the cross-
sectional area at 1.3 m above the ground level of all trees
on a plot expressed as m
2
ha
)1
(Van Laar & Akc¸a, 2007):
G¼p
4sX
n
i¼1
d2
i;ð2Þ
diis the diameter (in cm) of the ith tree of the plot; sis
defined as above.
The mean diameter of the tree (D), i.e. the diameter of
the tree with the mean basal area expressed in cm (Van
Laar & Akc¸a, 2007):
D¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
nX
n
i¼1
d2
i
s;ð3Þ
nis the number of trees found on the plot and dithe
diameter of the ith tree in cm.
The Lorey’s mean height (H, in m), i.e. the average
height of all the trees found in the plot, weighted by their
basal area (Philip, 2002):
H¼P
n
i¼1
gihi
P
n
i¼1
gi
with gi¼p
4d2
i;ð4Þ
giand hiwere the basal area (in m
2
ha
)1
) and the total
height (in m) of the tree i; the diameter diwas expressed
in meters. This mean height is more stable than an
Africa
200 000 300 000 400 000 500 000 600 000
200 000 300 000 400 000 500 000 600 000
1 400 000 1 300 000
NIGER
Malanville
Kandi
BURKINA FASO
Natitingou
Parakou
Lokossa
Cotonou
ATLANTIC OCEAN
Bohicon
TOGO
NIGERIA
Wari-Maro
Forest Reserve
N
S
WE
Savè
1 200 000 1 100 000 1 000 000 900 000 700 000800 000
1 400 000
1 300 000
Benin
1 200 000
1 100 000
1 000 000
900 000
700 000 800 000
Fig 1 Location of the Wari-Maro forest
reserve
646 Achille Ephrem Assogbadjo et al.
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

unweighted mean height because it is less affected by
mortality and harvesting of the smaller trees and consti-
tutes an important index for woody species management.
The species richness (S) is the number of species
recorded in the whole stand.
The Simpson’s reciprocal Index of Diversity (ID) is com-
puted using the following formula:
ID¼1
P
sp
i¼1
niðni1Þ
nðn1Þ
ð5Þ
niis the number of trees of species i,nis the overall number
of trees inventoried in the plot, and spis the number of
species found in the considered plot.
The Simpson’s evenness (Eq) measures the diversity
degree of a stand compared with the maximum possible:
Eq ¼ID
Imax
with Imax ¼S:ð6Þ
In (6), I
max
is the maximum value of the Simpson’s
diversity index and S, the number of tree species recorded
on the considered plot.
The structural parameters defined above (except the spe-
cies richness) were computed for each plot and the four
dendrometric parameters (N, D, G, H) were computed for
A. leiocarpa trees separately. Moreover, basal area contribu-
tion (Cs, in percent) and Blackman index were only computed
for A. leiocarpa trees. The first parameter is defined as part of
A. leiocarpa trees in the overall basal area of trees in the plot:
Cs ¼100 Gp
G:ð7Þ
Gp is the basal area of the A. leiocarpa trees and Gis the
basal area for the whole plot.
The Blackman index IB characterizes the spatial distri-
bution of trees in a stand. It was computed for A. leiocarpa
trees in each of the two types using the formula (8):
IB ¼r2
N
lN
:ð8Þ
In (8), IB is the Blackman index; r2
Nand l
N
are
respectively the variance and mean of the A. leiocarpa tree-
density of the stands; the value of IB may be less than 1
(regular distribution), equal 1 (random distribution) or
more than 1 (aggregation distribution of the trees).
The mean and coefficient of variation for all the
parameters (except the species richness that was computed
for the two whole stands) were computed for plots both
under high and minimal pressure. The two types of stands
were compared by performing the Student t-test on each of
the above-cited structural parameters. The Simpson
diversity and evenness indices were however not normally
distributed. In such cases, the non parametric Mann-
Whitney test was performed using the Minitab software.
Moreover, to analyse the possible shifting in the species
composition of each of the two types of stands, the
Importance Value Index (IVI) of each species was com-
puted (Curtis & Macintosh, 1951). For a species a, the IVI
was computed using formula (9):
IVIa¼RDaþRFaþRCa:ð9Þ
RDa¼na=X
k
i¼1
na
and is called the relative density of the species a;
RFa¼fa=X
k
a¼1
fa;fa¼ja=k;fa
is the frequency of the species a;jais the number of plots at
which the species awas counted, and kis the total number
of plots. RFais the frequency of the species aas a pro-
portion of the sum of the frequencies for all species.
RCa¼Ca=X
n
a¼1
Ca
is called the relative coverage for the species a.
Ca¼aaNa=na;a
ais the basal area of the species a;N
ais
the tree-density of the species aand nais the total number
of individuals sampled for that species. The coverage Caof
the species ais the proportion of the ground occupied by a
vertical projection to the ground from the aerial parts of
the plant.
The value of IVI may range from 0 to 3 and is referred to
as the importance percentage. It gives an overall estimate
of the level of importance of a plant species in the com-
munity. The coefficient of correlation of the IVI values
between the two types of pressure stands was computed.
Cluster analysis was performed using SAS software and the
four established groups were projected in the system axes
of the Principal component analysis on the IVI values in
the two types of stands.
To establish the stem diameter structure of A. leiocarpa
stands, all the trees were grouped into 5 cm class diam-
eter in order to obtain enough diameter classes (at least
10) to allow the adjustment of the theoretical distribution
Description of A. leiocarpa dominated stands 647
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to the observed shape. A histogram was established for
the whole stand and for A. leiocarpa trees separately.
Based on the shape of the structure, the truncated Nor-
mal distribution is adjusted for the stem diameter struc-
ture of A. leiocarpa tree in low-pressure stands. For the
whole stand, a truncated negative exponential distribu-
tion was adjusted according to the following model
(Meyer, 1942; Philip, 2002):
fðdcÞ¼aeadc:ð10Þ
In (10), fðdcÞis the frequency of the centre dcof a given
diameter or height class; dc¼ðLinf þLsupÞ=2 where L
inf
and
L
sup
are the inferior and superior limits of the class. The
constant areflects the stocking of smallest trees and a
governs the relative frequencies of successive diameter
classes.
The height–diameter relationship was established for
A. leiocarpa trees in each of the two types of stands by
adjusting the model that gives the best fit:
Ln H ¼aþbLnD:ð11Þ
In (11), His the height of the tree (in meter); Dis its
diameter (in centimetre); ais intercept and b, the loga-
rithmic slope of the relationship that describes the form of
the trees.
Results
Structural characteristics of the stands
The mean values and the coefficient of variation of the
parameters, for low- and high-pressure stands are pre-
sented in Table 1. The Simpson’s diversity and evenness
indices, the overall tree-density and the tree-density of
A. leiocarpa showed no significant difference between low-
and high-pressure stands. The other parameters exhibited
significant different values between the two types of stands.
The value of Blackman Index computed for these stands
was low in low-pressure stands compared with the one
computed for stands under high pressure.
Diameter class structure
The structure of the A. leiocarpa mixed stands in the Wari-
Maro forest reserve is described through the stem diameter
structure of A. leiocarpa trees and the overall diameter
structure of the stand. The stem diameter distribution of
A. leiocarpa was established for the low- and high-pressure
stands and presented in Fig. 2. The truncated Gaussian
distribution was not well adjusted to the observed shape of
the tree-diameter in low-pressure stands. The best pre-
Table 1 Mean, coefficient of variation (CV)
and probability (P) for the structural
parameters of A. leiocarpa stands
Parameter
Low-pressure
stands
High-pressure
stands
PMean CV (%) Mean CV (%)
Whole stands
Tree density, N(trees ha
)1
)315.33 24.54 316.67 43.78 0.978
Basal area, G(m
2
ha
)1
) 19.56 20.00 12.52 41.81 0.003
Mean diameter, D(cm) 29.46 15.70 22.73 34.08 0.029
Mean height, H(m) 18.45 16.68 11.95 42.48 0.003
A. leiocarpa trees
Tree density, N(trees ha
)1
)108.67 45.91 80.00 85.17 0.297
Blackman Index, IB 19.0 – 60.7 – –
Basal area, G(m
2
ha
)1
) 9.78 20.75 3.55 142.44 0.002
Mean diameter, D(cm) 36.92 23.05 23.59 54.76 0.014
Mean height, H(m) 22.37 27.19 15.50 35.52 0.016
Basal area contribution, Cs(%) 48.01 40.74 28.49 55.77 0.024
Ecological parameters
Species richness, S(species) 34 – 33 2.81 –
Simpson reciprocal diversity, I
D
7.43 6.53 7.05 3.32 0.820
Simpson’s evenness, Eq 0.55 9.93 0.55 21.06 0.976
648 Achille Ephrem Assogbadjo et al.
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

sented diameter class (28.6%) comprised the smallest trees
of diameter less than 20 cm.
In high-pressure stands, the shape of the observed
structure was far from the bell shape that is character-
istic of the structure of tree-species population (Philip,
2002); the distribution of the diameters had an inverse
‘J’ shape. In these stands, smallest trees were more
common. The frequency of the other diameter classes
however, decreased rapidly with the increase in the class
centre. Moreover, trees with diameter more than 50 cm
were very scarce. The stem diameter structure of the
whole stand illustrated by the Fig. 2 had an inverse ‘J’
shape in low- and high-pressure stands, however, the
relative frequencies of successive diameter classes fell
rapidly in the high-pressure stands. The equations ob-
tained by adjusting the frequency data to the negative
exponential distributions were:
Lowpressure :37:49 expð0:31dÞðRsquare ¼0:95Þ;
Highpressure :63:02 expð0:49dÞðRsquare ¼0:94Þ
These equations showed that the parameters aand aare
higher in high-pressure (63.02 and 0.49) stands than in
low-pressure stands (37.49 and 0.31).
Tree-height structure
The tree-height distribution established for the A. leiocarpa
trees (Fig. 3) had a truncated bell shape for trees in low-
and high-pressure stands (the threshold being 5 m).
The observed distribution of tree-height structure in the
two types of pressure stand was well adjusted to the trun-
cated normal distribution that was characterized by single
species tree-stands. The more common height class in low-
pressure stands consisted of trees ranging from 23 to 26 m
tall, while those in the high-pressure stands were best pre-
sented at tree heights ranging from 14 to 17 m tall. For the
whole stand (Fig. 3), the tree-height distribution seemed to
be the same as for A. leiocarpa, however the 11–14 m height
classes were the best represented (16.4% of the trees in low-
pressure stands and 27.1% in high-pressure stands).
Low-pressure
12
17
12 11
13
7
10
55
34
0
3
6
9
12
15
18
21
24
27
30
55–60
50–55
45–50
40–45
35–40
30–35
25–30
20–25
15–20
10–15
> 60
55–60
50–55
45–50
40–45
35–40
30–35
25–30
20–25
15–20
10–15
Diameter classes (cm)
Percent
Low-pressure
21 22
15
12
976
2212
0
5
10
15
20
25
30
35
Diameter classes (cm)
Truncated negative
exponential distribution
55–60
50–55
45–50
40–45
35–40
30–35
25–30
20–25
15–20
10–15
> 60
Percent
High-pressure
30 30
16
11
55
1111 0
0
5
10
15
20
25
30
35
Diameter classes (cm)
Truncated negative
exponential distribution
55–60
50–55
45–50
40–45
35–40
30–35
25–30
20–25
15–20
10–15
> 60
Percent
Percent
Truncated normal
distribution
High-pressure
28
30
13
11
6
8
1212
0
3
6
9
12
15
18
21
24
27
30
Diameter classes (cm)
Truncated negative
exponential distribution
A. leiocarpa
Whole stands
Fig 2 Stem diameter distributions of
A. leiocarpa dominated stands
Description of A. leiocarpa dominated stands 649
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

Moreover, as far as the species composition of the stands
was concerned, the coefficient of correlation between the
two types of stands was 0.72. Four groups of species were
established from the cluster analysis performed on the IVI
values in the two types of stands and 81.6% of the infor-
mation on the IVI values of the species in low- and high-
pressure stands was saved. The projection of the four
groups in the system axes obtained from the principal
component analysis performed on the IVI values showed
that the first axis discriminated the groups of species with
high IVI values from those of low values of IVI irrespective
of the level of anthropogenic pressure (Fig. 4). Therefore,
the cluster 1 is constituted of species of high IVI values
(A. leiocarpa,Pterocarpus erinaceus,Vitellaria paradoxa,
Lannea barteri and Terminalia schimperiana), while cluster 2
is constituted of species with low IVI values. Among them
are Vitex doniana,Bombax costatum and Bridelia ferruginea.
The second axis distinguished groups of species according
to their value of IVI in each type of pressure stands.
Therefore, cluster 4, consisted of Daniella oliveri,Ficus
congensis and Parinari curatellifolia which have a relatively
high value of IVI in high-pressure stands than in low-
pressure stands. The opposite trend was observed for the
group 3 that consisted of Detarium microcarpum,Afzelia
africana and Burkea africana.
Height–diameter relationship
For the two types of stands, the relationship had a loga-
rithmic shape. However, the logarithmic slope of the
relationship was higher in low- than in high-pressure
stands (Fig. 5).
Discussion
The present study analysed the possible effect of anthro-
pogenic pressure on the structure of forest stands by
focussing on the stands dominated by A. leiocarpa which is
one of the most important forest species in Benin.
The analysis of the possible impact of anthropogenic
pressure on the dendrometric parameters of the A. leio-
carpa dominated stands revealed that the overall basal
A. leiocarpa
Whole stand
Low-pressure
10 11
16
10
14
11 11
8
5
3
0
5
10
15
20
Height classes (m)
Truncated normal
distribution
High pressure
18
16
26
20
13
22 3
0
5
10
15
20
25
30
Height classes (m)
Truncated normal
distribution
Low-pressure
44 3
8
15 14 15
10
6
0
5
10
15
20
25
32–35
29–32
26–29
23–26
20–23
17–20
14–17
11–14
8–11
5–8
32–35
29–32
26–29
23–26
20–23
17–20
14–17
11–14
8–11
5–8
26–29
23–26
20–23
17–20
14–17
11–14
8–11
5–8
26–29
23–26
20–23
17–20
14–17
11–14
8–11
5–8
Height classes (cm)
Percent
PercentPercent
Percent
Truncated normal
distribution
High-pressure
6
11
17
31
14
66
9
0
5
10
15
20
25
30
35
Height classes (m)
Truncated normal
distribution
Fig 3 Tree-height structure of A. leiocarpa
dominated stands
650 Achille Ephrem Assogbadjo et al.
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

area, mean height and diameter, basal area and con-
tribution in basal area of A. leiocarpa presented higher
values in low-pressure stands than in high-pressure
stands of A. leiocarpa. Moreover, the present study indi-
cated higher value of the tree-density of A. leiocarpa
while the overall tree-density of all the species found in
the stands has the same value in the two types of
stands. It is possible that the selective uncontrolled
logging targeting the big trees of A. leiocarpa in high-
pressure stands led to these situations. The foliage
pruning for traditional medicine, pasture and feeding
purposes could have also reduced the growth of the trees
in high-pressure stands. Bayer & Water-Bayer (1999)
noted that pruning strongly influences the quantity of
leaves and interferes with the growth of trees. In the
present study however, the dendrometric parameters
calculated both in low- and high-pressure stands were
slightly superior to the ones computed for the same
species in the Pendjari Biosphere Reserve in northern
Benin (Houehanou et al., 2007). Mean diameter was
17.8 cm in the reserve, while mean height was 9.5 m.
These low values of dendrometric parameters could be
explained by the difference in climate between the two
forests (Wari-Maro is in the sudanian zone while the
Pendjari reserve is in sudano–sahelian zone).
Moreover, in the two stands, there was a high density
of individuals of all species in the 10–20 cm dbh class
indicating better ecological conditions for the transition
Fig 4 Projection of the four groups of
species according to the IVI values in the
system axes of PCA
Low pressure
H = 11.87 LnD – 19.47
R2 = 0.69
0
5
10
15
20
25
30
35
40
0 102030405060708090
Diameter D (cm)
Height H (m)
0 10203040506070
Height H (m)
High pressure
H = 9.77 LnD – 15.27
R2 = 0.65
0
5
10
15
20
25
30
35
40
Diameter D (cm)
Fig 5 Height-diameter relationship of A.
leiocarpa trees under low and high-pres-
sure.
Description of A. leiocarpa dominated stands 651
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, Afr. J. Ecol.,48, 644–653

from saplings to young trees (10–20 cm dbh). However,
in the high-pressure stands, the frequency of trees in the
diameter classes greater than 20 cm dropped rapidly. The
parameter aof the negative exponential distribution (Eq.
10) indicates the probability of the transition of trees from
the lower diameter classes to the high diameter classes
(Philip, 2002). This parameter has higher value in high-
pressure stands (0.49) than in low-pressure stands (0.39).
This situation could be explained by pasture and fire
frequency which are more common in high-pressure
stands. This impacted negatively on the development of
the saplings and their transition to the higher diameter
classes. The relatively high density of young trees of
A. leiocarpa in the two stands is explained by the fact that
A. leiocarapa has, among many other tree-species the
highest potential to break through the buffering mecha-
nism of fire as its regenerations in savanna sites were
moderately fire resistant (Hennenberg et al., 2005).
Therefore, the rapid fall in frequencies from lower diam-
eter classes to higher diameter classes observed for A.
leiocarpa in high-pressure stands could be due to other
factors like pasture or young leaves pruning for medicine
purposes. Several authors (Paradis & Houngnon, 1997;
Cunningham, 2001; Sokpon & Biaou, 2002) have used
the diameter and height size-class distribution as a field
method to assess the impact of harvest practices on the
regeneration of the species. However, it is important to
take into consideration the species character and the
stage of development of the population when analysing
its diameter or height size class distribution (Sinsin et al.,
2004). As stated by Philip (2002), pioneer species like
A. leiocarpa often presented high density of regenerations
and rapid fall of the density in the successive diameter
classes (Hennenberg et al., 2005). This could explained
the almost ‘J’ shape obtained for its diameter structure in
low-pressure stands.
The value of the logarithmic slope of the height–diam-
eter relationship was lower in high pressure indicating a
lanky shape probably due to slower growth in height of the
species in high-pressure stands.
As far as the species composition of the stands was
concerned, the study revealed that, generally, the two
types of stands contained the same tree species with regard
to the high value obtained for the coefficient of correlation
between the two stands. However, A. africana,Detarium
microcarpum and Burkea africana had less influence in high-
pressure stands than in low-pressure stands. These three
species were among the most valuable ones because they
are even more preferred than A. leiocarpa. Therefore, in
stands without management the exploitation of forest
resources often targeted these multipurpose species and
reduces their influence in the considered areas. Other
species such as Uapaca togoensis,Maranthes polyandra and
Combretum collinum as revealed by the present study
become more dominant in terms of relative coverage,
frequency and density.
Our results thus suggest that effective conservation is
needed for A. leiocarpa stands under high pressure by
limiting human perturbations and developing appropriate
growing strategy for restoration purposes. In situ con-
servation measures of the species should be enhanced in
areas where the species is facing low pressures. In
addition, measures focusing on the enrichment of the
poorest natural stands mainly composed of the species
should be one of the major concerns of Benin forest
managers.
Acknowledgements
This research was supported by a grant of the British
Ecological Society through the Overseas Bursaries.
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