Physical and Mechanical Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin

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Journal of Materials Science and Surface Engineering, 8(1): 992-1000
ISSN (Online): 2348-8956; DOI: https://doi.org/10.52687/2348-8956/815
Corresponding Author: Montcho Crépin Hounlonon, Tel: +229 97504727 © 2021 INScienceIN. All rights reserved
Email: mcrepin.hounlonon@gmail.com
Contents lists available at http://www.jmsse.in/ & http://www.jmsse.org/
Peer Reviewed
Physical and Mechanical Properties of Acacia Auriculiformis
A. Cunningham Ex Benth Used As Timber in Benin
Montcho Crépin Hounlonon1,2 · Clément Adéyèmi Kouchadé1,2 · Basile B. Kounouhéwa1,2,3
1Department of Physics, Faculty of Science and Technology, University of Abomey-Calavi, Abomey-Calavi, Republic of Benin.
2Laboratoire de Physique du Rayonnement, (LPR/FAST-UAC) 01 BP 526 Cotonou (Benin).
3Institut des Recherches Industrielles, Technologies et en Sciences Exactes (IRITESE/CBRSI) 03 BP 1665 Cotonou (Benin).
ABSTRACT
Acacia auriculiformis has been adopted in Benin as an energy wood through the firewood project since
1985. Increasingly, people use it as timber due to scarcity of conventional species and its availability while
its physical and mechanical parameters are little known. It was determined the percentage of heartwood,
density, modulus of elasticity and shear modulus of this wood in order to appreciate its current use as
timber and to estimate its exploitable age. 18 trees of 5; 8-10 and 20 years were sampled from plantations
of Sèmè, Pahou and Itchèdè-Toffo. It appears that from 8 years onwards, the percentage of Acacia
auriculiformis heartwood is above 70%. With medium module of elasticity, the density and the mechanical
parameters of Acacia auriculiformis have low variability between 8 and 20 years. Compared to species
conventionally used, its current use is justified and suggest an exploitability age of 15 years to be refined.
ARTICLE HISTORY
Received 05-04-2021
Revised 20-05-2021
Accepted 21-05-2021
Published 16-06-2021
KEYWORDS
Dendrometry
Heartwood
Modulus of Elasticity
Shear Modulus
Exploitability
© 2021 · INScienceIN. All rights reserved
Introduction
Forests provide several important ecosystem services,
including wood production. These services expose them to
strong anthropogenic pressures [1] that contribute to
deforestation. Between 2010 and 2015, the net annual
decrease in global forest area was 3.3 million hectares [2].
Wood remains the main product of forestry. As a result, it
constitutes an accessible raw material for populations with
limited incomes and contributes substantially to job
creation because it is produced and processed by
essentially local channels, including artisans. Thus, the
provision of multi-purpose forest resources by fast-
growing plantations is a major alternative to mining-type
harvesting in natural tropical forests. With a few
exceptions, plantations were initially established to
provide local fuelwood or supply local factories with
timber [3-4]. Once mature, some of these trees, which were
not intended for timber, are used in the more remunerative
production of timber due to increased demand [4].
Acacia auriculiformis is a widely planted tropical species in
the world [5]. In Benin, in order to deal with the problems
of global warming and the energy problems of the
population, wood energy plantations were set up in 1985.
To this end, the "Firewood Plantations in Southern Benin"
project, which was to be called the "Firewood Project" a
few years later, was initiated and implemented to address
the dendro-energy deficit in Benin's large urban areas.
After fourteen (14) years of implementation (1986 to
1998), the project has achieved plantations covering nearly
10078 ha. The species most represented in each of these
plantations are Acacia auriculiformis, Eucalyptus sp and
Anogeissus leiocarpa; Acacia auriculiformis is the most
abundant species. In the framework of this work, the
particular case of Acacia auriculiformis interests us. Indeed,
Acacia auriculiformis is a species introduced into Benin
with an initial vocation of energy wood. It is a forest species
originating from Australia (Northern Territory,
Queensland), Papua New Guinea and Indonesia [5-6] but
African countries are among the most recent to import it
between 1960 and 1980 [5-8]. For use as fuel wood, the
exploitable age is 4 years, whereas it is 10 years for timber
[9]without any precision on the quality of the wood. Acacia
auriculiformis is widely planted in southern Benin with an
area of about 2000 ha to be planted in 2019. In Benin,
Acacia auriculiformis is widely used in reforestation for its
rapid growth allowing for high productivity and an average
diameter increase of 2.3 cm per year at 6 years [10].
Although this species was initially intended for purely
energy and fertiliser use in agroforestry systems [10], it is
increasingly used for timber due to the scarcity of
conventional species with high economic potential. It is
now being used for furniture and construction. This
information is confirmed by the work of Tonouéwa et al
[10], Akouehou et al [8], Avikpo et al [11] who identified
certain plantations of the species that are treated as high
forest and exploited as timber, and Tonouéwa et al [12]
who noted that the wood sold as timber is generally at least
8 years old with a diameter of more than 20 cm.
The only problem is that in this change in the behaviour of
craftsmen towards this species, no technological reference
has yet been established to justify the reasoned choice of
its use in these different works, knowing that the use of a
wood as a timber is also determined by its physical and
mechanical properties [13].
In view of this specific context of Acacia auriculiformis,
research questions arise: do the physico-mechanical
characteristics of Acacia auriculiformis wood allow its use
as timber?

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
JMSSE Vol. 8 (1), 2021, pp 992-1000 ©2021 INScienceIN. All rights reserved
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To answer this question, the overall objective of this work
was to determine some physical and mechanical
characteristics of Acacia auriculiformis wood.
Specifically, this involved to :
 determine the density, elasticity and shear modulus of
Acacia auriculiformis wood taken from three state
plantations in southern Benin of three different ages;
 compare the physical and mechanical characteristics of
Acacia auriculiformis with those of conventional species
usually used and highly prized in Benin in order to
assess the probable age of exploitability as timber by
analyzing these properties in the radial direction.
Experimental
Study area
This study took place in southern Benin. This area has a
Sudano-Guinean climate characterized by two dry seasons
(December to February and August to September) and two
rainy seasons (April to July and October to November). The
average temperature is about 27°C with a high relative
humidity and an average annual rainfall of over 1100 mm.
Pahou has a tropical ferruginous soil, Sèmè a hydromorphic
sandy soil and Itchèdè-Toffo a ferralitic soil.
Table 1: Sampling scheme
Department
Plateau
Ouémé
Atlantique
Sector
Itchèdè-
Toffo
Sèmè-
Podji Pahou
Plots of 1000 m
2
installed 3 3 3
Number of trees cut
6
6
6
Number of trays cut
18
18
18
Figure 1: Picture showing the cutting of the samples and their
numbering from pith to sapwood
Measurement of parameters
Dendrometric characteristics
The dendrometric parameters determined are: stand
density (N), mean tree diameter (d), mean bole height (h)
and mean total height (ht).
Stand density (N)
The density of a stand represents the number of stems
present on a given surface. It is generally expressed in
number of stems per hectare.
S
n
N1000


(1)
Where S is the area of the sample plot (in m2) and n is the
number of trees or the number of feet in the plot.
The arithmetic mean diameter of the trees
 
d
The arithmetic mean diameter is obtained by the following
formula:
id
n
d
n
i



1
1 (2)
Where n is the total number of individuals in the stand (or
density), di = diameter of the shaft i.
The average total height
 
h
The average total height of the trees on a plot is obtained
by the following formula:



n
i
i
h
n
h
1
1 (3)
Where n is the total number of individuals in the stand (or
density), hi = height of tree i.
Density
The density of the sample is the ratio of the mass to the
volume of the specimen stabilized at 12% water content. It
is expressed in kilograms per cubic meter (kg/m3). The
mass of the test piece is determined by a balance with a
resolution of 0.01 g and the dimensions (width and
thickness), which allow the volume to be calculated, are
measured using an electronic calliper with a resolution of
0.01 mm. The length is measured with a tape measure
accurate to 0.05 cm. Measurements are made in a
temperature and relative humidity-controlled chamber,
which allows the samples to be stabilized at 12% water
content.
Proportion of heartwood
The proportion of heartwood is influenced by the age of the
wood and the growth rate of the tree. To measure the
percentage of heartwood, the radii from pith to bark (R1,
R2, R3, R4) and from pith to heartwood boundary (r1, r2, r3,
r4) were measured (Figure 2). The total cross-sectional
area (total pellet area noted St) and heartwood area
(heartwood area noted SHW) are calculated for each pellet
using the root mean square of at least four orthogonal radii
and up to eight if the cross-section is very irregular [14-15].
Figure 2: Picture showing orthogonal radii inscribed on the cross-
section of a wooden slab
993

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
JMSSE Vol. 8 (1), 2021, pp 992-1000 ©2021 INScienceIN. All rights reserved
Contents lists available at http://www.jmsse.in/ & http://www.jmsse.org/
Table 2: Percentage of heartwood and bark thickness according to the position of the wood in the tree
of Acacia auriculiformis seedlings in the state plantations of Sèmè, Pahou and Itchèdè-Toffo
Sector
Sèmè
Pahou
Itchèdè-Toffo
Wood
position
Percentage of
heartwood (%)
Bark thickness (cm)
Percentage of
heartwood (%)
Percentage of
heartwood (%)
Bark thickness (cm)
m
se
m
se
m
se
m
se
m
se
5m
59.27
6.21
0.8
0.08
67.94
2.12
46.05
3.27
0.18
0.03
1m30
69.71
2.65
1.04
0.11
69.14
1.79
51.27
1.52
0.35
0.04
Base
71.91
1.93
1.18
0.11
71.89
1.22
56.64
1.4
0.35
0.08
p
0.08ns
0.04*
0.29ns
0.01*
0.06ns
5m = at 5m height in the tree, 1m30 = at 1m30 height, Base = at the base of the tree, m= mean; se = standard error, *significant difference at
the 5% level, ns= non-significant difference at the 5% level
Figure 3: Picture showing "BING" experimental device
Thus, for the total area we have:
4
4
1
2








i
i
R
St

(4)
For the heartwood surface we have:
4
4
1
2







i
i
r
SHW

(5)
The percentage P of heartwood was calculated in relation
to the total surface area of the disc by the relationship:
100
St
SHW
P (6)
Measurement of modulus of elasticity and shear modulus
The modulus of elasticity and the shear modulus were
measured on the same specimens as those used to measure
the density of the wood. The device shown in figure 3 is
used for the test. It consists of a microphone (LEM, Type
EMU 4535), a data acquisition card (Pico Technology Type
PicoScope 3224), a computer, an elastic system on which
the wood material to be characterized is placed. The
system is driven by the BING (Beam Identification by Non-
destructive Grading) software, initially developed to
evaluate wood material, in its version 9. The method was
developed by CIRAD [16]. The principle of the
measurement was largely developed in the work of
Baillères et al. [16] cited by Hounlonon et al [17] and
Kouchadé et al [18] and is based on the spectral analysis of
natural bending vibrations.
Method of statistical analysis
The data were analyzed using excel and R software for
ANOVA. A comparison of the means was made according to
the radial position of the wood in the tree and the study
station.
Results and Discussion
Dendrometric parameters and percentage of
heartwood
The statistical analysis of some dendrometric parameters
and the proportion of heartwood of Acacia auriculiformis
from the three study sites is summarized in Table 2.
The study of the wood slices collected shows that the
percentage of heartwood at man's height is on average
about 70% for the Sèmè and Pahou plantations and 52%
for the Itchèdè-Toffo plantation.
The plantation of Sèmè, 20-year-old seedlings is a mixture
of Acacia auriculiformis and Eucalyptus camaldulensis, with
Acacia auriculiformis dominating. The density of Acacia
auriculiformis plants is 143 plants per hectare and that of
Eucalyptus camaldulensis is 53 plants per hectare. Several
illegal cuttings are noted. The litter is thick. The average
diameter of Acacia auriculiformis trees is 28.3 cm, the
average height of the shafts is 17.4 m and the total height is
24.2 m. Eucalyptus camaldulensis trees have an average
diameter of 59.2 cm, the bole height is 22.9 m and the total
height is 28.8 m.
The study of cut wood trays shows that the percentage of
heartwood and bark thickness of Acacia auriculiformis
species decreases from the base to the crown as shown in
Figure 4. The percentage of heartwood at man's height is
about 70%.
994

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
JMSSE Vol. 8 (1), 2021, pp 992-1000 ©2021 INScienceIN. All rights reserved
Contents lists available at http://www.jmsse.in/ & http://www.jmsse.org/
Figure 4: Percentage of heartwood (a) and bark thickness (b) of
Acacia auriculiformis in Sèmè
The Pahou plantation is pure, but there are a few old
Eucalyptus camaldulensis plants that were spared during
the cuttings, and some Acacia mangium and palm plants
are also present. The Acacia auriculiformis plantations are
young, with an average age of 8 to 10 years. The density of
Acacia auriculiformis plants is 516 plants per hectare. The
average diameter of the trees is 23 cm, the height of the
bole is 8 m and the total height is 12.2 m.
Acacia auriculiformis trees in this plantation have an
average heartwood percentage of 70%. This percentage of
heartwood (figure 5) decreases from the crown to the base
of the tree, but there is no significant difference between
the variations of these parameters from the base to the top
of the tree.
Figure 5: Percentage of heartwood of Acacia auriculiformis in
Pahou
At Itchèdè-Toffo, the plantation is pure (i.e. not mixed with
any other species) with a density of 790 trees per hectare.
The trees have an average diameter of 17.2 cm, an average
bole height of 7 m and a total height of 12.5 m. These
plantations are very young at 5 years old.
The average percentage of heartwood of the samples taken
in these plantations is 52% for an average bark thickness of
0.35 cm. Analysis of these results (Figure 6) shows that
there is a significant difference between the percentages of
heartwood in Acacia auriculiformis trees sampled in the
Itchèdè-Toffo state plantations.
Figure 6: Percentage of heartwood (a) and bark thickness (b) of
Acacia auriculiformis in Itchèdè-Toffo
A comparison of the Acacia auriculiformis plantations in
Pahou and Sèmè shows that the growth of Acacia
auriculiformis in Pahou is the highest. The two plantations
being subjected to the same rainfall regime, this difference
in growth could be explained by the type of stand and/or
the type of soil or even the age. In Sèmè, Acacia
auriculiformis is grown in a mixed stand, the competition
effect of Eucalyptus camaldulensis which is very demanding
in water could explain this difference.
Acacia auriculiformis wood has a high percentage (70%) of
heartwood in the Sèmè and Pahou forests, which are
respectively 20 years old and 8 to 10 years old, whereas it
is only 52% in the 5-year-old Itchèdè-Toffo stand. These
results show that the hardening of Acacia auriculiformis is
early and that it evolves very little between 8 and 20 years
of age. The same findings were made by Tonouéwa [10] on
stands ranging from 4 to 29 years old. Thus, from 8 years
onwards, the percentage of heartwood of Acacia
auriculiformis varies very little. It is therefore possible that
this visually perceptible characteristic is the trigger for the
current use of this wood by craftsmen. Similarly, the high
and stable proportion of heartwood in relatively old trees
could also allow us to project the probable age of
exploitability to between 8 and 20 years, the age of the
Sèmè and Pahou plantations. Tonouéwa [19] already
proposed a 15-year rotation for well-followed Acacia
Auriculiformis plantations on specific soils. However,
physical-mechanical characteristics and durability remain
indispensable criteria for proposing species for a given use.
Indeed, durability also remains an important criterion.
Although not assessed in this work, Acacia auriculiformis
has a moderate natural durability of the heartwood [20].
This durability can be seen in the dark colour of its
heartwood and the presence of molecules such as phenol,
tannins and other aromatic compounds in the wood [20].
Trials in India show that its wood is resistant to fungi from
8 years onwards [21]. Further tests in Bangladesh confirm
the species' high natural durability to fungi [22].
Physical and mechanical characteristics
The density, elasticity and shear moduli of Acacia
auriculiformis in the different sectors are presented in the
graph in figure 7.
995

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
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Figure 7: Density, elasticity and shear moduli of Acacia auriculiformis according to sectors
Table 3: Density, modulus of elasticity and shear modulus of Acacia auriculiformis wood collected
in the Sèmè, Pahou and Itchèdè-Toffo plantations
Sector
Density (kg/m
3
)
Modulus of elasticity (MPa)
Shear modulus (MPa)
m
se
m
se
m
se
Sèmè
825.16a
3.41
14991.17a
110.87
1532.70b
55.23
Pahou
616.57b
2.11
14066.78b
120.71
1284.34a
33.48
Itchèdè-Toffo
722.92c
5.83
13164.47c
197.73
1726.13b
140.46
P value
0.000***
0.000***
0.000***
m = mean, se= standard error; ***significant difference at 0.1% level
Table 4: Summary of data on the density and modulus of elasticity of some of Benin's prized wood species
Species Density (kg/m3)
Modulus of elasticity
(MPa) Authors
Acacia auriculiformis
825.17
14 991.17
This study
Afzelia africana
700 - 880
800
13700
17020
[27]
[28]
Milicia excelsa (Iroko) 640 12840 [28]
Kaya senegalensis (cailcedrat)
780
11650
[28]
Kaya ivorensis (African mahogany)
570
11820
[28]
Teak (Tectona grandis)
800
670
550 à 660
12913 - 14628
13740
12280
[17]
[28]
[29]
Gmelina arborea
490
400 à 510
440 à 510
9120
5500 - 10800
7500 - 9100
[28]
[1]
[30]
Anthonotha fragrans
688
14160
[18]
The analysis of these parameters presented in Table 3
shows that there is a significant difference at the 0.01%
threshold between the density, modulus of elasticity and
shear modulus of Acacia auriculiformis wood from the
different plantations studied.
Acacia auriculiformis wood from Sèmè has the best density
and the best modulus of elasticity. Acacia auriculiformis
wood has a density ranging from 616.57 to 825.17 kg/m3, a
modulus of elasticity varying between 13164.47 and
14991.17 MPa for a shear modulus varying from 1284.35
to 1726.13 MPa.
The physical and mechanical characteristics determined in
the course of this work confirm the advantages of wood
from this species. Indeed, the Acacia auriculiformis woods
tested have a density varying between 617 and 825 kg/m3,
a modulus of elasticity oscillating between 13164 and
14991 MPa for a shear modulus varying from 1284 to 1726
MPa. MacDicken and Brewbaker [23] found a density of
600-750 kg/m3 for Acacia auriculiformis in India. In
Malaysia, an average density of 610 kg/m3 decreasing
along the trunk from 630 to 580 kg/m3 was found for
Acacia auriculiformis [24]. In Japan, on Acacia
auriculiformis wood from Indonesia, Hasegawa et al [25]
found a mean density of 679 kg/m3. Kumar et al [26] found
a density of 722 kg/m3 for a modulus of elasticity 160500
kg/cm2 or about 15740 MPa for Acacia auriculiformis in
India. The same trends were confirmed by Tonouéwa [19].
The enthusiasm of craftsmen for using it as timber cannot
be assessed without comparing it to other species
conventionally used in Benin. Indeed, compared to these
highly prized species in Benin (Table 4), Acacia
auriculiformis has very appreciable physical and
mechanical characteristics. The physical and mechanical
properties of Acacia auriculiformis compared to those of
teak show that this wood is of high quality. Indeed, Acacia
auriculiformis wood is easy to work and offers a good finish
with sharp tools [31]. The heartwood of Acacia
auriculiformis is hard, medium-heavy and durable and is
valued in cabinet making. It is suitable for pulp, kraft and
semi-chemical paper [6]. Orwa et al [32] found that this
996

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
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wood is excellent for making toys, handicrafts, various
furniture, tool handles and construction if wood of
appropriate circumference is available. Tonouéwa [19]
found that from a young age (4-7 years), Acacia
auriculiformis has high basal densities in the order of 500-
600 kg/m3, comparable to those of mature wood of several
timber species such as Tectona grandis and Khaya
senegalensis. The physical and mechanical characteristics of
mature Acacia auriculiformis in Benin are also comparable
to those of Pterocarpus erinaceus and Afzelia africana. The
average modulus of elasticity of Acacia auriculiformis is
about 14594 MPa, a value largely superior to the moduli of
iroko which is 12840 Mpa, Kaya senegalensis which is
11650 MPa, and okoumé which is 9670 Mpa [27]. These
values found in our study are in the range of the moduli of
elasticity of some woods such as teak (13740 MPa),
Eucalyptus grandis (15200 MPa), African ebony (15500
MPa), barwood also called kosso (15670 MPa) [28].
With a density higher than that of iroko, cailcedrat, African
mahogany, teak and Gmelina arborea, the Acacia
auriculiformis wood studied has a modulus of elasticity
higher than those of the same species mentioned above.
The use of this wood species as timber by our populations
and craftsmen (Figure 8) is therefore not without interest.
Figure 8: Picture showing some works made with Acacia
auriculiformis wood
It would therefore be possible to use them in structural and
furnishing works, as some local wood material specialists
already do so well. It is therefore imperative to complete
this work by extending the range of samples and to carry
out studies on the hygroscopic behavior of this species
under high humidity and temperature gradients, especially
during the African monsoon.
The other reason why craftsmen could use it more and
more as timber is its availability. Indeed, this species has a
great ability to reproduce easily in a wide range of
ecological conditions. This has led to its reputation as an
invasive species in some continents, notably in Asia,
America and Africa [33]. Its invasiveness is also noted in
America and Asia in countries and states such as Florida,
India and Singapore [34-35]. It should be noted that the
species has a high capacity for natural regeneration. As a
result, it readily populates newly abandoned land and can
be found beyond its range but does not affect the integrity
of native forests [35].
Variation of technological characteristics according to
the radial position of the wood
The study of the radial variation of the physical-mechanical
characteristics of Acacia auriculiformis wood from the pith
to the sapwood (Table 5), shows that there is a significant
difference at the 0.1% threshold between the density and
the modulus of elasticity of Acacia auriculiformis wood
from the Pahou and Itchèdè-Toffo plantations according to
the radial position of the wood in the tree. As for the shear
modulus, there is no significant difference between these
data from the pith to the sapwood of the tree. The density
and modulus of elasticity evolve globally in a chronological
way from the pith to the sapwood, with the highest
densities and moduli near the sapwood as shown in figures
9 and 10.
Figure 9: Variation of density, shear modulus and modulus of
elasticity as a function of radial position (a, b); elasticity and shear
moduli as a function of density (c, d) of Acacia auriculiformis wood
in Pahou
997

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
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Figure 10: Variation of density, shear modulus and modulus of
elasticity as a function of radial position (a, b); elasticity and shear
moduli as a function of density (c, d) of Acacia auriculiformis wood
at Itchèdè-Toffo
Figure 11: Variation of density, shear modulus and modulus of
elasticity as a function of radial position (a, b); elasticity and shear
moduli as a function of density (c, d) of Acacia auriculiformis wood
in Sèmè
998

M. C. Hounlonon et al./ Physical and Mechani cal Properties of Acacia Auriculiformis A. Cunningham Ex Benth Used As Timber in Benin
JMSSE Vol. 8 (1), 2021, pp 992-1000 ©2021 INScienceIN. All rights reserved
Contents lists available at http://www.jmsse.in/ & http://www.jmsse.org/
Table 5: Density, modulus of elasticity and shear modulus of Acacia auriculiformis wood as a function of
the radial position of the wood in the tree of the Pahou, Itchèdè-Toffo and Sèmè state plantations
Sector Radial position of the wood 1 2 3 4 5 P value
Itchèdè-Toffo
(5 years)
Density (kg/m3) m 657.85 689.23 738.79 793.88 751.75 0.000***
se 17.57 9.15 11.91 13.31 4.9
Modulus of
elasticity (MPa)
m 10596.4 13443.3 13681.2 13881 13312.3 0.001**
se 583.81 18.29 429.72 482.02 569.88
Shear modulus
(MPa)
m 2842.24 1021.55 1363.32 1468.96 2916.77 0.33ns
se 684.96 156.96 92.8 116.04 558.16
Pahou
(8 to 10 years)
Density (kg/m3) m 657.85 689.23 738.79 793.88 751.75 0.000***
se 17.57 9.15 11.91 13.31 4.9
Modulus of
elasticity (MPa)
m 10596.4 13443.3 13681.2 13881 13312.3 0.001**
se 583.81 184.29 429.72 482.02 569.88
Shear modulus
(MPa)
m 2842.24 1021.55 1363.32 1468.96 2916.77 0.33ns
se 684.96 156.96 92.8 116.04 558.16
Sèmè
(20 years old)
Density (kg/m3) m 744.42 821.81 833.77 856.42 856.86 0.000***
se 12.65 5.85 4.16 7.6 13.47
Modulus of
elasticity (MPa)
m 13542.3 15790.2 14853.3 16162.6 12853.7 0.156ns
se 357.35 163.14 191.42 264.54 231.48
Shear modulus
(MPa)
m 1981.68 1666.02 1487.3 1365.15 1083.48 0.000***
se 294.24 89.8 85.15 120.51 99.24
m = mean, se= standard error; ***significant differ ence at the 0.1% level, ns= non-significant difference
Thus, there is a significant difference at the 0.1% threshold
between the density and shear modulus of Acacia
auriculiformis wood from the Sèmè plantations as a
function of the radial position of the wood in the tree,
whereas there is none between the density and modulus of
elasticity. Thus, despite the increasing evolution of density
from pith to sapwood at the Sèmè site, the modulus of
elasticity fluctuates (Figure 11).
In the evaluation of the age of exploitability, we had to
analyze the radial variation of density, modulus of elasticity
and shear modulus, from the heartwood to the sapwood.
The results show the progressive evolution of these
parameters in the tree at the level of the youngest stands
(Pahou, 8 to 10 years old and Itchèdè-Toffo, 5 years old), on
the other hand, for the wood coming from Sèmè with a
stand of 20 years old, very little variation is noted in the
radial direction. This can be explained by the excessive
presence of juvenile wood in the younger stands. Thus,
depending on the radial growth of the tree, there is the
formation of juvenile wood close to the pith and mature
wood close to the sapwood [36]. Juvenile wood generally
has inferior characteristics to mature wood although the
differentiation between these two zones in the wood varies
from species to species e.g., for Pinus brutia, the
differentiation is found at the twelfth growth ring [37]. The
growth of the tree during its first years of life is rapid and
involves a low density of wood. This density increases as
this growth decreases. The tree invests in surviving in its
environment as an adult with high wood density. Wood
density increases from pith to sapwood [38-39], but there
are species whose density decreases from pith to sapwood
[38]. However, the characteristics of the sapwood are
generally lower than those of the heartwood. Wood density
explains the mechanical behavior of the tree and is a major
functional trait of the tree [38]. The radial increase in
density from the heartwood to the sapwood for the young
stand explains that the mature wood forms gradually and
reaches its stability between 15 and 20 years of age as
observed in the Sèmè stand. This age range could be ideal
for the exploitability of a plantation of Acacia
auriculiformis. The variation in radial wood density can
also be explained by changes in the anatomical
characteristics of the wood, which vary during the growth
of the tree, especially in the early years. For example, in
Heliocarpus appendiculatu, 85% of the variation was
explained by the differentiation of axial parenchyma into
fibres [38]. Pending a large-scale study on plantations of
different ages, from 10 years, the exploitable age of Acacia
auriculiformis can be estimated at 15 years as proposed by
Tonouéwa [19].
Conclusions
The study of the percentage of heartwood, density,
elasticity and shear moduli of three different ages of Acacia
auriculiformis was undertaken in three regions of southern
Benin (Pahou, Sèmè and Itchèdè-Toffo). This study shows
that the percentage of heartwood is high after 10 years and
is strongly related to the values of the physico-mechanical
parameters. The physico-mechanical parameters obtained
are acceptable and better than those of conventionally used
species. This justifies the use of Acacia auriculiformis as
timber. The low variation in density at all points of the
heartwood of Acacia auriculiformis from the age of 10 years
onwards suggests an exploitable age of 15 years on average
for its use as timber.
Apart from the physico-mechanical characteristics, the
durability and availability of the species certainly played a
role in its use as timber. However, the age of exploitability
remains to be refined by further studies.
Acknowledgements
We thank the Office National du Bois du Bénin (ONAB) for funding
this work through the Fonds d'Aide à la Recherche Forestière. We
also thank the Technical Forest Management Units (CTAF) of
Pahou and Sèmè and the General Directorate of Forests and
Natural Resources (DGFRN) of Benin.
Conflicts of Interest:
In view of this submission the authors declare that there
are no conflicts of interest.
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