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AS IA N JO URN AL OF FO RE ST RY
Volume 8, Number 2, December 2024 E-ISSN: 2580-2844
Pages: 173-182 DOI: 10.13057/asianjfor/r080208
Natural regeneration of woody species in Acacia mangium and
A. auriculiformis stands in Anguédédou, Abidjan, Côte d'Ivoire
KOUASSI RICHARD KOUADIO1,♥, MÉNÉKÉ DISTEL KOUGBO1, SOUNAN GATIEN TOURÉ1,2, BRAHIMA
COULIBALY1, ANATOLE KANGA N’GUESSAN1, ADAMA BAKAYOKO3,4
1Forest and Environment Program, National Center for Agronomic Research. Côte d'Ivoire, 01 BP 1740 Abidjan 01, Côte d’Ivoire.
Tel.: +225-27-22-4-89624, ♥email: richard.kouadio@cnra.ci
2Climate Change, Biodiversity and Sustainable Agriculture Laboratory, University Félix Houphouët-Boigny. 01 BPV 34 Abidjan 01, Côte d'Ivoire
3Ecology and Sustainable Development Laboratory, University Nangui Abrogoua. 02 BP 801 Abidjan 02, Côte d'Ivoire
4Natural Resources Conservation and Valuation Group, Swiss Center for Scientific Research in Côte d’Ivoire. 01 BP 1303 Abidjan 01, Côte d’Ivoire
Manuscript received: 17 September 2024. Revision accepted: 2 December 2024.
Abstract. Kouadio KR, Kougbo MD, Touré SG, Coulibaly B, N’guessan AK, Bakayoko A. 2024. Natural regeneration of woody species
in Acacia mangium and A. auriculiformis stands in Anguédédou, Abidjan, Côte d'Ivoire. Asian J For 8: 173-182. In the current context
of climate change, forest landscape restoration is promoted to reverse forest ecosystem degradation. In Côte d'Ivoire, leguminous plants,
notably Australian Acacias, have been introduced since 1980 at Anguédédou to restore the fertility of degraded farmland. The
introduction of Acacias was seen as a potential disturbance to the local flora, as these non-native species are sometimes invasive.
However, observation of these Acacia-based landscapes revealed good regeneration of woody species. The aim of this study was to
assess the natural regeneration of woody plants under Acacias stands and its relation with stand ages. We assessed the floristic
composition and studied the dynamics of natural regeneration of local woody species in four Acacia stands as a function of age. The
results showed that the most widespread family of naturally regenerating plants in Acacia stands is Fabaceae. We noted an increase in
the number of species as a function of stand age. The number of species rose from 20 (3-year-old stand) to 51 (27-year-old stand), with
28 species and 24 species in the 8-year-old and 11-year-old stands respectively. In all stands, mesophanerophytes represent the dominant
plant life form. The Shannon-Wiener diversity index of natural regeneration increased from 1.66±0.44 (3-year-old Acacia stand) to
2.45±0.36 (27-year-old Acacia stand). In contrast, as the Acacia stands aged, the regeneration index decreased, with values of 1 (for the
3-year-old and 8-year-old Acacia stands), 0.94 (for the 11-year-old Acacia stand) and 0.81 (for the 27-year-old Acacia stand). This study
shows that Acacias improve the local flora by promoting natural regeneration and the development of woody species.
Keywords: Acacias, forest restoration, natural regeneration
INTRODUCTION
The health of the world's forests is a major global
concern as maintaining their ecological functions has a
positive impact on living beings. Forests play an important
role in mitigating climate change effects. Forests are the
major ecosystems that maintain the microclimate and act as
carbon sinks by sequestering and storing carbon (Verkerk
et al. 2022). Every year, almost a third of global carbon
emissions -2.6 billion tonnes of carbon dioxide- from fossil
fuel combustion are absorbed by forests (UICN 2022). In
addition, forests deliver a wide range of ecosystem goods
and services (Gosain et al. 2015; Awasthi et al. 2022a).
These include the provision of timber and non-timber
products, the protection of biological diversity, the supply
of food and medicines and the maintenance of cultural and
recreation services (Nakajima et al. 2017; Akujärvi et al.
2021; Mason et al. 2022; Hu et al. 2022).
Despite their importance role, many forest ecosystems
in the world are threatened by anthropogenic activities
(Trumbore et al. 2015) including agricultural expansion
and intensification, invasion of invasive species and
infrastructure development (Htun et al. 2011; Bargali et al.
2019; Fartyal et al. 2022; Negi et al. 2024; Pandey et al.
2024). Over the past three decades, global forest cover has
declined by 420 million hectares, although the rate of
deforestation fell from 16,106 ha per year in the 1990s to
10,106 ha per year between 2015 and 2020 (FAO 2020).
Various disturbances on forest ecosystems cause
biodiversity loss, disturb microbial activities and nutrient
cycling and reduce forest productivity (Manral et al. 2020;
Vibhuti et al. 2020; Padalia et al. 2022).
The loss of plant diversity is a major challenge faced by
forest ecosystems in maintaining ecological sustainability
(Hua et al. 2022; Bisht et al. 2023). To address such
problem, reforestation efforts are promoted in tropical
regions where deforestation and land degradation remains
the major threats (Chazdon 2014; García et al. 2014). As a
result, numerous international reforestation initiatives are
being carried out in tropical countries. The aim of these
commitments is the resilience of forest functions by halting
deforestation and forest degradation and increasing forest
cover (Curtis et al. 2018; Song et al. 2018; Chazdon et al.
2020). The downward trend in deforestation is therefore
partly the result of global forest recovery through the
regeneration of forest species in deforested areas (Garcia et
al. 2020).
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8 (2): 173-182, December 2024
174
There are many approaches in forest restoration, from
large-scale reforestation, agroforestry, natural regeneration,
artificial regeneration and so on. All these practices have
the same objective, namely to re-establish the ecological
mechanisms that accelerate the recovery of forest
formation, ecological functioning and biodiversity toward a
climax forest (Elliot et al. 2013). However, across forest
restoration efforts globally, they differ in planning,
implementation and financial resources. Clearly, large-
scale reforestation projects are very costly and therefore
difficult to implement in many countries around the world.
On the other hand, natural forest regeneration does not
involve high economic costs (Fagan et al. 2020; Garcia et
al. 2020; Holl and Brancalion 2020). Thus, forest
restoration using natural regeneration approach seems
indispensable, because it minimizes the implementation
budgets (Garcia et al. 2020). Several methods are used to
stimulate the natural regeneration of forest species. The
process of natural regeneration is vital for forest
ecosystems, as it enables these biotopes to renew
themselves and conserve biodiversity. Natural regeneration
impacts the stability and evolutionary succession of forests
(Jin et al. 2018; Johnson et al. 2021; Zhang et al. 2022).
In Côte d'Ivoire, Australian Acacias were introduced in
1982 at Anguédédou to restore wasteland abandoned due to
the loss of fertility caused by agricultural overexploitation
(Kouadio et al. 2016). These wastelands originate from
clandestinely established cultivation stands in the classified
forest of Anguédédou. Thus, these Acacias were intended
to contribute to forest reconstitution, but their presence has
raised concerns for the local flora. Indeed, these
leguminous plants can be invasive for native species and
prevent their development. However, good plant diversity
has been observed under Acacia stands in Anguédédou,
Côte d'Ivoire (Kouadio et al. 2018). This raises questions as
to whether Acacia stands promote the regeneration of
woody species and whether the floristic diversity of natural
woody regeneration improves with stand age.
The aim of this work is to analyze the floristic
composition of the natural regeneration of woody flora and
to characterize the regeneration potential of woody species
under two Acacias species (Acacia mangium Wild and
Acacia auriculiformis A.Cunn. ex Benth.). Specifically, the
objectives of the present study were to assess (i) the
diversity of woody flora and their regeneration pattern and
(ii) characterize the effect of the age of Acacia stands on
the natural regeneration process of woody species.
Ultimately, this article will demonstrate that leguminous
trees can contribute to forest restoration by promoting
spontaneous regeneration of forest woody species.
MATERIALS AND METHODS
Study area
This study was conducted in the classified forest of
Anguédédou, located in the District of Abidjan (southern
Côte d'Ivoire) at the coordinates of 5°22'-5°26' N and
4°04'-4°13' W (Figure 1). The dominant vegetation in the
study area is dense evergreen rainforest (Guillaumet and
Adjanohoun, 1971). The climate is tropical equatorial,
characterized by abundant annual rainfall (around 2,000
mm) and four seasons: a long rainy season (April-July), a
short dry season (August-September), a short rainy season
(October-November) and a long dry season (December-
March). Average monthly temperatures range from 24.2°C
to 27.4°C, with average monthly relative humidity ranging
from 78 to 87% (Bi et al. 2010). The area's relief is marked
by high plateaus (40 to 50 m and 100 to 120 m), mid-
altitude plateaus (8 to 12 m), plains and deep valleys
ranging from 12 to 40 m (Kablan 2016).
Figure 1. Map of study area in the classified forest of Anguédédou, District of Abidjan, Côte d'Ivoire
KOUADIO et al. – Natural regeneration of woody species in Acacia stands
175
Data collection
A system of rectangular stands (50 × 35 m) was set up
in the Acacia stands to collect data by means of floristic
inventories. Four stand classes were defined based on the
age of the Acacia species, i.e. 3 years (parc3), 8 years
(parc8), 11 years (parc11) and 27 years (parc27). On each
rectangular stand, five square plots (6x6 m) were installed,
one at each corner and in the center of the stand. We
combined the area survey (in the square stands) and the
roving inventory (in the rectangular stands). All species
present were first inventoried, then diameter measurements
were taken on individuals at breast height, i.e., 1.30 m
above ground level. To study natural regeneration, all
individuals with a diameter of less than 5 cm were taken
into account. This dendrometric threshold, already used by
Assédé et al. (2015), makes it possible to minimize the
inclusion in regeneration of mature individuals of shrub
species predominantly present in the stands studied.
Species were determined in the field or by laboratory
identification (herbarium) of plant species samples
collected. The nomenclature adopted was APG IV (2016).
Floristic analysis
Floristic analysis of natural regeneration focused on
floristic composition and three regeneration parameters,
namely specific regeneration rate, stand regeneration index
and stand regeneration importance value. The life cycle of
a tree is characterized by three stages following Baboo et
al. (2017) as adults (dbh ≥ 10 cm), perches (3.2 ≤ dbh < 10
cm) and seedlings (dbh < 3.2 cm; height ≥ 30 cm). In this
study, regeneration individuals have a diameter of less than
5 cm and a height greater than or equal to 30 cm.
Floristic composition
Floristic composition expresses the total number of taxa
(families, genera, species) recorded, their taxonomic
distribution and their biological and ecological
characteristics. The parameters of floristic composition be
considered in this study are floristic richness and biological
types.
Floristic richness
In the sample of a plant community studied or in an
ecosystem, the number of species, genera and families
encountered represents floristic richness (Marcon 2015).
Plant life-form
This study considers only phanerophytes according to
Raunkiaer (1934). The interest of addressing these plant
life-form for natural regeneration is the prediction of the
vertical structure of the future regenerated forest stand
under acacias. Depending on the height of the species, a
distinction must be made between: (i) megaphanerophytes
(MP), trees with a height of over 30 meters ; (ii)
mesophanerophytes (mP), trees between 8 and 30 meters
tall ; (iii) microphanerophytes (mp), shrubs between 2 and
8 meters tall ; (iv) nanophanerophytes (np), shrubs less than
2 meters high.
Species diversity index
The species diversity of natural regeneration was
addressed using the Shannon-Wiener index (Shannon-
Wiener 1963). This is a diversity index used to compare
distinct plant communities. It associates the number of
species and the relative abundance of each species in a
given community. Its value commonly varies between 1.5
and 3.5 (Magurran 1988). The Shannon Index (H') is
calculated by the following formula:
Where:
S: Total number of species present,
pi: Abundance percentage of species present (pi =
ni/N),
ni: number of each present species individuals
N: total number of all species individuals ;
log2: base-2 logarithm
Species regeneration rate (SRR)
The specific regeneration rate is used to assess the
relative abundance of a species within the natural
regeneration of a group of stands. It is expressed as a
percentage and is obtained by dividing the number of
regenerating individuals of a species by the total number of
regenerating individuals in the stand or stand concerned.
Regeneration individuals are young individuals, i.e. those
with a diameter of 5 cm or less. The Specific Regeneration
Rate (SRR) is determined using the formula below:
Where :
n: Number of seedlings of each species,
N: Total number of surveyed seedlings
Regeneration index (RI)
The value of each stand's regeneration index (RI) is
determined by the ratio of the number of regenerating
individuals (diameter ≤ 5 cm) to the number of individuals
of all diameters. This index is used to assess the age of the
stand and has range between 0 and 1. The older the stand,
the more the RI tends towards 0, while the younger the
stand, the more the RI tends towards 1. The formula for
calculating the Regeneration Index is as follows:
Where:
r: number of regeneration individuals (diameter ≤ 5 cm)
T: number of all diameters individuals
Regeneration Importance Value (RIV)
The Regeneration Importance Value (RIV) was
developed in this study to assess the weight of natural
regeneration within each stand type. It is an index that
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176
integrates the number of species resulting from natural
regeneration and the regeneration rate of each species. For
every stand, the higher the RIV, the more remarkable the
level of natural regeneration. The RIV formula is as
follows:
Where:
r: number of regeneration individuals (diameter ≤ 5 cm)
T: number of all diameters individuals
sp: number of natural regeneration species in the stand
Statistical analysis
All data were entered into an Excel spreadsheet and
processed using R software. The mean Shannon-Wiener
diversity indices of the different stands were compared
using a one-factor analysis of variance (ANOVA 1), and a
Fisher test was performed to distinguish statistically
different means. The significance level chosen for this test
was 5%. A factorial analysis of correspondences (FAC)
was carried out to observe the distribution of woody
regeneration species according to the age of the Acacia
stands. This distribution is based on the relative abundance
of species expressed by the specific regeneration rate.
RESULTS AND DISCUSSION
Floristic richness
In total, the natural regeneration of woody flora
included 67 species belonged to 55 genera and 30 families.
The Fabaceae family was the best represented with 14.93%
of species, followed by Annonaceae (10.45%),
Sapindaceae (10.45%), Meliaceae (5.97%) and Moraceae,
Olacaceae and Rubiaceae with 4.48% of species each
(Figure 2). The remaining 31 families accounted for
44.76% of species in natural regeneration.
At Parc3, the natural regeneration contained 20 species
belonged to 18 genera and 12 families. This stand was
dominated by Fabaceae and Sapindaceae, each accounting
for 15% of species (Figure 3.A). These two families were
ahead of four others, which individually accounted for 10%
of species, namely the Apocynaceae, Malvaceae, Meliceae
and Olacaceae (Figure 3.A). Parc8 consisted of 28 natural
regeneration species belonged to 27 genera and 17 families.
In this stand, the Fabaceae was the most dominant with
20.69% of species, followed by Sapindaceae with 10.34%
(Figure 3.B). In third place, there were five families with
6.9% of species, namely Annonaceae, Apocynaceae,
Lecythidaceae, Malvaceae, Meliaceae and Rubiaceae
(Figure 3.B). At Parc11, the natural regeneration comprises
24 species belonged to 22 genera and 14 families. This
stand was heavily populated by the Fabaceae and
Sapindaceae families, which accounted for 20.83% and
16.67% of species respectively (Figure 3.C). These families
were followed by Annonaceae, Apocynaceae and
Euphorbiaceae, which each accounted for 8.33% of species
(Figure 3.C). At Parc27, there were 51 naturally
regenerating species belonged to 43 genera and 26 families.
The dominant families were Annonaceae and Fabaceae,
each with 11.76% of species (Figure 3.D). These two
families were followed by the Sapindaceae with 9.8% of
species, the Meliaceae with 7.84% and the Apocynaceae
with 5.88% (Figure 3.D).
Plant life-form
The most widespread plant life-form in all four stand
age groups were mesophanerophytes (Figure 4). They
accounted for almost half of the species found on the other
stands. At Parc3, 50% of species were mesophanerophytes,
while at Parc8, Parc11 and Parc27, respectively 41.38,
41.67, and 47.06% of the species recorded were
mesophanerophytes (Figure 4). Next in order of
preponderance were microphanerophytes with 35% at
Parc3, 37.93% at Parc8 and at Parc11 with 37.5% of
species (Figure 4). This order is overturned at Parc27,
where microphanerophytes ranked third with 21.57% of
species (Figure 4). Finally, the extremes
(nanophanerophytes and megaphanerophytes) were the
least represented in the stands. These two plant life-forms
accounted for 15% of species at Parc3, with 10%
nanophanerophytes and 5% megaphanerophytes (Figure 4).
At Parc8, we counted 6.9% nanophanerophytes and
13.79% megaphanerophytes, while Parc11 contained
12.5% nanophanerophytes and 8.33% megaphanerophytes.
At Parc27, we recorded 7.84% nanophanerophytes, with
the exception of megaphanerophytes, which ranked second
with 23.53% of species (Figure 4).
Species diversity index
The Shannon-Wiener diversity index of the stands was
less than 2.5. It was estimated at 1.66±0.44 for Parc3;
2.09±0.14 for Parc8; 1.98±0.37 for Parc11 and 2.45±0.36
for Parc27 (Table 1). Statistical analysis of these values
showed significant differences between them (F = 7.308; P
= 0.001) and three groups emerged (Table 1). The lowest
index was for Parc3, and the highest was for Parc27.
Between these two values, we had the second group, made
up of roughly equal Shannon indexes, corresponding to
Parc8 and Parc11 (Table 1).
Figure 2. The proportion of naturally regenerating species of each
family across all stands
KOUADIO et al. – Natural regeneration of woody species in Acacia stands
177
Figure 3. The proportion of naturally regenerating species of each family at each stand age. A. Parc3; B. Parc8; C. Parc11 ; D. Parc27
Figure 4. Plant life-form of natural regeneration vegetation at each stand. Note: MP: Megaphanerophytes, mP: Mesophanerophytes, mp:
Microphanerophytes, np: Nanophanerophytes
Table 1. Shannon-Wiener species diversity index of naturally
regenerating plant at each stands
Shannon index
Parc3
1.66±0.44b
Parc8
2.09±0.14ab
Parc11
1.98±0.37ab
Parc27
2.45±0.36a
Statistical parameters
F=7.308 ; P=0.001
Table 2. Eigenvalues and percentage inertia of factorial analysis
of correspondences (FAC) main axes
Axes
dim1
dim2
dim3
Eigenvalue
0.362
0.285
0.174
Percentage of variance (%)
44.08
34.70
21.22
Cumulative percentage of variance
44.08
78.78
100
A
B
C
D
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Figure 5. Diagram of species distribution on the factorial plane (F1 and F2)
Specific regeneration rate (SRR)
At Parc3, the dominant naturally regenerating species
was Funtumia africana with SRR of 21.62%, followed by
Baphia nitida (16.22%), Microdesmis keayana (12.16%)
and Rauvolfia vomitoria (10.81%). These species were the
most important in the natural regeneration of Parc3, with a
cumulative SRR of 60.81%. At Parc8, M. keayana
dominated with a SRR of 22.22%, followed by B. nitida
(15.15%), Cola heterophylla (11.45%) and Angylocalyx
oligophyllus (7.74%). These four species largely dominated
the other twenty-five species at this stand, which had a
cumulative SSR of 43.43%. At Parc11, B. nitida was the
most widespread naturally regenerating plant with an SRR
of 15.83%, followed by R. vomitoria (14.17%), Macaranga
barteri (10%) and Millettia zechiana (7.5%). At Parc27, B.
nitida was the most abundant with a SSR of 10.18%,
followed by C. heterophylla (9.12%), Chrysophyllum
subnudum (7.02%) and M. keayana (7.02%), while the
remaining 47 species had an accumulative SSR of 66.66%.
Species distribution by stand age
Three factorial axes (Dim1, Dim2 and Dim3) explained
the distribution of species within stands, which varied from
0.174 to 0.362, i.e. from 21.22 to 44.08% (Table 2). The
FAC revealed that the distribution of species within the
stands is mainly represented by the factorial plane formed
by the Dim1 and Dim2 axes (Figure 5). The observation is
therefore made on these two axes, which accounted for
78.78% of total inertia. The Dim2 axis (34.7%)
discriminated the oldest stands (Parc11 and Parc27) from
the youngest (Parc3 and Parc8) and the Dim1 axis
(44.08%) separated the oldest stands from each other
(Figure 5). The FAC revealed nine groups of species
distributed according to the age of the Acacia stands.
Species such as Myrianthus libericus, Antiaris welwitschii,
Chytranthus setosus and F. africana are mainly found at
Parc3. Species such as Vismia guineensis, Massularia
acuminata, M. keayana, Heisteria parvifolia, Hannoa
klaineana, Xylopia parvifolia and A. oligophyllus were
found at Parc8. Ceiba pentandra, Monodora tenuifolia, M.
zechiana, M. barteri, Blighia unijugata, Ficus exasperata,
Macaranga beillei, Albizia adianthifolia were more widely
distributed at Parc11. Finally, species such as
Piptadeniastrum africanum, Sphenocentrum jollyanum,
Entandrophragma angolense, Antiaris africana and
Petersianthus macrocarpus were more common at Parc27.
Regeneration index (RI) of the stand
At Parc3, we recorded 74 young individuals out of a
total of 74 individuals in this stand, giving a value of 1 as
the stands regeneration index (Table 3). The same trend
was observed at Parc8, where the RI was equal to 1,
suggesting that all the individuals inventoried were young
individuals (Table 3). At Parc11, we recorded 129
individuals of all diameters, including 121 young
individuals, giving us a RI of 0.94 for Parc11 (Table 3).
Finally, the RI for Parc27 is estimated at 0.81, i.e. 285 of
the 350 individuals in this block are young (Table 3). The
regeneration index in the stands regressed with age (Figure
6).
KOUADIO et al. – Natural regeneration of woody species in Acacia stands
179
Figure 6. Trend of regeneration index according to stand age
Table 3. Assessment of regeneration indices for the different blocks of stands with varying ages
Stand3
Stand8
Stand11
Stand27
Young individuals (diameter ≤5 cm)
74
300
121
285
Total individuals (individuals of all diameters)
74
300
129
350
Regeneration index (RI)
1
1
0.94
0.81
Table 4. Summary of regeneration importance values for the
different stand blocks
Stand3
Stand8
Stand11
Stand27
R
74
300
121
285
T
74
300
129
350
Sp
20
29
24
51
RIV
4.47
5.39
4.6
5.8
Stand regeneration importance value (RIV)
The natural regeneration of the flora was most
important at the oldest stand (Parc27) where the RIV was
5.8 (Table 4), while the lowest RIV (4.47) was recorded at
the youngest stand (Parc3). The other two stands, Parc8
and Parc11, had RIV of 5.39 and 4.6 respectively (Table 4).
Discussion
In the Acacia stands, we have recorded the typical
species inventoried by Tiébré et al. (2015). All the stands
differ in species composition and regeneration pattern
because in the dissected landscapes, bioclimatic conditions
change rapidly and may vary within short distances
resulting in a pronounced heterogeneity of soil types
(Bäumler 2015; Awasthi et al. 2022b) hence influence the
distribution of vegetation and their regeneration pattern
(Bargali et al. 2019; Manral et al. 2022). Vegetation cover
in any ecosystems varies in space and time because of
variation in topography, climate, weathering processes,
physico-chemical properties of soil and microbial activities
(Paudel and Sah 2003; Manral et al. 2023) and several
other biotic and abiotic factors (Pandey et al. 2023).
Vegetation and its regeneration therefore vary within short
distances according to parent rocks, soil types and land use
pattern. This proves the restoration capacity of the local
flora under the Acacias, which seem to offer the soil seed
stock optimal germination conditions. At all stands, the
flora of the natural regeneration is dominated by the
Fabaceae family. These results are in line with those of
Ameja et al. (2022), who conclude that regenerating
environments are dominated by Fabaceae. The abundance
of species in this family would depend on their effective
and successful dispersal strategies on the one hand, and
their high potential for adaptation in more varied
ecosystems on the other (Yemata and Haregewoien 2022).
After this family, we have, to a lesser extent, the
Sapindaceae and Apocynaceae as preponderant families.
Commonly, these families appear as the most important in
forests of the same study area with Apocynaceae
concerning the Anguédédou forest (Tiébré et al. 2015) and
Fabaceae and Apocynaceae in the Mabi forest (Amba et al.
2021). Our results show that natural regeneration under
Acacias retains the characteristics of dense evergreen
forests in Côte d'Ivoire.
Analysis of the Shannon-Wiener index recorded in the
various stands suggests that woody regeneration under
Acacias is moderately diverse. Indeed, in all stands, around
20% of the species inventoried account for more than half
of the individuals found. As pointed out by Barmo et al.
(2019), the Shannon-Wiener index is minimal when the
stand is dominated by one species and other species are
poorly represented. However, natural regeneration becomes
increasingly diverse as the age of Acacia stands increases.
This observation can be explained by the proliferation of
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woody species over time, encouraged by the reduction in
herbaceous cover due to the presence of arborescent
legumes. Thanks to their role in fixing atmospheric
nitrogen, leguminous trees exert control over grasses and
weeds (Kouadio et al. 2018). Indeed, a two-thirds reduction
in herbaceous cover was observed after thirteen years of
leguminous presence in native grasslands in Uruguay
(Jaurena et al. 2016). Acacia stands are unfavorable
environments for the propagation of herbaceous flora,
creating ideal conditions for the regeneration of woody
species. Furthermore, some authors (Carnus et al. 2006;
Paquette and Messier 2013) claim that the aging of forest
tree plantations is a factor reinforcing the availability of
ecological niches as well as the high and diverse presence
of species. All Acacia stands, whatever their age, are home
to all the biological types of arborescent phanerophytes,
which are varied forms of woody plants. This observation
follows the logic of the floristic composition of tropical
forests, since phanerophytes make up the majority of the
flora at the expense of other biological types, which are in
reduced proportions. Acacias are arborescent leguminous
plants that control herbaceous flora. Moreover, thanks to
their tree cover, these species create a microclimate that
prevents the colonization of the environment by herbaceous
species. The variability of vascular plant species is proof
that Acacia stands provide a suitable environment for the
regeneration and growth of woody species. Also, some
authors (Keil and Chase 2019; Liang et al. 2022) have
established a link between environmental conditions and
the diversity of woody species and forest tree species. In
other words, when the undergrowth is rich and diverse, the
environment becomes favorable to interactions and the
presence of several plant species (Yang et al. 2023).
Moreover, the diversity of tree species in forest
communities is fundamental to the conservation of
ecosystem services such as carbon storage, groundwater
protection, wood supply and soil stabilization (Esquivel et
al. 2020; Hua et al. 2022; Duan et al. 2023).
Over time, the diversity of naturally regenerating plants
has improved. This result could be explained by an average
accumulation of nitrogen in the soil from the nitrogen fixed
by the roots of these tree legumes. This activity of the
Acacia rhizosphere leads to an improvement in soil
productivity in these Acacia stands. Indeed, total nitrogen
is an edaphic factor influencing vegetation growth,
regeneration and the development of a plant stand (Qian et
al. 2014). When the nitrogen stock in the soil is moderate,
it stimulates the uptake and activity of soil nutrients (Luo et
al. 2022), resulting in improved vegetation productivity
(O'Sullivan et al. 2019). The ultimate goal of vegetation
recovery is to increase biodiversity and ecosystem stability
(Midolo et al. 2019; Li et al. 2021). Natural regeneration is
an effective method of restoring vegetation, helping to
store soil carbon, fix soil nitrogen, restore degraded
ecosystems and improve soil quality (Hu et al. 2021). It
helps to increase the diversity of plants in the understory
and ensures the sustainable restoration of the forest canopy
(Wang et al. 2019). In addition to this, vegetation
restoration helps to increase the soil carbon sink, and
especially the reservoir of biodegradable carbon contained
in the soil (Liu et al. 2020; Hu et al. 2021). Soil carbon is
very important, as it is involved in the nitrogen fixation
process by Acacia roots. There is a symbiosis between the
Rhizobiums (soil nitrogen-fixing bacteria) and the roots of
leguminous plants, which provide these bacteria with
carbon as a source of energy. In return, the Rhizobiums use
their energy source and become active, fixing nitrogen for
the leguminous roots that host them. Previous studies
(Ferguson et al. 2019; Roy et al. 2020; Yang et al. 2022)
have explained that legume root nodules host Rhizobium.
Supplied with nutrients by legumes, these bacteria convert
atmospheric nitrogen into reduced forms that can be used
by host plants. It is worth pointing out that a forest's
resilience process relies on seed germination, seedling
establishment and survival (Taeroe et al. 2019), which in
reality depend on environmental conditions. Through their
role in capturing nitrogen, Acacias improve soil fertility,
creating edaphic conditions favorable to natural
regeneration. In the process of forest resilience, seedlings
resulting from seed germination survive and grow thanks to
numerous biotic and abiotic factors, including water,
nutrients and soil properties (Rozendaal et al. 2019; Zhang
et al. 2022). The other striking fact is the decline in the
regeneration index as the stands age. This finding would
mean that there is a progressive maturity of regenerated
woody flora under the Acacias marked by the increasing
proportion of mature individuals. This proves that the
individuals that regrow in the natural regeneration process
initiated under the Acacias are developing well.
In conclusion, we noticed that the presence of Acacias
on a degraded potion of the Anguédédou Forest has
restored soil fertility and tree cover in these areas. The new
environmental conditions created by the Acacias have
encouraged the regeneration and development of woody
species. Therefore, there is a good natural regeneration of
woody species under Acacias planted in a degraded forest
landscape. We noted an increase in the species richness and
an improvement in the specific diversity of the regenerated
woody flora as the Acacia stands aged. These results show
that Acacias, in addition to their agronomic benefits, can be
used to naturally regenerate a forest at lower cost. Our
study leads us to conclude that tree legumes in general, and
Acacias (notably A. mangium and A. auriculiformis) in
particular, are useful species both in agricultural systems
and in forestry. These Acacias species therefore appear to
be strategically important species in the management of
agroforestry systems and forest restoration.
ACKNOWLEDGEMENTS
The authors would like to thank the CNRA and its
Forest and Environment Program for the availability of the
study site during data collection. We are also grateful for
the logistical support provided during this work.
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