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Folk perception of sexual dimorphism, sex ratio, and spatial repartition: Implications for population dynamics of Sclerocarya birrea [(A. Rich) Hochst] populations in Benin, West Africa

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In Sub-Saharan Africa, indigenous fruit trees play vital roles in nutrition and food security particularly, in food shortage times. Sclerocarya birrea subsp. birrea, an indigenous dioecious fruit tree is such a resource with strong multipurpose use characteristics in semi-arid zones of West Africa. We assessed sex ratio, spatial distribution among male and female adult trees using second-order spatial statistics and assessed folk perception of dioecism among the natural populations in protected areas and surrounding agroforestry systems. A field survey showed that 55% of interviewees were aware of sex separation in the species. Some used bark appearance to make distinction between sexes, but this morphological criterion was not consistent with statistical results. The sex ratio did not deviate significantly from 0.5 in any of the districts or land use types. Bivariate spatial analysis with pair correlation function revealed no spatial association between male and female individuals. Moreover, a strict spatial segregation of sexes was not observed even though some individuals of the same sex could sometimes be found together. Results confirmed the functional dioecy of the species and showed that the species did not display any apparent sex-specific dimorphism outside the reproduction period or any apparent sex-specific requirement for environment conditions. KeywordsAgroforestry–Spatial analysis–Local perception–Dioecious species–Spatial segregation of sexes–Protected area
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Folk perception of sexual dimorphism, sex ratio, and spatial
repartition: implications for population dynamics
of Sclerocarya birrea [(A. Rich) Hochst] populations
in Benin, West Africa
Gerard Nounagnon Gouwakinnou
Anne Mette Lykke
Bruno Agossou Djossa
Brice Sinsin
Received: 12 February 2010 / Accepted: 17 January 2011 / Published online: 30 January 2011
Ó Springer Science+Business Media B.V. 2011
Abstract In Sub-Saharan Africa, indigenous fruit
trees play vital roles in nutrition and food security
particularly, in food shortage times. Sclerocarya
birrea subsp. birrea, an indigenous dioecious fruit
tree is such a resource with strong multipurpose use
characteristics in semi-arid zones of West Africa. We
assessed sex ratio, spatial distribution among male
and female adult trees using second-order spatial
statistics and assessed folk perception of dioecism
among the natural populations in protected areas and
surrounding agroforestry systems. A field survey
showed that 55% of interviewees were aware of sex
separation in the species. Some used bark appearance
to make distinction between sexes, but this morpho-
logical criterion was not consistent with statistical
results. The sex ratio did not deviate significantly
from 0.5 in any of the districts or land use types.
Bivariate spatial analysis with pair correlation func-
tion revealed no spatial association between male and
female individuals. Moreover, a strict spatial segre-
gation of sexes was not observed even though some
individuals of the same sex could sometimes be found
together. Results confirmed the functional dioecy of
the species and showed that the species did not
display any apparent sex-specific dimorphism outside
the reproduction period or any apparent sex-specific
requirement for environment conditions.
Keywords Agroforestry Spatial analysis Local
perception Dioecious species Spatial segregation
of sexes Protected area
Introduction
As part of their livelihoods, people living near or in
forest make use of plant resources which fulfill
different roles in their subsistence and allow them to
live with less cash (Vedeld et al. 2007). In Sub-
Saharan Africa, indigenous fruit trees play vital roles
in food and nutritional security, especially during
periods of famine and food scarcity (Chirwa and
Akinnifesi 2008), and they are becoming increasingly
important as a main source of food to supplement
diets even in better times. Sclerocarya birrea is an
indigenous fruit tree species, and its fruit is subject of
a significant trade in the Sahel (Diallo et al. 2006).
Although three subspecies are distinguished, fruits,
leaves, bark, kernels, and wood of the species are
widely used by local people irrespective of subspecies
G. N. Gouwakinnou (&) B. A. Djossa B. Sinsin
Laboratory of Applied Ecology, Faculty of Agronomic
Sciences, University of Abomey-Calavi, 01 BP 526
Cotonou, Benin
e-mail: gougerano@yahoo.fr
A. M. Lykke
Department of Terrestrial Ecology, National
Environmental Research Institute, Aarhus University,
8600 Silkeborg, Denmark
123
Agroforest Syst (2011) 82:25–35
DOI 10.1007/s10457-011-9371-x
through its distribution range (Glew et al. 2004;
Gouwakinnou et al. 2009a; Muok and Owuor 2005;
Shackleton et al. 2002). The species is widely
described in literature as dioecious. However, Diallo
et al. (2006) have found the Sahel populations in
Senegal to be morphologically androdioecious, and
Hall (2002) have reported the presence of occasional
female flowers in the most proximal inflorescences of
the shoots on predominantly male trees. Diallo et al.
(2006) found that pollens of Sclerocarya birrea
‘hermaphrodites’ were viable, but they did not carry
out any selfing experience. Androdioecy is a rare
sexual system in plants and animals in which males co-
occur with cosexuals (Charlesworth 1984; Pannell
2002). Although androdiocecy has been reported in
certain plants, functional analysis has revealed that
nearly all those species reported to be androdioecious
are in fact dioecious (Charlesworth 1984; Anderson
and Symon 1989) with hermaphrodites that functioned
as females. Whether strictly dioecious or androdioe-
cious, there has been a widely reported sex-specific
cost of reproduction in plants. This cost of reproduction
is reflected in females (hermaphrodites) being less
likely to survive in stressful habitats resulting in spatial
segregation of the sexes (Bierzychudek and Eckhart
1988; Dawson and Ehleringer 1993) or females co-
occurring with males but defraying the costs of
reproduction by delaying reproductive maturity or by
reducing the photosynthetic activity and lifetime
growth. From all these above mentioned studies and
investigations, it is obviously clear that the type of
reproduction and mating system is one of the factors in
shaping the dynamics of a given plant species at
individual or population level. Thus, the natural
balance of relative proportion of male and female
individuals is crucial. Sex ratio is known to affect both
the growth rates and the evolutionary trajectories of
wild populations (Sapir et al. 2008) given that it affects
the probability of a female to mate successfully. While
genetic factors and environmental conditions are often
referred to as the primary proximate determinants of
individual sex and population-level sex ratios, anthro-
pogenic impacts are also likely to shape the population
sex ratio.
Dioecious species are more represented among
trees and mainly occur in the tropics (Renner and
Ricklefs 1995), but there are very few attempts to
assess the proportion of males and females individ-
uals in their populations in Africa. Moreover, factors
such as the distance between male and female
regulate the successful reproduction in plants in
general and in dioecious plants in particular (Gibson
and Menges1994; Percy and Cronk 1997), but data
on sex ratio and spatial distribution of sex in
dioecious species are scanty for African dry land
tree species although they are of crucial interest as
far as the population dynamics, the evolution and
the biological conservation of these species are
concerned.
The overall aim of this article is to analyze the
population dynamics in relation to sex of S. birrea.
First, we aim to assess the level of local people
perceptions on dioeciousness of Sclerocarya birrea
subsp. birrea (hereafter S. birrea) and to assess the
criteria used to make the distinction in plant sexes.
This is to understand if people’s perception of
dioeciousness influences human impact on the
dynamics of the species based on the hypothesis that
farmers take into account the sex of the individual
tree during removal activities such as burning,
falling, and ring-barking. Second, we aim to assess
any sex ratio bias within populations of S. birrea and
the relative spatial structure of males and females to
detect any possible spatial segregation of sexes.
Materials and methods
Study species
Sclerocarya birrea (Anacardiaceae) is a fast growing
tree. Three subspecies of Sclerocarya birrea are
known. The subspecies caffra occurs mainly in the
southern part of Africa and is known as marula. The
subspecies multifoliolata is restricted to Tanzania and
possibly the neighboring part of Kenya and the
subspecies birrea is present in Western and Central
Africa (Nghitoolwa et al. 2003). Flowering takes
place in the dry season when trees are leafless. The
major pollinators (or flower visitors) of Sclerocarya
birrea are honey bees. Secondary pollinators include
flies and wasps (Chirwa and Akinnifesi 2008).
Sclerocarya birrea bears plum-sized stone fruits with
a thick yellow peel and translucent white flesh. Many
are eaten fresh, but most are processed into products
such as beverages, jams, and jellies. Regardless of
taste (sweet-and-sour or tart), the juice is reported to
be nutritionally important containing as much as four
26 Agroforest Syst (2011) 82:25–35
123
times the vitamin C of orange juice (National
Research Council 2008). The kernels are eaten as
snack or the oil extracted; the leaves are browsed by
livestock and have medicinal uses, as does the bark.
The wood is carved into utilitarian items such as
mortars, agricultural tools, spoons, and plates as well
as decorative animal figures (Glew et al. 2004;
Gouwakinnou et al. 2009a; Shackleton et al. 2002).
Data collection
A survey based on structured interviews was con-
ducted in two districts in northern Benin. The study
involved 29 informants in Karimama district and 31
informants in Tanguieta district (Fig. 1) These two
districts are the main distribution range of S. birrea in
Benin (Adomou et al. 2006) and present different
climatic and soil conditions (Table 1). The main
socio-ethnic groups involved were Gourmantche
around W National Park (Karimama) and Gourmant-
che and Waama around Pendjari National Park
(Tanguieta). Interviews focused on the awareness of
sex separation and differentiation within the species.
When differentiation was made, the criterion used to
distinguish male and female individuals, if any, was
recorded. It was also noted whether these criteria are
taken into account during land clearing (felling, ring-
barking or burning). Informants’ reported age ranged
from 23 to 105 years. However, priority was given to
older respondents (60% of informants were over
40 years old) as we assumed them to be the most
knowledgeable about the issue (Gilchrist et al. 2005).
To assess the sex ratio, we established three
transects of 2–3 km length along which all the adult
individuals of S. birrea were recorded. Two transects
were laid in Karimama District (KD), one in the
protected area, W National Park, and one in agrofor-
estry systems whereas only one was laid in Tanguieta
District (TD) in agroforestry systems as S. birrea
individuals were scant and scattered in the Pendjari
National Park. The field survey was undertaken from
late February to early May, corresponding to the
Fig. 1 The study area (Karimama and Tanguieta districts and the tree sampling points in agroforestry systems and in protected area
(W National Park)
Table 1 Characteristics of the study sites
District Karimama Tanguie
´
ta
Location 2°17–3°17 E and
11°24–12°25 N
1°3–1°58 E and
10°26–11°29 N
Average rainfall (mm) 650 1,000
Mean temperature (°C) 30 27
Type of climate Sudano-sahelian Sudanian
Agroforest Syst (2011) 82:25–35 27
123
reproductive season of the species. Thus, recognition
of individual trees as female, male, or uncertain sex
was made possible by the presence of female or male
flower on trees because flowers were dimorphic
(Fig. 2a, b), Some rare adult individuals that did not
flower during the reproductive season was identified
as female with the presence of old kernels beneath the
tree or considered as unidentified. In each population,
we recorded the sex of each individual tree and the
criteria mentioned by local people to recognize the
sex.
Plots of 1–2.25 ha were laid out on each site
(allowing the mapping of an entire sub-population) in
which the relative position of each reproductive
individual was mapped using a GPS receiver.
Data analysis
Interview data analysis
The level of awareness of sex separation was
calculated as the percentage of respondents giving
approving answers out of total respondents. A G-test
was performed to check for an association of age
category of respondents and level of perception and
to test for matching of folk perception and scientific
basis of identification of trees sex.
Sex ratio data analysis
Sex ratio was expressed in each population as the
proportion of male in a sample, i.e. ratio of males/
(males ? females) because the expression of sex
ratio sensu-stricto can lead to errors in interpretation
(Wilson and Hardy 2002). Deviations of sex ratios
from 0.5 were tested using Fisher’s exact test of
goodness-of-fit instead of G test-of-goodness of fit as
our sample size was relatively small. The test was
made using a ‘weight’ parameter in PROC FREQ
with SAS (SAS 2004).
Spatial distribution analysis
Spatial distribution of male and female at population
level was assessed using the pair correlation function
g(r) which is a non-accumulative version of Ripley’s
K-function (Stoyan and Stoyan 1994; Wiegand and
Moloney 2004). The bivariate g
12
(r) is the normal-
ized density of neighboring male trees (=pattern 2) as
a function of distance r from an average female trees
(=pattern 1) (Wiegand and Moloney 2004). To
determine statistical significance of the observed
g
(r)
, 1% simulation envelopes of a random labeling
hypothesis null model (as opposed to independence
hypothesis) were generated by 999 replicates Monte
Carlo simulations of the null model (Goreaud and
Fig. 2 Flowers of S. birrea, a normal female flower, remark only one flower per peduncle, b male flower, remark many flower on the
peduncle (raceme), c flower from a tree bearing both pistil and stamen, remark flower in raceme
28 Agroforest Syst (2011) 82:25–35
123
Pelissier 2003; Wiegand and Moloney 2004). If g
(r)
for a given scale r, was outside the simulation
envelopes, the null hypothesis was rejected at this
scale. The g
12
(r) was used to assess whether females
individuals are positively correlated with males. We
also examined probable spatial segregation of sex by
assessing the pairwise difference g
12
(r) - g
11
(r) \ 0
which indicates whether female individuals are
positively correlated with other females and
g
21
(r) - g
22
(r) \ 0 which indicates whether male
individuals are positively correlated with others
males.
Programita (Wiegand and Moloney 2004) was
used to perform spatial analyses. A grid size of 1 m
2
and a ring width of 2 m were used for all analyses. To
account for low density in agroforestry systems from
Karimama district, the two plots laid out were
combined into one overall, mean weighted pair-
correlation function (Diggle 2003; Riginos et al.
2005).
Results
Local perception of sexual dimorphism in S.
birrea population
About 55% of respondents were aware of sex
separation within S. birrea, even though this is more
notable in KD (67%) than in TD (41%). A significant
difference in awareness was found among age
category (v
2
= 7.7; DF = 1; P = 0.006). Among
informants below 40 years old, only 28% were aware
of sex separation while the percentage was 67% for
informants over 40 years. Within informants who
were aware of sex separation, 50% reported to be able
to make distinction between sexes but others were not
(Fig. 3). The presence of holes on the bark was the
most distinctive criteria reported in both districts to
be used outside of reproductive period (Fig. 4).
Further analysis on whether there was an association
of the presence of holes on the bark and the sex of the
individual tree as suggested by folk perception
showed no significant relation (G = 1.516; DF = 1;
P = 0.218) suggesting that this perception was not
consistent.
Sex ratio
The detail of the number of trees surveyed per sex
and per land use type is presented in Table 2. The
minimum flowering diameter at breast height
(130 cm above ground level) recorded in the sample
was 8.7 cm. Although the number of trees was either
female or male biased in some subpopulations, there
was no statistical evidence that the sex ratio observed
globally differed from 0.5 per district or per land use
type. Apart from female and normal male individuals,
Fig. 3 Criteria reported by some of local people to distinguish
male and female trees of S. birrea in Karimama and Tanguie
´
ta
district
Fig. 4 Photographs of
barks of S. birrea.
a individual with small
holes indicating male
individual according to the
perception of some local
people. b individual without
holes indicating a female
individual
Agroforest Syst (2011) 82:25–35 29
123
we recorded few male individuals (4 out of 301) with
hermaphroditic flowers, and they could then bear
some scant fruits (Fig. 2c).
Spatial distribution
The intertype analysis among male and female
individuals showed that the g
12
(r) function did not
overlap the simulation envelops suggesting no spatial
association between male and female individuals
(Fig. 5). Thus, the position of male individuals
relative to the position of female individuals suggests
a random repartition in the stands of S. birrea.
Moreover, the pairwise difference analysis
revealed that they were not significantly different
from zero except in TD where the pairwise difference
g
21
(r) - g
22
(r) was slightly above the confidence
interval at the scale 7–8 m. These results demon-
strated that there was no strict spatial segregation of
sex at population level in S. birrea individuals in both
districts.
Discussion
In this study, we investigated how local people
perceive sexual differentiation in S. birrea, the sex
ratio among the species populations, and the relative
spatial distribution of male and female individuals.
Table 2 Details of tree sampling and statistical analysis of sex
ratio
Sites Male Female Sex ratio Fisher’s exact test
95% CI P
KD 152 149 0.50 0.447–0.563 0.908
P 86 109 0.44 0.370–0.513 0.115
TD 89 91 0.49 0.419–0.569 0.941
KD Agroforestry systems in Karimama district, P Protected
area, TD Agroforestry systems in Tanguieta district
Fig. 5 Spatial distribution of males and females of S. birrea.
In the notation, the letters TD, KD, and P represent,
respectively Tanguieta district, Karimama district, and the
protected area. The numbers 1, 2, and 3 represent g
12
, g
12
-
g
11
and g
21
- g
22
functions, respectively. g(r) values are
represented in solid lines (); the 999 simulations confidence
envelopes are represented in dashed lines (----)
30 Agroforest Syst (2011) 82:25–35
123
Our findings suggested that local people’s perception
do not correlate with scientific definitions. The sex
ratio in the population of the species did not
significantly deviate from 0.5, and there was not
any evidence of spatial segregation of sexes.
Perception of S. birrea dioecism by local people
According to one group of farmers, it is not easy to
differentiate between male and female trees. Follow-
ing other perceptions, trees bearing small holes on
their bark (not having a smooth bark) are male
individuals. However, statistical analysis revealed
that this structure was not specifically related to the
sex of the species and highlights an inconsistency in
the perception of the second group of farmers. Some
skepticism about local ecological knowledge has
sometimes been raised in formal scientific commu-
nities (Pierotti and Wildcat 2000) and our results
support this point. Nowadays, the fruits of the species
are reported to be only little exploited by adults who
were involved in this study in both districts (Gouwa-
kinnou, personal communication). If the fruits were
matter of high exploitation, then this would have been
a factor of frequent presence of people around female
trees for fruit collection and would certainly contrib-
ute to dispelling of the misuse of back criteria.
The importance of local communities’ knowledge
in many other aspects of sustainable forestry is
nowadays acknowledged, and the general trend is to
integrate this kind of knowledge in the formal forest
and natural resources management actions (Berkes
et al. 2000, Gaoue
´
and Ticktin 2009). The foundation
is that people have managed forest for decades by a
way that seems sustainable, and this knowledge has
been transmitted from generation to generation
(Gadjil et al. 1993). Although their usefulness is
acknowledged, this study suggests, in accordance
with Huntington et al. (2004), that there is a need to
carefully compare specific observations from local
communities with those from formal science when-
ever possible because local perceptions could also
lead to unsound forest management on some aspects
(Gilchrist et al. 2005).
Sex ratio
Our results showed that the global sex ratio did not
deviate significantly from 0.5, although there was
evidence of male or female bias in some subpopu-
lations mainly in TD. Many other studies involving
tropical or neo-tropical dioecious species have
reached similar conclusions (Morellato 2004). The
sex ratio of 0.5 found in S. birrea population was
theoretically shown by Fisher (1930) to be a stable
strategy. in a population of diploid organisms, where
each individual has exactly one father and one
mother. However, other authors have reported a
male-biased sex ratio significantly different from 0.5
in other dioecious plants (Queenborough et al. 2007;
Thomas and LaFrankie 1993; Yamashita and Abe
2002) and have concluded that this trend is common
within dioecious species. This assumption is sup-
ported on one hand by the fact that male investment
ends at flowering while females continue investment
in fruit and seed production, which may weaken the
female plants and inhibit future growth and repro-
duction. On the other hand, males having more
resources for vegetative growth gain a competitive
advantage which may lead to male-biased sex ratio
(Korpelainen 1994; review in Obeso et al. 1998).
Female-biased adult sex-ratio is rare in the liter-
ature even though classic sex allocation theory
predicts a female-biased seed sex ratio with sib-
mating (Klinkhamer and de Jong 2002). However,
some cases on this aspect have been reported
(Melamphy and Howe 1977; Morellato 2004; Opel
and Bawa 1978). Among the mechanisms that can
lead to female-biased sex ratio are sexually differen-
tial mortality, agamospermy, and vegetative repro-
duction. Selective tree removal (Maranz and
Wiesman
2003) which is one of the most important
causes of changes in agroforestry systems of African
drylands is likely to influence adult sex ratio and has
been reported to induce female-biased sex ratio in
dioecious species (Nghitoolwa et al. 2003; Verdu
´
and
Garcı
´
a-Fayos 1998). The more useful is a given tree
species, the higher is the selection within individuals
of its population. This is the case of shea butter tree
(Vitellaria paradoxa) (Djossa et al. 2008; Maranz and
Wiesman 2003) although the issue of sex does not
arise with this species. This type of selection is likely
one of the main causes of biased sex ratio that we
found in sub-populations of the species in agrofor-
estry systems.
The species S. birrea is known to be dioecious,
and the most-studied subspecies (S. birrea subsp.
caffra) in southern and central Africa has largely
Agroforest Syst (2011) 82:25–35 31
123
been reported in the literature as dioecious. However,
the female flowers also bear staminodes which have
been proved to have viable pollen and suggest that
the species is morphologically androdioecious (Diallo
et al. 2006). Yet, no selfing experience was carried
out to verify whether these supposed hermaphrodites
flowers were self-compatible, or pollinated other
plants in nature. Such an androdiocism found in
Fraxinus ornus (Domme
´
e et al. 1999) was contro-
versial although it was self-compatible. Indeed, the
observed 0.5 sex ratios did not fit well with the
theoretical expectation of hermaphrodite-biased sex
ratios of the androdioecy models and suggest func-
tional dioecy (Charlesworth 1984; Pannell 2002;
Verdu
´
2004). Similarly, the sex ratio that we found
was proved to be statistically not different from 0.5
and suggests that S. birrea is functionally dioecious.
We found that some male individuals in agrofor-
estry systems in KD (4/301) produced functionally
hermaphroditic flowers (different from those of
females) and few fruits (Fig. 2c). This trend was
previously documented in the S. birrea subsp. caffra
(Hall 2002; Nghitoolwa et al. 2003) where those
flowers were assimilated to female flowers. However,
keen observation revealed that those flowers were
hermaphroditic. These results suggest that the mating
system of this important fruit tree species remain less
understood. Further controlled pollination experience
coupled with a paternity analysis using molecular
marker will be necessary to elucidate the mating
system of the species particularly S. birrea subsp.
birrea.
Spatial distribution
Two main results were drawn from the spatial
analysis. First, no attraction or repulsion among
males and females individuals within population of
the species was found in any of the land use types
suggesting no spatial association between sexes. As
the proximity of mates is known to influence mating
opportunities and the quantity and quality of off-
spring, especially in dioecious plant species (Stehlik
et al. 2008), this kind of random distribution of male
and female individuals found would depict a con-
straint in pollen flow among mates. However, with
regard to the species’ reported major pollinators
which include honey bees, flies, and wasps (Chirwa
and Akinnifesi 2008), the presence of male and
female individuals within short distance in natural
stands suggests that the pollen flow between individ-
ual is not a constraint as far as reproductive success is
concerned. This constraint, however, could arise in
farmland where the species is represented in a low
density (Gouwakinnou et al. 2009b) with relatively
increased distance among individuals.
Second, the analysis suggested an absence of
spatial segregation of sexes in the populations of
S. birrea. Such a spatial segregation of sex would
occur in dioecious plants under one of the following
given conditions. First, sex differential mortality in
different environment patches due to difference in
reproduction biology; second, sex choice (i.e., ability
to vary the sex according to their physiological
conditions or environment); and finally, an active
habitat selection through male and female propagules
with different dispersal property or direct vegetative
growth, or male and female seeds having different
germination requirements (Bierzychudek and Eckhart
1988). In the case of S. birrea subsp. birrea, our
results suggest that none of the above mentioned
feature is involved in the dynamic process which
shapes the spatial distribution.
Implications for population dynamics of S. birrea
The description of a spatial and temporal pattern of a
community is usually not enough, but it is rather the
beginning of a process that gives insight into natural
system complexity, and which, in turn, generates new
ecological hypotheses that need to be tested either by
experiments or by modeling (Fortin and Dale 2005
).
The spatial pattern is a result of the past temporal and
spatial dynamics of the stand and can be used to infer
some information on this dynamics.
The absence of segregation according to sex in
S. birrea populations coupled with the balanced sex
ratio suggests that some environmental factors, such
as soil conditions or microhabitat partitioning, sea-
sonal and annual wild fires do not have specific sex-
related mortality on this plant species. Moreover, in
open farmlands where human intervention would
interact with the intrinsic dynamics of the species, our
results revealed that there is no evidence of selective
logging which might lead to a biased sex ratio or a
pattern including only individuals of the same sex in
natural populations. This balance is important in
32 Agroforest Syst (2011) 82:25–35
123
genetic and conservation context because unbalanced
sex ratios operate to reduce effective population sizes
(Ackerly et al. 1990).
In dioecious plant, females are known to invest
more in reproduction than males, because in addition
of flower production, they produce seeds, fruits, and
associated structures (Dawson and Ehleringer 1993;
Wheelwright and Logan 2004). A spatial segregation
of sexes with females preferring the richest and less
stressful habitat would be expected for an optimal
production. This would also favor compensation as
far as soil’s chemical nutrients invested in fruit
production are concerned. Although this absence of
spatial segregation of sexes in our study can suggest
that females do not display specific environmental
conditions requirement for performing reproduction
function, this investment can be prejudicial to their
other physiological functions such as photosynthesis
as demonstrated by Wheelwright and Logan (2004)
leading to a reduced growth rate for females.
Sclerocarya birrea is a multipurpose use species in
which the wood, fruits, and bark represent the main
used parts of the species (Gouwakinnou et al. 2009a;
Shackleton et al. 2002). Currently, local people do
not take sex into account for wood harvest for carving
activities, and this finding is consistent with the sex
ratio found in agroforestry systems. The findings of
this study suggest that wood and overall plant part
harvest should be more orientated toward male
individuals to reduce threats on females, and hence,
allow their optimal fruit production. However,
pressure in male individuals should be to such an
extent that it does not negatively impact on the
pollination activities. Further studies on the required
proportion of male in a given population would be
necessary to guaranty an optimal and sustainable use
of S. birrea and consequently other dioecious species
of similar biology in agroforestry systems. Our results
also showed that there is not yet any reliable apparent
criterion used by local people for sex distinction in
the species out of the reproduction period. Further
study involving the other criteria such as the form of
the leaves, as reported by some of respondents are to
be considered. The importance of a reliable criterion
is that if a specific management policy target a
specific sex out of reproduction period, confusion
could occur with the current perception, leading to
mismanagement.
Acknowledgments This study was supported by IFS
(International Foundation for Science) through a grant to
GNG (No: D/4794-1) and by European Union (FP6 INCO-dev
031685) through SUN Project (Tools for Management and
Sustainable Use of Natural Vegetation in West Africa). The
authors acknowledge the support of farmers from Tanguie
´
ta
and Karimama districts.
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... Furthermore, reports in subspecies birrea in Kenya [14] indicated a few female flowers having some pollen-bearing anthers but self-pollination studies were not conducted to confirm whether these suspected hermaphrodites' flowers were self-compatible or not. In addition, Gouwakinnou et al. [15] also reported some individuals of male trees in an agroforestry system producing functionally hermaphroditic flowers with few fruits yet the flowers were different from those of female trees. The development of female flowers by male trees was previously documented in S. birrea subsp. ...
... The presence of monoecy, occurrence of bisexual flowers in males, and sound pollen-bearing anthers in females of S. birrea [12,14] suggest the possibility of self-pollination within the species. Consequently, Gouwakinnou et al. [15] recommended the need for further investigation on the breeding system of S. birrea specifically in the area of controlled pollination experiment combined with a parenthood analysis in order to ascertain the mating system of the species. ...
... The development of fruits and viable seed through selfing treatment is an indication of hermaphrodite flowers with functional male and female organs on the same flower [15]. Even though S. birrea is generally described as dioecious tree species reports have indicated the existence of isolated cases of monoecy within the species [11,33]. ...
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Mating system of a species is critically important both genetically and ecologically in developing plans for breeding and gene conservation. This study was conducted to assess twenty provenances of Sclerocarya birrea (A. Rich.) Hochst. planted in Malawi. The trial was assessed for mating system and sex ratio at eighteen years of age. The results revealed that the mating system in S. birrea occurred from selfing, insect, and wind-mediated pollination. There were no significant ( P >0.05) differences on seed germination percentage among the three mating systems. The germination percentages were 47%, 44%, and 43% for insect, wind, and self-pollinations, respectively. This implies that the seeds were viable in all the three mating systems. Production of viable seed from selfed flowers ruled out the possibility of apomixes in S. birrea . Most frequent flower visitors were the orders Hymenoptera and Lepidoptera with peak visitation period being from 7:00 to 11:30 hours in the morning and then 15:30 to 18:00 hours in the afternoon (+2 GMT) when temperatures were cooler. There were significant ( P <0.05) variations in sex ratio among the provenances. Five provenances (Marracuene, Magamba, Tanzania pooled, Ngundu, and Matebeleland South) did not deviate significantly from sex ratio equality. The other provenances showed male-biased sex ratios.
... In particular, at the level of a given plant species population, spatial structure informs on interactions between ontogenetic stages (seedlings, saplings, juveniles and adults) and their environments and could be used to infer demographic pattern ). This spatial structure may however be altered by biotic and/or abiotic stresses of which land use change (Djossa et al. 2008;Gouwakinnou et al. 2011;Somanathan and Borges 2000), leading to either population decline, growth or stable demography (Schmidt et al. 2011). ...
... Land use change is recognized as one of the most important drivers of changes in ecosystems' structure and particularly structure of plant populations (Martínez-Ballesté and Mandujano 2013; Schumann et al. 2011;Venter and Witkowski 2013). Therefore understanding how human disturbances affect the spatial structure of plant population has become a crucial applied ecology and conservation issues (Djossa et al. 2008;Gouwakinnou et al. 2011;Idohou et al. 2016;Kelly et al. 2004a). However, studies of effects of land use change on the spatial structure of trees have often been limited into whether structure is random, aggregative, or regular for univariate spatial pattern (SP) or whether there exists an association, independency or repulsion between different ontogenetic stages for bivariate SP (Abdourhamane et al. 2017;Djossa et al. 2008;Gouwakinnou et al. 2011;Idohou et al. 2016). ...
... Therefore understanding how human disturbances affect the spatial structure of plant population has become a crucial applied ecology and conservation issues (Djossa et al. 2008;Gouwakinnou et al. 2011;Idohou et al. 2016;Kelly et al. 2004a). However, studies of effects of land use change on the spatial structure of trees have often been limited into whether structure is random, aggregative, or regular for univariate spatial pattern (SP) or whether there exists an association, independency or repulsion between different ontogenetic stages for bivariate SP (Abdourhamane et al. 2017;Djossa et al. 2008;Gouwakinnou et al. 2011;Idohou et al. 2016). While such information has important ecological meaning, they explicitly lack providing key information such as neighborhood distance. ...
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Spatial pattern (SP) and neighbourhood distance (ND) of trees are crucial for pollination services, in particular for dioecious species. However, land use types through human disturbances may affect the natural SP and ND and possibly have a negative effect on pollination services. Though several studies have focused on the effect of land use types on SP of trees, few have reported on ND. In this study, we compared SP and ND of the dioecious Borassus aethiopum Mart. between two land use types (protected areas vs farmlands) in two contrasting climatic regions (semi-arid vs sub-humid) in Benin. Trees were mapped in twelve plots from six populations. Pair-correlation function was used to generate univariate and bivariate SP and ND. Next, ANOVA was used to compare ND. While supporting the overall trend towards a less aggregated pattern along plant life-cycle, the study showed that the SP of B. aethiopum was altered from aggregated and spatial association in protected areas toward random and independence patterns in farmlands with increased ND among individuals, particularly between adult males and females. In addition, differences in ND between land use types varied across climatic regions, the differences being higher in the drier semi-arid region, thus suggesting more intense human activities in this region and climatic region-specific management. Management actions should mainly aim at reducing or not further increasing ND, particularly between female and male adult populations in farmlands in the semi-arid region through planting new individuals of B. aethiopum trees or limiting their removal but at the same time account for other tree species to maximise diversity of farmlands’ functions and services. Further studies should examine whether the observed increased ND due to human-disturbances in farmlands is detrimental for the species pollination services, fruit production and whether it affects the species spatio-temporal population genetic structuring.
... Dans le système agroforestier, les arbres sont délibérément sauvegardés par les agriculteurs et épargnées par des feux de végétation saisonniers tandis que dans la zone protégée, les individus de S. birrea sont situés en savane où ils sont soumis aux feux précoces et/ou tardifs de végétation. Des études antérieures ont montré un sex-ratio équilibré dans les deux formes d'utilisation des terres avec, cependant, une distribution plus agrégée dans la zone protégée où la densité des individus adultes était près de 9 fois plus élevée que celle des systèmes agroforestiers (Gouwakinnou et al., 2009(Gouwakinnou et al., , 2011b. La zone d'étude est située dans la région semi-aride avec une période sèche de sept à huit mois et une pluviométrie annuelle moyenne de 730 mm et une température annuelle moyenne de 31 ° C (Hijmans et al., 2005). ...
... Le très faible succès ou l'échec de fécondation des fleurs ensachées sans apport de pollen (HS) dans les deux milieux suggère que les organes mâles portés par les fleurs femelles de S. birrea (Diallo et al., 2006;Gouwakinnou et al., 2011b;Munjuga, 2000), ne sont pas autant fonctionnels que les fleurs mâles. Ceci suppose donc que les fleurs femelles (désignées comme hermaphrodites dans cette étude) sont en majorité fonctionnellement femelles. ...
... Cependant, le système de repro-duction de l'espèce semble plus complexe qu'il n'apparaît. En effet, des observations antérieures dans les populations naturelles de l'espèce ont révélé l'existence des individus mâles portant les fleurs en inflorescence (traits caractéristiques des fleurs males) mais portant de pistils qui sont capables de produire de fruits donc des fleurs parfaitement hermaphrodites (Gouwakinnou et al., 2011b). Ce phénomène de dioécie avec fuite ou dioécie imparfaite ( désigné par « leaky dioecy » par les anglo-saxons) a été observé sur d'autres espèces de plantes (Baker & Cox, 1984;Humeau et al., 1999;Venkatasamy et al., 2007). ...
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Mode de reproduction et phénologie florale d'une espèce dioïque, Sclerocarya birrea (Anacardiaceae), en relation avec l'utilisation des terres Abstract: Understanding reproductive biology of plant species is fundamental to plant breeding and improvement program. Knowing plant population ecology is also an essential prerequisite for sound conservation design. In this study, we investigated the reproductive biology of Sclerocarya birrea (Anacardiaceae), a socioeconomically important tree species in two contrasting land use systems in Northern Benin (sudani-an zone). The species has been previously described as dioecious. Yet, female flowers present some males organs suggesting an androdiocecy in its natural populations. However, it is not clear whether pollen from the male organ on female flowers is able of fertilizing. We performed controlled pollination on 45 female trees (30 in agroforestry systems and 15 in the protected area) and monitored flowering phenology as well as main flowers visitors. Considering female flowers as hermaphrodites, we performed three kinds of treatments: Male-hermaphrodite cross (MH); Hermaphrodite selfing (HS) and open pollination (NT). Hand pollination (MH) and pollen enclosure (HS) experiments, analysed using a GLM, revealed a pollination failure in most of the bagged female inflorescences without pollen supply (HS) (2 successes out of 45) showing that female flowers are mostly functionally dioecious. Male trees flowered at least one week earlier than females. Insects belonging to Hymenoptera (Apidae, Vespidae) and Diptera (Muscidae, Syrphidae) orders were the main flower visitors. The analysis of the results from the controlled pollination suggests a cryptic androdioecy in the populations of S. birrea. The implications of this breeding system for the domestication of the species have been discussed. Résumé : La compréhension de la biologie reproductive des espèces végétales est fondamentale pour les programmes d'amélioration des plantes. La connaissance de l'écologie des populations végétales est aussi une condition indispensable pour la conception de bons programmes de conservation. Nous avons étudié la biologie de reproduction de Sclerocarya birrea (Anacardiaceae), une espèce d'importance socio-économique dans deux différentes formes d'utilisation de terres (systèmes agroforestiers vs aires protégées) au nord du Bénin. L'espèce a été décrite auparavant comme dioïque. Pourtant, les fleurs femelles présentent des organes mâles suggérant une androdioecie dans les populations naturelles de l'espèce. Cependant, il n'est pas clair si le pollen des fleurs femelles est capable de féconder. Nous avons effectué la pollinisation contrôlée sur 45 arbres femelles (30 dans les systèmes agroforestiers et 15 dans la zone protégée) et surveillé la phénologie de la floraison, mais aussi les principaux visiteurs de fleurs. Considérant les fleurs femelles comme hermaphrodites, nous avons réalisé trois types de traitements : mâle-hermaphrodite par pollinisation manuelle (MH) ; autofécondation des hermaphrodite (HS) et la pollinisation libre (NT) comme témoin. Les données de pollinisation manuelle (MH) et d'autofécondation (HS), analysées à l'aide d'un GLM, a révélé un défaut de pollinisation dans la plupart des inflorescences femelles ensachées sans apport de pollen (HS) (2 succès sur 45), montrant que les fleurs femelles sont pour la plupart fonctionnelle-ment dioïque. Les arbres mâles ont fleuri au moins une semaine plus tôt que les femelles et la floraison a été plus précoce dans le système agro-forestier que dans les aires protégées. Les insectes appartenant aux Ordres des hyménoptères (Apidae, Vespidae) et des diptères (Muscidae, Syrphidae) ont été les visiteurs principaux des fleurs. L'analyse des résultats de la pollinisation contrôlée suggère une androdioecie cryptique dans les populations de S. birrea. Les implications de ce système de reproduction pour la conservation et la domestication des espèces ont été discutées.
... Crop harvesting is primarily done by women and young people. A comparison of results from this study with previous studies in the same area (Fandohan et al., 2010b;Gouwakinnou et al., 2011aGouwakinnou et al., , 2011b, where elders and women were more familiar with the study species, should be done with caution. From N'zebo et al. (2018) for instance, the age and gender balance significantly affect knowledge of Tetrapleura tetraptera fruit, and women had a higher level of knowledge than the men. ...
... Crop harvesting is primarily done by women and young people. A comparison of results from this study with previous studies in the same area (Fandohan et al., 2010b;Gouwakinnou et al., 2011aGouwakinnou et al., , 2011b, where elders and women were more familiar with the study species, should be done with caution. From N'zebo et al. (2018) for instance, the age and gender balance significantly affect knowledge of Tetrapleura tetraptera fruit, and women had a higher level of knowledge than the men. ...
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In a context of fragmentation of forest landscape and sustainable use of orchid species, an analysis of the local knowledge base is necessary for developing conservation strategies of orchid habitat. This study aims at i) assessing factors affecting local perception on conservation status of orchids, and ii) determining degradation factors of their habitat in Sudanian woodland and the consequences. We used questionnaire-based survey, to cross knowledges on orchids and habitat degradation, from the four mains socio-cultural groups, including the age, the sex and the level of education, translating into 390 respondents. We used the binomial and multinomial logistic regression to assess the local perception on conservation status of orchid habitats with reference to different ethno-demographic groups. We also used the negative binomial generalized linear modelling (GLM) to test for an existing dependence relation between the local perceptions of the “conse-quences” of the degradation of gallery forest (as response variable), and the ethno-demographic variables. Ethnicity and level of education significantly influence knowledges on orchids, as well as the perception on their habitat con-servation status (p<0.05). The socio-cultural groups Fulani, Gourmantché and Waaba perceived more the degrada-tion of their gallery forests, habitat of orchid species. The three main factors of degradation from local view are tree cutting (65.64%), shifting agriculture (44.10%) and wildfires (39.49%). Whatever the considered GLMs, there is not a significant influence of the ethnodemographic variables on local perception of the consequences of the gallery forests degradation (p< 5%). In Sudanian woodland, education and ethnicity significantly influence perception on the conservation status of orchid habitat. Assess how this local perception translated into quantitative data in the field is crucial to develop adaptive strategies for long term conservation of orchid species and sustainable use of their habitats.
... plantations. In contrast, Gouwakinnou et al. [59] reported a folk perception about how some farmers use the appearance of the back of a tree to determine its sex. Unfortunately, this turned out to be a false clue likely to result in the poor management of Sclerocarya birrea (A. ...
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Background: Ecosystems provide humanity with goods and services known as ecosystem services. The value of these services represents a basis for political decision-making. To be sure that these decisions are made on a valid basis, policymakers require an understanding of the biophysical processes involved. This study was carried out around two forest reserves (Alibori-Supérieur and Ouénou-Bénou) in Northern Benin. It aimed to highlight the knowledge of the surrounding communities and their perceptions about the importance of the ecosystem services provided by these forest reserves as well as the factors that influence their knowledge and perceptions. Methods: Primary data were collected from 25 group discussions in 25 villages surrounding the forest reserves based on predefined ecosystems services of the Millennium Ecosystems Assessment (MA). Multiple linear regression models were used to examine how socio-economic characteristics of the communities influenced the ecosystem services identification rate. Perceptions of importance, levels of satisfaction, and trends of services provided were analyzed using descriptive statistics. Results: Our results showed that education level, poverty index, household size, and proximity to forests played an important role in the variation in knowledge of ecosystem services (P < 0.05). Provisioning services (such as crops supply, fuelwood, lumber, wild food, and medicinal plants) were mostly identified by the poorest villages located very close to the forests (P < 0.05). The importance of the provided services for well-being has been unanimously recognized. The most recognized cultural services were education and knowledge facilitation (84%) and spiritual value (76%). Climate regulation (84%) and pollination (84%) were the best-known regulating services. However, supporting services (soil formation and pest regulation) that are important for improving production systems were unknown to the communities. Conclusion: Education level, poverty index, and village proximity to the forest were important predictors of regulating and supporting services identification. But use of non-tangible services by local rural communities will require more emphasis on targeted environmental education specifically designed according to the needs of each group.
... Recent developments in spatial pattern modelling have illustrated their relevance for explaining biological or environmental factors underlying a given point process (see Baddeley and Turner, 2005). In this respect, spatial patterns analysis has been used to test for association or repulsion between life stages as an indicator of anthropogenic disturbances (Djossa et al., 2008), and to check for gender-specific spatial patterning of trees in dioecious species, that could have resulted from gender-specific environment requirements (Gouwakinnou et al., 2011). Understanding natural spatial patterning of individual timber or non-timber forest product tree species and how this is influenced by environmental conditions is important not only to account for this effect when designing forest/agroforest enrichment plans but also for decision making towards intervention or specific conservation actions/strategies (i.e., ex situ, in situ). ...
Article
Full-text available
Domesticating indigenous agroforestry species is gaining interest as a potential option for conservation and production. Yet, spatial patterning of key species and how it is altered by environmental variations, which are important to design plantation schemes in forest and agroforestry systems, are still poorly documented. The pair-correlation function was used to assess spatial pattern of Tamarindus indica and its variation under contrasting environmental conditions (vegetation cover and soil degradation). Tamarind seeds being dispersed by zoochory and barochory, we hypothesized positive association within and among life stages (adults-Adults, juveniles-juveniles, and adultsjuveniles). Variations in environmental conditions did not significantly affect density and overall spatial pattern of either adult or juvenile trees. Adults and juveniles showed clumped patterns irrespective of environmental conditions. However, juveniles showed positive association with adults under low canopy cover and/or soil degradation, and independence from adults under dense canopy. This could be due to the shade intolerant status of this species and allelopathic effect of adults on juveniles under dense canopy. On the contrary, soil degradation favored attraction between adults and juveniles, presumably by inducing coppicing. Tamarind trees proved to withstand land degradation and could be used to restore degraded areas. To this end, we suggest introducing juveniles in patches of 40 m radius using a 10 m x 10 m planting grid, and at least 30 m from mature trees.
... Field observations of hermaphrodite flowers and fruit on rare male plants, corroborated the allegation of some farmers that male plants produced rare fruits in some seasons. Gouwakinnou et al. (2011) also noted that male plants of hermaphrodite flowers bore fruits. This study has specifically shown that there exist hermaphrodite plants that bear a few male flowers. ...
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Sclerocarya birrea is a major component of agroforestry parks of Sudano Sahelian zones, which is exploited by rural populations in Burkina Faso. Recently, Sclerocarya birrea kernel marketing has increasingly developed in Ouagadougou (capital of Burkina Faso). This study aimed at evaluating the productivity, understanding the socio-cultural uses and socio-economic importance of S. birrea among local populations of Burkina Faso. The study has shown that S. birrea is a very fructiferous plant. The number of fruits yielded per tree ranges from 136 to 4256, with mean fruit weight and diameter that are respectively 12.66 and 26.71 mm. The number of kernels per fruit ranged from 1 to 3, with a mean diameter of 4.75 mm. All is not clearly separated to multi-purpose uses. All organs are used in 36 different ways. The pulp and kernels of the fruit are the only by-products marketed by women and mainly girls who dropped out of school.
... valere.sala-ko@labef-uac.org production, especially for dioecious species (Berry, 2006;Gouwakinnou et al., 2011). In dioecious species particularly, factors such as sex ratio and the distance between males and females determine the probability of successful pollination (Berry, 2006). ...
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... valere.sala-ko@labef-uac.org production, especially for dioecious species (Berry, 2006;Gouwakinnou et al., 2011). In dioecious species particularly, factors such as sex ratio and the distance between males and females determine the probability of successful pollination (Berry, 2006). ...
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