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Estimation of cultivable areas for Irvingia gabonensis and I. wombolu (Irvingiaceae) in Dahomey-Gap (West Africa)

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Cultivation of priority plant species ensures their sustainable management. African bush mango trees (Irvingia gabonensis and I. wombolu) are the most exploited Irvingiaceae species. Experts disagree on the status of these very similar taxa, as taste remains the only character by which they can be distinguished in the field. We combined occurrences and environment data in ecological niche models to assess suitable areas for the two species. Irvingia gabonensis presented a wider occurrence area due to cultivation across contrasting ecological areas. Irvingia wombolu does not appear to be cultivated and only occurred in southwestern Togo. These differences in range is likely determined by phenological limitations of I. wombolu, reinforced by differences in local management systems, thus confirming the failure of market development to impact useful plant species’ conservation significantly. Highly suitable areas for I. wombolu were in the Volta Forest, where I. gabonensis saw low suitability, while out of this inverse situation was observed, as regard environmental suitability. These differences are significant, implying different ecological adaptation. However, anthropogenic influences, related to domestication history, are also important. Therefore, updated genetic investigations and field trials in contrasting ecological areas are required for understanding the origin of differences between these two forms.
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Estimation of cultivable areas for Irvingia gabonensis and I.
wombolu (Irvingiaceae) in Dahomey-Gap (West Africa)
Romaric Vihotogbe
´.Rodrigue Idohou .Jens Gebauer .Brice Sinsin .
A. Townsend Peterson
Received: 23 January 2017 / Accepted: 8 January 2018
ÓSpringer Science+Business Media B.V., part of Springer Nature 2018
Abstract Cultivation of priority plant species
ensures their sustainable management. African bush
mango trees (Irvingia gabonensis and I. wombolu) are
the most exploited Irvingiaceae species. Experts
disagree on the status of these very similar taxa, as
taste remains the only character by which they can be
distinguished in the field. We combined occurrences
and environment data in ecological niche models to
assess suitable areas for the two species. Irvingia
gabonensis presented a wider occurrence area due to
cultivation across contrasting ecological areas. Irvin-
gia wombolu does not appear to be cultivated and only
occurred in southwestern Togo. These differences in
range is likely determined by phenological limitations
of I. wombolu, reinforced by differences in local
management systems, thus confirming the failure of
market development to impact useful plant species’
conservation significantly. Highly suitable areas for I.
wombolu were in the Volta Forest, where I. gabonensis
saw low suitability, while out of this inverse situation
was observed, as regard environmental suitability.
These differences are significant, implying different
ecological adaptation. However, anthropogenic influ-
ences, related to domestication history, are also
important. Therefore, updated genetic investigations
and field trials in contrasting ecological areas are
required for understanding the origin of differences
between these two forms.
Keywords Bush mango Cultivation Ecological
niche modelling Non-timber forest products
R. Vihotogbe
´J. Gebauer
Faculty of Life Sciences, Rhine-Waal University of
Applied Sciences, Marie-Curie-Straße 1, 47533 Kleve,
Germany
R. Vihotogbe
´R. Idohou (&)
Laboratoire de Biomathe
´matiques et d’Estimations
Forestie
`res, Faculte
´des Sciences Agronomiques,
Universite
´d’Abomey-Calavi, 04 BP: 1525 Cotonou,
Republic of Benin
e-mail: rodrigidohou@gmail.com
R. Vihotogbe
´
Ecole de Foresterie et d’Inge
´nierie du Bois, Universite
´
d’Agriculture de Ke
´tou, 01 BP 6779 Cotonou, Republic of
Benin
R. Idohou A. T. Peterson
Biodiversity Institute, University of Kansas, Lawrence,
KS 66045, USA
B. Sinsin
Laboratoire d’Ecologie Applique
´e, Faculte
´des Sciences
Agronomiques, Universite
´d’Abomey-Calavi,
01 BP 526 Cotonou, Republic of Benin
123
Agroforest Syst
https://doi.org/10.1007/s10457-018-0193-y
Introduction
Sustainable use and preservation of plant genetic
resources are considered as key pathways to ensure
their conservation (Klimas et al. 2012). Despite
application of such measures in many tropical coun-
tries, overexploitation of non-timber forest products
(NTFPs) is causing structural destabilization and
erosion of diversity in natural ecosystems (Neumann
and Hirsch 2000; Onomu and Akeem 2014). There-
fore, relying on exploitation and commercialization of
NTFPs in forest management programs provides
economic opportunities (Banjade and Paudel 2008)
but does not guarantee conservation success (Sheil and
Wunder 2002; Rai and Uhl 2004).
NTFP cultivation in traditional agrosystems is
developed by local residents, and promoted by gov-
ernments and international institutes to meet growing
demands for biological products (Wiersum 2004;
Schreckenberg et al. 2006; Trauernitch et al. 2006).
The idea is to reforest landscapes, enhance livelihood
and reduce overexploitation of natural ecosystems
(Leakey 2010). These natural reservoirs may have
limited supply capacity in the face of the growing
market for NTFPs (Ticktin 2004; McLain and Jones
2005). Cultivation thus acknowledges traditional
agroforestry as a vital biodiversity conservation sys-
tem (Jose 2012; Klimas et al. 2012). In tropical Africa,
integration of indigenous fruit tree species is key to
sustaining small–scale productive systems (Leakey
et al. 2005a) and the need for large-scale plantations as
priority NTFP enterprises has been signalled (Leakey
2012; Hodgdon and Martı
´nez 2015). These strategies
for indigenous fruit tree species development are
important to protecting and restoring soil fertility, and
mitigating global climate change effects, thus con-
tributing to a healthier physical environment (Leakey
2010).
African bush mango trees (ABMTs; Irvingia) are
the most economically profitable species in the family
Irvingiaceae (Harris 1996; Asaah et al. 2003). Their
slimy and oily kernels rank among the highest-priority
Sub-Saharan African NTFPs (Clark and Sunderland
2004; Leakey et al. 2005b). Ground kernels are used to
make much–prized thick flavoured African ogbono or
dika sauces (Akusu and Kiin-Kabari 2003; Bamidele
et al. 2015). These species are in increasing demand in
regional and international trade based mostly on
collections from natural forests (Lowe et al. 2000;
NRC 2006). Because global kernel demand exceeds
natural production, cultivation of Irvingia trees is
becoming as an important initiative in West and
Central Africa (Asaah et al. 2003; Vihotogbe
´et al.
2013).
ABMTs produce sweet or bitter fruits, from plants
that are named taxonomically as I. gabonensis (Aubry-
LeComte ex O’Rorke) Baill and I. wombolu Vermoe-
sen, respectively (Harris 1996). Disagreement among
experts regarding these names is fierce, because fruit
taste, not a typical botanical character, and local expert
opinion, are the only ways to separate these taxa
(Harris 1996; Vihotogbe
´et al. 2013). In this context,
detailed information about the distinctiveness of I.
gabonensis and I. wombolu is much needed.
Ecological niche modeling (ENM) is a set of
approaches designed to estimate environmental
requirements for species to maintain populations,
and in turn allows researchers to identify potential
distributional areas for those species (Pearson 2007).
This technique has been applied recently to evaluate
cultivability of several tropical plant species (Miller
and Knouft 2006; Sanchez et al. 2010; Idohou et al.
2017). Evaluation of differences in characteristics of
potential cultivable areas might indicate differences in
ecological adaptability (Scheldeman et al. 2007;
Tocchio et al. 2015). Here, we used ENM to link
occurrence data for I. gabonensis and I. wombolu to
corresponding environmental characteristics to (1)
determine the potential geographic distribution of
cultivable regions for each taxon, and (2) test and
explain differences between sweet- and bitter-tasting
bush mango trees, regarding the geographic distribu-
tion and environmental features of their suitable areas
in the Dahomey Gap.
Materials and methods
Study area
In West Africa, the Dahomey Gap covers southern
Benin and Togo (up to about 8°N latitude; Fig. 1). It is
a mosaic of small deciduous forest patches (protected
by national forest administrations or local communi-
ties), vast savannahs, plantations of exotic tree species
(e.g., Tectona grandis, Lamiaceae; Terminalia spe-
cies, Combretaceae; Gmelina arborea, Verbenaceae),
and cultivated fields (Nagel et al. 2004). No consensus
Agroforest Syst
123
exists regarding the causes (natural or anthropogenic)
of the gap, established since the Late Holocene
between the Lower and Upper Guinean forest blocks.
However, this region, which today is drier (rainfall:
1200–900 mm) and hotter (temperature: 33–25 °C),
was previously covered by evergreen forest (Duminil
et al. 2013) suitable for ABMTs. Hence, this area is the
focus of our analyses.
Occurrence data
We used existing occurrence data of bitter- and sweet-
fruiting ABMTs (I. wombolu and I. gabonensis,
respectively: Sossa 2005, Vihotogbe
´et al.
2013,2014a,2014b). These data were supplemented
with those collected during field works in 2015–2016.
Every district in this ecoregion was visited, and local
residents were interviewed to identify areas of ABMT
occurrence not sampled in previous studies. Thus,
ABMTs were detected and documented along 10 km
long transects, with trees detected within 500 m of the
transects. Three to seven transects (59 in total) were
surveyed in each of the ten districts, depending on its
ecological heterogeneity (diversity of land use sys-
tems and soil types) and geographic extent.
Because determination of potential cultivable lands
is our goal, GPS positions of wild and planted ABMTs
were recorded within every land use system: natural
Fig. 1 Current occurrence
pattern of Irvingia
gabonensis and I. wombolu
in the Dahomey Gap (West
Africa)
Agroforest Syst
123
forests, forest gardens, cultivation lands, plantations,
and home gardens. Isolated presence points, not on
transects but detected when moving in the study area
or mentioned by local residents, were included. To
avoid misidentifications, we used local expertise and
the clear difference in fruit pulp taste to separate these
taxa in the co-occurrence areas (Harris 1996). Along
transects, we required that consecutive occurrences for
each taxon be separated by 1 km, unless they were
related to different ecological situations. Because
online occurrence data (e.g., from GBIF) could present
misidentifications and less-accurate georeferencing
(Weigend et al. 2016), they were avoided in this study,
particularly so considering the rapid landscape
changes in tropical Africa (Lambin et al. 2003;
Hartemink 2010). In total, 2701 and 233 occurrences
were recorded for I. gabonensis and I. wombolu,
respectively (Fig. 1). Occurrence data of each type for
ABMT were further submitted to duplication removal
in ENMTools (www.ENMTools.com: Warren et al.
2010) considering 1 km
2
grid cells, which left 857 and
230 presence points for I. gabonensis and I. wombolu,
respectively.
Environmental data
Investigation of ecological niches, adaptation, or
cultivability for a species requires detailed climatic
data (Miller and Knouft 2006; Scheldeman et al. 2007;
van Andel et al. 2015). Idohou et al. (2017) recently
argued that, at higher spatial resolutions, pervasive
spatial autocorrelation in climatic data prevent them
from incorporating fine-resolution substrate informa-
tion (Linder et al. 2005; McCormack et al. 2010). As
such, we used 2013-2014 imagery to achieve finer
spatial resolution (1 km
2
). These data summarized
gross primary productivity (GPP), land surface tem-
perature (LST), and photosynthetic mass (vegetation
indices; VEG), which are responsive to environmental
variation at fine spatial resolutions, with more direct
impacts on plant species’ occurrence and persistence
(https://reverb.echo.nasa.gov/reverb/; Ganguly et al.
2014). Continuous value data on soil characteristics at
the same spatial resolution were obtained from the
ISRIC World Soil Information Database (http://www.
isric.org): soil organic carbon (g/kg), sand content
(%), silt content (%), clay content (%), pH in H
2
O,
cation exchange capacity (cmol/kg), and bulk density
(tons/m
3
) for six depth horizons: 0–5, 5–15, 15–30,
30–60, 60–100, and 100–200 cm.
Model development
We combined wild and cultivated occurrence data for
each species as both represent sites where the species
are able to persist, and integrated them with the
environmental data in a maximum entropy algorithm
(Phillips et al. 2006). This algorithm ranks among the
most robust alternatives available for modeling niches
(Polce et al. 2013). Occurrence data for each species
were split into calibration (50%) and evaluation (50%)
subsets five times at random, to permit assessment of
predictive capacity of the models. To reduce dimen-
sionality of environmental spaces and create compos-
ite environmental variables with minimal collinearity,
principal component analyses (PCA) was performed
on each of the environmental data sets (e.g., GPP,
LST, SOIL, VEG; Dormann et al. 2013). All combi-
nations of the four sets of environmental layers were
considered: GPP, LST, GPP ?LST ?SOIL,
GPP ?SOIL ?VEG, GPP ?VEG, etc. The combi-
nation that generated the best-evaluated model (see
below) was selected. Initially, 10 bootstrap replicate
models were developed for each of the 5 random
subsamples for each species with 14 environmental
data combinations, for 1400 total model runs.
We used partial receiver operating characteristic
(pROC) curves to evaluate performance of the models
in identifying areas suitable for cultivation across the
Dahomey Gap ecoregion (Peterson et al. 2008; Samy
et al. 2014). One hundred random subsampling
iterations of the partial ROC were produced using an
acceptable error rate E=5%, and the resulting AUC
ratios were compared to unity, the performance of a
random classifier (Peterson et al. 2008). The combi-
nation with the highest ratio was used to map potential
cultivation areas for I. gabonensis and I. wombolu.
Resulting maps were thresholded to create binary
maps to avoid effects of overfitting that is common in
Maxent models (Peterson et al. 2007). We sought the
highest (i.e., most restrictive) threshold that included
95% (= 1–E) of the occurrence data used in model
calibration, a modification of the least training pres-
ence thresholding approach (Pearson 2007). This
approach weights omission error more heavily than
commission error which is appropriate for niche
Agroforest Syst
123
estimation challenges (Fielding and Bell 1997; Peter-
son 2014).
Differences between ABMTs
ENMTools was used for background similarity tests to
quantify the level of similarity or dissimilarity
(Wooten and Gibbs 2011; Moo-Llanes et al. 2013)
between I. gabonensis and I. wombolu, regarding the
geographic distribution and environmental character-
istics of their suitable areas in the Dahomey Gap.
Maxent-generated suitability indices assigned to grid
cells in the Dahomey Gap were used to compare
models between I. gabonensis and I. wombolu. The
similarity between these models was compared with
those from 100 random sets generated in ENMTools
and the Schoener’s Dsimilarity index was calculated
(Warren et al. 2008). We tested the null hypothesis that
niches were not different than expected by chance
between the two taxa.
To understand the roles of individual environmen-
tal factors in differentiating between I. gabonensis and
I. wombolu, we used a canonical discriminant analysis
to clarify the differences observed (Park et al. 2006). A
stepwise binary (I. gabonensis =0 and I. wom-
bolu =1) logistic regression was used to select the
most significantly discriminative vegetation and tem-
perature indices. The significant variables were used in
a canonical discriminant analysis to assess how they
separate the two taxa in multivariate analyses
(Scheldeman et al. 2007).
Results
Pattern of suitability
Among the occurrence data, I. gabonensis was not
recorded in the wild (i.e. all records were of plants
under cultivation). However, cultivated trees occur at
the edge of natural forests in coastal areas. Irvingia
gabonensis was also found in some community forests
in southern Benin, and was the most commonly
cultivated ABMT in the Dahomey Gap: 6.2–9.6°N
latitude. In contrast, I. wombolu occurred only in
south-western Togo, in the Volta region (Fig. 1).
There, very few cultivated individuals (seedling stage)
were recorded, the majority being wild trees preserved
in severely degraded forests, and in traditional cocoa
and coffee-based agroforestry systems.
The partial ROC analyses indicated that among the
four groups of predictive factors tested for the
modelling, all combinations of environmental variable
sets were significant in modelling the geographic
distribution of cultivable lands of I. wombolu and I.
gabonensis. However, the combination LST ?VEG
yielded the highest AUC ratio for I. wombolu (1.642)
and the second-highest for I. gabonensis (1.611), such
that it appears to represent an optimal suite of
variables for these taxa.
Suitable areas for I. gabonensis were distributed
across all the Dahomey Gap coastal area, north to
7.5°N latitude. Farther north, a patchy potential
distributional pattern was observed, and patchiness
increased northwards, north to 10.8°N latitude
(Fig. 2). In contrast, suitable areas for I. wombolu
were concentrated in the Volta region. Outside of this
region, only small patchy extents of suitable areas
were mapped for I. wombolu in coastal areas (Fig. 2).
Differences between I. gabonensis and I. wombolu
The geographic range estimated for I. gabonensis was
broader than that of I. wombolu across the study area.
The Volta forest region was identified as the main
suitable area for the cultivation of both ABMTs in the
Dahomey Gap (Fig. 2). The Dsimilarity index
indicated around 64% overlap of suitable areas
between I. gabonensis and I. wombolu. Tests compar-
ing niches of the two species rejected the null
hypothesis of niche similarity (P\0.05; Fig. 3),
which is to say that models for I. gabonensis and I.
wombolu were more different from one another across
the Dahomey Gap than would be expected by chance.
Thus, specific suitable areas existed for each of the
species (Fig. 3).
The environmental variables selected by the step-
wise logistic regression as differentiating the two taxa
were five of the first principal components based on
VEG and seven of the first components based on LST.
The canonical discriminant analysis showed that the
first axis summarized all (100%, Fig. 4) of the
information in the initial matrix, and that this axis
significantly discriminated I. gabonensis from I.
wombolu (LR test, F=1653.2, P\0.001). All
factors were significant (P\0.001) in discriminating
the two taxa except LST 5 (F=1.41, P=0.235).
Agroforest Syst
123
This confirms wider current cultivation areas for I.
gabonensis.
Discussion
This study demonstrated that, in the Dahomey Gap, I.
gabonensis has a broader and more diverse occurrence
pattern in contrasting ecological regions (mainly in
cultivation) than does I. wombolu. Increasing alloca-
tion of productive space to NTFPs in the region is
intended to enhance livelihood based on new crops
with higher economic profitability (Pandit and Kumar
2010). Therefore, based on its two major NTFPs (the
sweet edible fleshy mesocarp and the kernel, which are
equally valued for both ABMTs), I. gabonensis
appears to present a greater economic opportunity.
However, this higher economic opportunity is decep-
tive, because kernels from I. wombolu are more
economically valuable, in light of their supposedly
superior organoleptic properties (Jamnadass et al.
2011). Moreover, ABMTs are cultivated mostly for
the kernels (Asaah et al. 2003; Preedy et al. 2011;
Vihotogbe
´et al. 2013); the increasing frequency of
Fig. 2 Geographic
distribution of suitable areas
for Irvingia gabonensis and
I. wombolu revealed by
ecological niche modeling:
broader range for I.
gabonensis
Agroforest Syst
123
parasites alters the organoleptic properties of the sweet
mesocarp (Vihotogbe
´et al. 2008), further reinforcing
this business orientation.
In fact, knowledge of useful indigenous plant
species within a cultural food system in a particular
region and identification of desired and economically
profitable phenotypes are important leading forces in
any traditional domestication process (Gonza
´lez-
Soberanis and Casas 2004; Lira et al. 2009; Fuller
2011). Unlike in the Lower Guinean Forest region
(Asaah et al. 2003), such characteristics have not yet
been defined for I. wombolu in West Africa, where no
traditional domestication or selection exists, to allow
this taxon to extend into new areas in the rest of the
region (Vihotogbe
´et al. 2013,2014a). Irvingia
wombolu occurs mostly in its natural range, and is
clearly not facing much change in the Dahomey Gap,
because this strong, bitter-fruited Irvingia tree remains
unknown to most local inhabitants and research
institutions in this ecoregion (Boateng et al. 2007;
Ayivor et al. 2011) for two main reasons. First,
Irvingia kernel-based diets (and other important
traditional uses, such as healthcare) do not belong to
the cultural system there, and they are not still fully
accepted in the Volta region (Vihotogbe
´et al. 2014a),
while these forested lands are being cleared for cocoa
and coffee-intensive production (Asare 2005). Hence,
apart from marginal commercialization of kernels by
women, no place appears to exist for I. wombolu
plantations either locally or regionally, in West Africa.
Second, I. wombolu fruits only once, in the dry
season, across its entire range, including in the Volta
region, where seeds are intensively collected for
kernel commercialization (Clark and Sunderland
2004; Vihotogbe
´et al. 2014b). This phenological
behavior matches poorly with ecologically favorable
times for natural regeneration both inside and outside
the Volta region, thus preventing this taxon from
broadening its range. Nevertheless, abundant regen-
eration was observed by Vihotogbe
´et al. (2014a), at
least on seasonally flooded soils, a subset of its full
ecological niche. In addition to this limited regener-
ation, and continued clearing for agricultural land
preparation, increasing wood commercialization
Fig. 3 Results of background similarity tests based on the
Schoener’s Dindex indicating significant niche differentiation:
similarity of niches of I. gabonensis and I. wombolu was lower
than expected given the background similarity of the accessible
areas of the two species in the Dahomey Gap region. Observed
niche overlap value is shown with an arrow; null distribution is
shown as a histogram
Fig. 4 Canonical scores significantly discriminating I. gabo-
nensis from I. wombolu
Agroforest Syst
123
(Djikpo 2016) diminishes population size, putting I.
wombolu in a critical conservation state (Miller-
Rushing et al. 2010).
Irvingia gabonensis has a broader suitable area
across more diverse ecological conditions. This
difference, in size and diversity of cultivable lands,
between these two taxa, is significant in the Dahomey
Gap. The Dahomey Gap perhaps offers more possi-
bilities for cultivation of I. gabonensis, which has
already proven its contribution to enhancing liveli-
hoods in this ecoregion. However, these differences
are obviously linked to those in domestication history
because of unequal traditional uses and knowledge
between I. gabonensis and I. wombolu.
Ecological niche models have proven valuable in
evaluating environmental associations, potential dis-
tributional areas, and population differentiation
(Scheldeman et al. 2007). The significant ecological
differences demonstrated here are in line with evi-
dence from phenological (Vihotogbe
´et al. 2014b) and
Random Amplified Polymorphic DNA-based genetic
data (Lowe et al. 2000). All three lines of evidence
support taxonomic distinction between I. gabonensis
and I. wombolu (Harris 1996; Schluter and Conte
2009). This implies different ecological adaptability
between both types of ABMTs, since there is no
demonstration of local disinclination to domesticate I.
wombolu (Vihotogbe
´et al. 2014b). Climatic hetero-
geneity within the Dahomey Gap might be the most
explanatory cause of the observed ecological differ-
ence. However, irreproducibility of RAPDs (Kumari
and Thakur 2014) and influences of human-induced
changes in the current occurrence pattern of both taxa
waken any taxonomic conclusion in this study. Thus,
updated genetic data and ecophysiological investiga-
tion, integrating ecologically different materials are
still needed for unequivocal taxonomic conclusion
within ABMTs.
Conclusion
African bush mango trees (Irvingia gabonensis and I.
wombolu) show great potential to domestication,
although the two species appear to respond differently
to environmental conditions, which was confirmed in
our analyses. Irvingia gabonensis showed a broader
potential cultivation area across the country compared
to I. wombulu. The Volta forest region is the main
suitable area for the cultivation of both ABMTs in the
Dahomey Gap. Experiments are badly needed to
validate these model results, in identifying suitable ar-
eas for sustainable cultivation of ABMT species.
Acknowledgements The Alexander von Humboldt
Foundation founded this research (Humboldt Fellowship N°:
1154800 to Romaric Vihotogbe
´). Rosos Djikpo, Aubin
Amagnide, Judicael Vihotogbe
´and local farmers assisted in
occurrence data collection. We also thank Dr. Salako Kolawole
´
Vale
`re for his support in statistical analysis.
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... This results in poor regeneration of ABMTs across their distribution range (Lowe et al., 2000). Therefore, cultivation initiatives are increasing throughout their distribution range, with the higher domestication level in the Dahomey Gap (Vihotogbé et al., 2013), where Vihotogbé et al. (2018) assessed the geographical distribution of cultivable lands. Thus, clarification of the conflicting nature of occurrences (Harris, 1996;Vihotogbé et al., 2018) is needed, for a better ecological and taxonomical knowledge of these taxa. ...
... Therefore, cultivation initiatives are increasing throughout their distribution range, with the higher domestication level in the Dahomey Gap (Vihotogbé et al., 2013), where Vihotogbé et al. (2018) assessed the geographical distribution of cultivable lands. Thus, clarification of the conflicting nature of occurrences (Harris, 1996;Vihotogbé et al., 2018) is needed, for a better ecological and taxonomical knowledge of these taxa. ...
... Our study has now demonstrated that indeed the two taxa populate significantly different environmental niches. This is in line with Vihotogbé et al. (2018) who demonstrated significant differences in cultivable lands, meaning different ecological adaptability between both taxa. Since the occupation of a distinct niche is most likely caused by a genetic difference, we may conclude that most likely the two Irvingia taxa maintain a distinct genetic integrity, and as such should best be regarded as distinct taxa at the species level. ...
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African bush mango trees (Irvingiaceae) are priority food trees in West and Central Africa. There are bitter-and sweet-fruited species, which are difficult to distinguish based on morphological characters. This has led to a debate about their correct taxonomic status. Furthermore, it is unclear whether they are native to the Dahomey Gap, the dry and hot area, separating the two West African forest blocks. This study evaluates the ecological differences between bitter-and sweet-fruited species in tropical Africa, and the nature (wild vs. cultivated) of the occurrences in the Dahomey Gap, in order to discuss the current taxonomical opinions. Irvingia gabonensis and I. wombolu occurrence data were combined with climate and soil data in MaxEnt to produce environmental niche models. Environmental niche identity tests were carried out in ENM-Tools. Wild sweet-fruited trees were predicted in the Guinean-Congolian phytogeographical region, while the predicted occurrence of bitter-fruited trees extended to the Guineo-Congolia/Sudania and Lake Victoria transition zones. The related niche difference is significant, supporting the taxonomical opinion that bitter-and sweet-fruited species are two different taxa. We also conclude that bitter-fruited trees occur naturally in the Volta forests (Dahomey Gap). Moreover, our results support that I. gabonensis is not native to the Dahomey Gap. In historical times, they were probably introduced from Nigeria.
... However, tree establishment, growth and development require a specific set of environmental conditions also known to be affected by climate change, therefore impacting on their geographic distribution and performance. Studies conducted in west African countries have contributed to assessing germplasm adapatability to climate change through the evaluation of their response to environmental stresses (Bayala et al., 2017;Vihotogbe et al., 2018). Moreover, the potential impact of a changing climate to geographic distribution of some high value indigenous species was evaluated using climatic parameters-based niche modelling (Kakai et al., 2011;Djotan et al., 2018;Vihotogbe et al., 2018). ...
... Studies conducted in west African countries have contributed to assessing germplasm adapatability to climate change through the evaluation of their response to environmental stresses (Bayala et al., 2017;Vihotogbe et al., 2018). Moreover, the potential impact of a changing climate to geographic distribution of some high value indigenous species was evaluated using climatic parameters-based niche modelling (Kakai et al., 2011;Djotan et al., 2018;Vihotogbe et al., 2018). This knowledge is important to identify future planting areas, facilitate germplasm translocation potential, determine species plasticity potentials and therefore fulfil species response to climate change . ...
... From ecological niches modelling, it was observed that I. wombulu was highly suited to Volta forest of the Dahomey Gap while I. gabonensis showed very poor environmental suitability, implying different ecological adaptation for the 2 species. These results, together with genetic investigation and field trials in contratsting ecological conditions could better guide the future domestication strategy of this top priority species (Vihotogbe et al., 2018). ...
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... Many studies over the last decade have demonstrated farmers' interest and willingness to domesticate and cultivate local tree species in humid West and Central Africa [147,183,212,[228][229][230][231][232][233][234][235]. Moreover, many of these species can be found in farmers' fields, where they provide products and services useful for the livelihood and resilience of farmers [236][237][238][239][240][241]. ...
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... One of the forest productsbush mango Irvingia gabonensis (ogbono)appears to be the most valuable forest product in West Africa and Central Africa. Previous studies have shown that it is the most preferred non-timber forest product (NTFP) in the regions where they occur (Vihotogbé et al. 2019;Chah et al. 2014). While they occur in the rainforest as wild forest trees, they have increasingly been domesticated in all the zones as part of agroforestry systems, in backyards and in front of homes to form shades. ...
... Finally, it is worth noting that predicting the areas suitable for cultivation should take into consideration other factors such as domestication history, anthropogenic influences, etc., as their importance has been researched for underutilized crops (Vihotogbé et al., 2019). ...
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... Our choice was motivated by the large sampling effort and spatial resolution of occurrence data for the study area which are not available for the other countries where the species is reported. We also avoided extending the SDM to other countries because online occurrence data could present misidentifications and less-accurate geo-referencing (Vihotogbé et al. 2018;Weigend et al. 2016). In addition, our prediction did not account for dispersal, phenotypic plasticity and local adaptation of the species which may produce more pessimistic projections than models using the conventional assumption of homogeneously high plasticity across a species' range (Valladares et al. 2014). ...
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Predicted effects of climate change (CC) on plant species distribution have raised concerns on their conservation and domestication. Appropriate stand density may enhance species ability to adapt to CC. Therefore, combining species distribution modeling (SDM) and spatial pattern of density should provide insightful information for setting conservation actions. We combined geostatistical and SDM techniques to assess (1) current tree density spatial pattern and its relationship with bioclimatic zone (humid, sub-humid, and semi-arid), land-use type (protected areas vs. agrosystems), and soil type (eight types), and (2) present-day and future distributions of suitable habitats under low-RCP4.5 and high-RCP8.5 emissions scenarios for Borassus aethiopum, a declining agroforestry palm in Benin. Data were obtained from 2880 one-ha plots. Semivariogram and kriging were used to model spatial patterns of density while Maximum Entropy was used for SDM. Tree density followed an isotropic spatial model with a range of 2.15 km, indicating extremely fragmented density pattern. Tree density was 8-times higher in protected areas (PAs, 68.6 ± 5.09 trees ha⁻¹) than in agrosystems (8.4 ± 0.31 trees ha⁻¹) and greater on ferruginous soils. Though 80% of the country was currently highly suitable with similar trend for PAs and agrosystems, future predictions showed major habitat loss (20–61%), particularly under RCP8.5. While changes were similar between PAs and agrosystems, the decrease in habitat suitability was pronounced in the semi-arid zone where the species is currently widely-distributed with higher abundance. Very weak link was found between present-day abundance and present-day and future distribution. It is concluded that B. aethiopum has a fragmented density pattern and will be sensitive to CC. In-situ and circa-situ conservations or orchards establishment were suggested depending on the projected changes and the bioclimatic zone. The approach used here is exemplary for other agroforestry tree species.
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The ‘Key descriptors for Irvingia spp. (bush mango)’ consists of an initial minimum set of characterization and evaluation descriptors for this species. This strategic set aims at facilitating access to and utilization of these African indigenous fruit trees. This work has been done jointly with the World Agroforestry (ICRAF) and the FAO International Treaty on Plant Genetic Resources for Food and Agriculture.
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Yam species (Dioscorea spp.) constitute important staple foods for local communities in Sub-Saharan in Africa. Among yam species, Dioscorea dumetorum (Kunth) Pax, commonly consumed, has been reported to be among the most nutritious and well-appreciated local resource although it remains neglected and underutilized species in Benin. Here, we combined occurrence records and environmental data (bioclimatic and soil data) in ecological niche models to assess the suitable areas for the two locally acknowledged varieties of the species: the cultivated and the wild varieties. Results showed that the potential cultivable area of the cultivated variety is twofold, the one of the wild variety. Seven and five of the eight agro-ecological zones of Benin are potentially suitable for the cultivation of the cultivated variety and the wild variety, respectively. Differences in the two varieties of ranges could result from the adaptation of each variety to local conditions reinforced by intrinsic characteristics of each variety. These results constitute an important step towards scaling up the species valorization while developing conservation programs for both varieties. Further genetic investigation and field experiments could shed light on the origin of the differences between these two forms. Keywords Benin · Conservation · Domestication · Dioscorea dumetorum · Distribution
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Wild palms contribute significantly to food security and local economy in tropical areas, and particularly in sub-Saharan Africa. In light of this importance, eight palm species were explored [Borassus aethiopum (L.) Mart, Eremospatha macrocarpa (G. Mann et H. Wendl.) H. Wendl., Laccosperma opacum (G. Mann et H. Wendl.) Drude, Hyphaene thebaica (L.) Mart, Phoenix reclinata Jacq., Raphia hookeri G. Mann et H. Wendl., R. sudanica A. Chev., and R. vinifera P. Beauv.] as targets for conservation, domestication, and cultivation in Benin. Cultivation potential was evaluated in a coarse-resolution, first-pass effort using ecological niche models to relate known occurrences of each species to vegetation indices (VEG), gross primary productivity (GPP), and soil characteristics (SOIL), and model outputs were related to human distribution and land-use patterns. Results showed that wild palms responded differentially to different suites of environmental factors: some species showed best model performance with VEG + GPP + SOIL, others with GPP + SOIL or VEG + GPP, or with a single factor. Two species had broad potential distributions across the country; others had potential areas in the north (2 species) or the south (4 species). Raphia hookeri and R. vinifera showed greatest overlap in terms of ecology and distribution, whereas L. opacum and R. sudanica had the lowest similarity. These models constitute initial steps toward a sustainable scheme for planning exploration of the possibility of cultivation of these species.
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