Morphological Diversity within Pterocarpus erinaceus Poir. (Fabaceae), an Overexploited Species in the Savannahs of Côte d’Ivoire

Pterocarpus erinaceus is a high socio-economic tree of African savannahs. It is overexploited due to technological qualities of its wood, forage value and medicinal uses. The main objective of this study is the assessment of the morphological variability within this species in Côte d’Ivoire, in sight of rational management. Fourteen morphological traits were used to appraise the variability within 144 trees in production from six sites (Yalo, Moyenne Marahoué, Kahanso, Kouassi-Ndawa, Ouarigué and Téguirdouo). The results indicate that the variability reaches 6.60 for the height of the fruit and 45.99 for the weight of the fruit. The results reveal also that there are three morphological groups of P. erinaceus in Côte d’Ivoire. Of the four traits that revealed a distinction among the three morphotypes, only the width of leaves and the number of leaflets enabled their complete distinction. Trees of group 1 have long leaves with a high number of leaflets and large fruits. Trees of group 2 have small leaves with a reduced number of leaflets and large fruits. Then trees of group 3 have intermediate value of width of leaves and number of leaflet and small fruits. The three morphological groups could be useful for varietal selection of P. erinaceus in Côte d’Ivoire.


Introduction
The implementation of trees sustainable management and conservation strategy cannot be achieved without a good knowledge of the phenotypic and genotypic variability of the species, enabling individuals' differentiation [1]. Morphological variability study is appropriate for overall genetic improvement and tree varietal selection activities. It enables the identification of attractive morphological descriptors, identification of traits linked to the origin of seeds sources [2] and possible genetic groups. In Côte d'Ivoire, the variability of woody savannah species is poorly studied, when ecosystems that support them are highly threatened and native species are lost with their gene pools. P. erinaceus, commonly known as African rosewood, Senegal rosewood, African teak, African kino tree, is a species widespread in the savannahs of Côte d'Ivoire. This species, endemic to Guineo-Sudanian and Sudano-Sahelian areas, is multipurpose for local populations [3]. Indeed, it is sought by craftsmen for the manufacture of various musical instruments (balafons, n'goni and djembés). It is one of the most widely used woody species in the savannah of Côte d'Ivoire by farmers as forage [4]. Likewise, the bark, leaves and roots of the tree are used in pharmacopoeia to treat several illnesses including anemia, cough, dysentery, malaria and infant fever [5] [6] [7]. This species is sought-after, exploited and threatened in West Africa [8] for timber and is exported to Asian countries, mainly China. African teak provides a high-value timber for export used in cabinetmaking, building and armaments.
For these uses, the species is constantly mutilated and cut in its natural populations without any preservation measures. The high pressure exerted on the populations of P. erinaceus makes it one of the most threatened woody species in the savannahs of Côte d'Ivoire. In addition, studies carried out to date in west Africa on the species have only focused on structural variability and ethnobotany [9] [10] and those conducted especially in Côte d'Ivoire are only on germination tests [11]. Considering the threats on this species, it is appropriate to produce reliable data on its morphological variability, what is a prerequisite for detecting outstanding phenotypes and the development of optimal valorization plans and preservation of the species. The objective of the present study was to assess the genetic diversity among populations of P. erinaceus by morphological traits in Côte d'Ivoire. Specifically, this involves 1) identifying different morphological groups within P. erinaceus based on morphological traits and 2) determining the discriminating traits of morphological groups of P. erinaceus present in Côte d'Ivoire.

Description of the Study Areas
The study was carried out in the savannahs of Côte d'Ivoire (northern part of the country), the natural distribution area of P. erinaceus. According to variation in climatic factors and vegetations, Sudanian savannah, sub-Sudanian savannah and Guinean savannah can be distinguished [13]. In each of these savannahs, two sites were selected to characterize the morphological variability of natural populations of P. erinaceus (Figure 1).
In Sudanian savannah, the sampling sites are the Ouarigué protected area (58,000 ha) located between 9˚61' and 9˚65' North latitude and between 4˚89' and 4˚96' West longitude and the Téguirdouo site (village to 5 kms from Bouna) located between 9˚25' and 9˚28' North latitude and between 2˚92' and 2˚95' West longitude. The Téguirdouo site is subjected to many pressures including illegal grazing and bushfires. The Ouarigué protected area is under pressure from human activities such as uncontrolled land clearing and overgrazing. The climate in the area is Sudanese and typified by a long dry season (October to June) and a short rainy season (July to September). The annual rainfall varies from 700 to 1200 mm [14] with the intermittent presence of harmattan (fresh and dry wind) from December to February. Temperatures vary from 28˚C to 32˚C [13]. The vegetation consists of shrubby and grassy savannah with forest galleries along the watercourses [13]. The soils are of tropical ferralitic and ferruginous types [15]. In sub-Sudanian savannah, the protected areas of Kahanso (7400 ha) located at latitude 9˚17' North and longitude 7˚37' West, and Kouassi-Ndawa located at latitude 8˚13' North and longitude 2˚90' West (300 ha), constitute the sampling sites. The Kahanso site, which is poorly preserved, is typified by the presence of grazing. On the other hand, Kouassi-Ndawa site deprived of all human activities is a well preserved site. These sites are characterized by open forests and wooded savannahs that stretch over ferralitic soils [15]. The climate is Sudano-Guinean, under the influence of the harmattan, with two seasons, a rainy (4 to 5 months) and a dry (7 to 8 months). Annual precipitation ranges from 1200 to 1600 mm [14] and occurs mostly during July to September. The temperature varies from 16˚C to 36˚C with an average of 27˚C [16].
In Guinean savannah, the Yalo protected area (7˚80' -8˚10' North latitude and forests and savannahs. The climate is equatorial with a bimodal regime; it consists of two rainy seasons spaced by two dry seasons [13]. The average annual rainfall is 1200 mm with an average annual temperature of 27˚C [17]. The soils are ferralitic type [18].

Sampling Methods
Individuals of P. erinaceus from the same site were considered as one population, provenance or origin. In each population, trees separate at least 100 m from each other, with a diameter of 1.30 m from the ground (dbh) greater than 30 cm [1] [19] were selected for organs collection. Each individual is numbered with paint to prevent it from being inventoried twice, then georeferenced using a GPS system (Geographical Positioning System). A total of 144 fruit-trees of P. erinaceus from the six sites following the climatic gradient were sampled in this study (Table 1).

Data Collection
Fourteen quantitative traits were measured on trees, leaves, fruits and seeds of P. erinaceus [1] [19] [20]. The parameters measured on the productive trees are the diameter at breast height (dbh), the height of the bole and the total height. The heights (total and bole) were measured using a graduated pole. The  Table 2.

Data Analysis
Several statistical methods were used to evaluate and analyse the structure and distribution of phenotypic variability of P. erinaceus from the collected data. The modalities of the different parameters have been codified for the statistical analyses. The coded data were analyzed using XLSTAT Ecology software version 2018.5, according to the six sites of the three types of savannah. A descriptive statistic was performed to analyze all the variables of the six sites in order to highlight the mean, standard deviation, coefficient of variation (CV), maximum and minimum. The relationships between the quantitative traits were analyzed by submitting a data at a matrix correlation in order to determine the coefficients of correlation. A transformation of the reduced centred variables was performed for multifactor analysis followed by hierarchical ascendant classification (HAC). The discriminant function analysis (DFA) was carried out to structure the P. erinaceus of the trees according to morphological traits. This analysis also revealed the most discriminating traits. A dendrogram from the HAC was constructed using the Unweighted Pair-Group Method with arithmetic Average (UPGMA). A multiple analysis of variance was performed to compare all discriminating morphological traits together then one way analysis of variance (ANOVA 1) were carried out using the means of the discriminating variables to compare the different groups constituted. Significant ANOVA tests were followed by the multiple pair comparison (LSD) Fisher test at the 5% significance threshold.

Global Morphological Characteristics of Pterocrapus erinaceus
The results of the descriptive statistics carried out with all the morphological traits are in Table 3. Of the fourteen morphological traits, seven (diameter at breast height, bole height, leaf length and width, leaflet length and seed length) showed a fairly high variability with CV varying from 15% to 30%. The fruit weight is the most variable trait with CV upper than 45% whereas the fruit height and width are the less variable with CV lower than 10%. The number of leaflets per leaf and seed thickness showed moderate variability (10 < CV < 15).

Analysis of Correlations between Variables
The correlation matrix of the different variables analyzed is reported in Table 4.  weight is correlated positively to three characters that are the fruit width (r = 0.827), the seed weight (r = 0.63) and seed length (r = 0.728). In this case, only one of the variables of these pairs was considered. To avoid redundancy, the number of variables was reduced and five variables (leaf length, leaflet length, fruit width, seed weight and seed length) were removed.

Identification of Pterocarpus erinaceus Groups
The discriminant function analysis (DFA) was carried out using the mean values of the nine quantitative traits considered. The first factors (axes 1 and 2) of DFA represent the largest part of discrimination, accounting for 95.46% of the total variance (Table 5). These first two axes will therefore be used to describe the identified groups. The first axis accounts for 86.85% of the total variability. It

Determination of Discriminating Traits of Morphological Groups of Pterocarpus erinaceus
The multivariate analysis of variance (MANOVA) for the parameters of the identified groups showed very highly significant difference (F = 145.2; p = 0.038).
The results of the analysis of variance (ANOVA) for each of the four discriminating traits by factorial weight showed a very highly significant difference ( Table 6). Of the four distinctive traits, two (leaf width and the number of leaflet per leaf) revealed a complete distinction among the three groups. Regarding leaf width, group 1 has wide leaves (19.61 ± 2.14 cm) than those in group 3 (16.44 ± 2.23 cm) which are wide than those in group 1 (14.87 ± 1.32 cm). In terms of the number of leaflets per leaf, trees in group 2 have a high number of leaflets (10.48 ± 0.90) than those in group 3 (9.64 ± 1.25) which in turn are smaller than those in group 2 (8.90 ± 0.94). Two characteristics (fruit weight and height) revealed a partial distinction among the three groups. Thus, trees in group 1 and 2 are larger (0.65 ± 0.04 g and 0.65 ± 0.07 g respectively) than those in group 3 (0.27 ± 0.04 g). For fruit height, Group 1 and 2 trees have big fruits (69.48 ± 2.82 mm; 68.44 ± 2.68 mm respectively) than those in group 3 (64.64 ± 4.29 mm).

Morphological Descriptors Variability and Sustainable Conservation of Pterocarpus erinaceus
Descriptive statistics made from the fourteen morphological traits of P. erinaceus from savannahs of Côte d'Ivoire revealed significant variability for some of the traits studied such as height of the drum, diameter at breast height, weight of the fruit and weight of the seed. This variability could be related mainly to genotypic factors and to a lesser extent to anthropogenic and environmental factors.
There is a partial correlation between morphological variables and genetic data  [21]. Indeed, morphological characters are markers that reveal important variations between individuals of the same trees population. Thus, they enable to highlight genetic variation related to geographical origin [22]. The variation in morphological traits should be taken into account in the implementation of conservation measures for trees species. Phenotypic variability is essential for the selection of attractive genotypes for domestication purposes [23]. Intra-specific genetic variation is needed for "plus-trees" based on a large gene pool, assurance of species' ability to adapt to changes in the environment, to avoid the loss of interesting phenotypes for both local populations' uses and for wood machining. No scientific large-scale reforestation studies with P. erinaceus have yet been undertaken in Côte d'Ivoire. Through present study, some P. erinaceus trees of attractive and contrasting phenotypes should be used as "plus-trees" for the production of seedlings for the implementation of the species in agroforestry systems for its conservation.

Morphological Variability Structure of Pterocarpus erinaceus
Discrimant factorial analysis and Ward's ascending hierarchical classification grouped the P. erinaceus individuals sampled in the six studied sites into three morphological groups. Of the fourteen traits used for the distinction among the 144 samples, four were found to be discriminatory. Of the four, two (fruit weight and height) enabled partial distinction among the three groups and the other two (leaf width and number of the leaflet) enabled complete distinction among the three groups. These four morphological descriptors could be used as markers to identify phenotypic groups among individuals of P. erinaceus. This differentiation of P. erinaceus trees could be due to hereditary (genetic) and environmental (origin) factors. These results are consistent with those obtained by [24] on P. santalinus and [25] on Adansonia digitata. Apart from the probable effect of ecological factors on the differential vegetative development of the analyzed population of P. erinaceus, the spatial structuring of morphological diversity could be accentuated by the impact of human pressures (farms and pasture) [20]. In addition, leaf width and number of the leaflet traits can be used as a selection index for an improvement program to increase leaf size for forage production. Based on these results, it is preferable to make a selection from group 1 of P. erinaceus consisting of Kouassi-Ndawa trees for a valuation of the species.
Also, individuals from the three contrasting phenotypic groups could be chosen as seed producers to produce plants of different origins and phenotypes for provenance tests in the three types of savannah for varietal selection adapted to each type of savannah for different uses for local populations and industrial exploitation.

Discriminating Phenotypic Traits in Pterocarpus erinaceus
The discriminant factorial analysis identified the most discriminating traits for the morphological characterization of the P. erinaceus populations studied. The American Journal of Plant Sciences  [26]. It should be noted that the most sought-after aspect of P. erinaceus in Côte d'Ivoire is its production of wood (construction and export) and leaves (livestock feed). The discriminating traits identified constitute selection markers for routine identification of phenotypes adapted to different savannah ecosystems in Côte d'Ivoire.

Conclusion
The study of the morphological variability of P. erinaceus populations carried out in Côte d'Ivoire showed the existence of three phenotypic groups based on tree, leaf, leaflet, fruit and seed traits. Leaf width and number of leaflets enabled a complete distinction of the three groups. The assessment of morphological variability is an essential link in varietal selection to identify individuals that are in the interests of rural populations and domestication of the species. As part of this study, three groups of trees were defined. P. erinaceus trees from Kouassi-Ndawa (group 1) showed the best morphological characteristics. However, for the preservation of the species evolutionary potential, the three morphological groups must be considered to capture a broader base of the species variability. Morphological traits are not sufficient to capture the maximum diversity for genebank development because most morphological traits are influenced by the environment. Molecular analyses are therefore necessary to better understand the diversity and genetic structure within the sampled populations. Molecular markers will provide a better understanding of the genetic diversity of P. erinaceus. Seeds from the three morphological groups should be used for provenance tests primarily at the Kouassi-Ndawa, Moyenne Marahoué and one Sudanian savannah site.