Garcinia kola Heckel (Clusiaceae) is a medium size tree that grows up to 15-17 m high (Agyili et al., 2006). It is commonly known as “bitter kola” in  English,  “faux  kola” or “petit kola” in French. The tree is endemic to humid lowland rainforest vegetation of central and western Africa  (Akoegninou  et   al.,   2006;  Agyili   et  al.,  2007; Kanmegne and Omokolo, 2008). G. kola is one of the most valuable trees because of its socio-economic importance and medicinal attributes. The seed, commonly known as bitter kola, is a masticatory and a major kola substitute shared at social ceremonies. Bitter kola is a stimulant that has a bitter astringent and resinous taste when eaten (Yakubu et al., 2014). The seeds are often used as aphrodisiac. They are used in folk medicine and in many herbal preparations for the treatment of ailments such as laryngitis, liver disorders, and bronchitis (Farombi and Owoeye, 2011). Because of its high interest, harvesting of the different organs of G. kola has been very heavy (Akoegninou et al., 2006; Agyili et al., 2007; Assogbadjo et al., 2017), making it extinction-threatened in several West and Central African countries (Yakubu et al., 2014).

Garcinia kola is found in Benin and belongs to the top ten priority non-timber forest products in Benin (Assogbadjo et al., 2017). It is only found in the Southern part of the country which corresponds to the sub-humid Guinean zone (Akoegninou et al., 2006) where population density is the highest with the most destructive anthropogenic activities. Bitter kola is an IUCN globally vulnerable species but extinct in the wild in Benin (Neuenschwander et al., 2011). The genetic diversity of the species might therefore be strongly reduced in the future if no appropriate conservation measures are taken. From a phytogenetic viewpoint, characterization and evaluation of morphological and agronomic traits are essential to the identification, conservation and utilization of genetic resources (Gepts, 2006). Studies on morphological variation of G. kola are however scarce. Dah- Nouvlessounon et al. (2016) recently explored the intra specific variability in one of the occurring region of the species, namely the Ouémé Region and reported a strong variation. They suggested a strong potential of the species for domestication. However, intra-specific variation may be caused by several factors which if captured could optimize the species domestication.

So far, no quantitative assessment is available on factors that are responsible for the observed variation in trees and fruits in the species. Previous studies demonstrated that land-use types can affect population structure and morphometric traits of multipurpose trees such as V. paradoxa (Djossa et al., 2008; Akpona et al., 2015). An exploratory survey in its occurrence areas in Benin shows that G. kola is found in home gardens and fields close to houses (Dah-Nouvlessounon et al., 2015). Assuming that land-use could also have an impact on G. kola, the current study aims at understanding how  in  its area of distribution, land-use types affect tree and fruit characteristics of G. kola.

Garcinia kola Heckel (Clusiaceae) is a medium size tree that grows up to 15-17 m high (Agyili et al., 2006). It is commonly known as “bitter kola” in  English,  “faux  kola” or “petit kola” in French. The tree is endemic to humid lowland rainforest vegetation of central and western Africa  (Akoegninou  et   al.,   2006;  Agyili   et  al.,  2007; Kanmegne and Omokolo, 2008). G. kola is one of the most valuable trees because of its socio-economic importance and medicinal attributes. The seed, commonly known as bitter kola, is a masticatory and a major kola substitute shared at social ceremonies. Bitter kola is a stimulant that has a bitter astringent and resinous taste when eaten (Yakubu et al., 2014). The seeds are often used as aphrodisiac. They are used in folk medicine and in many herbal preparations for the treatment of ailments such as laryngitis, liver disorders, and bronchitis (Farombi and Owoeye, 2011). Because of its high interest, harvesting of the different organs of G. kola has been very heavy (Akoegninou et al., 2006; Agyili et al., 2007; Assogbadjo et al., 2017), making it extinction-threatened in several West and Central African countries (Yakubu et al., 2014).

Garcinia kola is found in Benin and belongs to the top ten priority non-timber forest products in Benin (Assogbadjo et al., 2017). It is only found in the Southern part of the country which corresponds to the sub-humid Guinean zone (Akoegninou et al., 2006) where population density is the highest with the most destructive anthropogenic activities. Bitter kola is an IUCN globally vulnerable species but extinct in the wild in Benin (Neuenschwander et al., 2011). The genetic diversity of the species might therefore be strongly reduced in the future if no appropriate conservation measures are taken. From a phytogenetic viewpoint, characterization and evaluation of morphological and agronomic traits are essential to the identification, conservation and utilization of genetic resources (Gepts, 2006). Studies on morphological variation of G. kola are however scarce. Dah- Nouvlessounon et al. (2016) recently explored the intra specific variability in one of the occurring region of the species, namely the Ouémé Region and reported a strong variation. They suggested a strong potential of the species for domestication. However, intra-specific variation may be caused by several factors which if captured could optimize the species domestication.

So far, no quantitative assessment is available on factors that are responsible for the observed variation in trees and fruits in the species. Previous studies demonstrated that land-use types can affect population structure and morphometric traits of multipurpose trees such as V. paradoxa (Djossa et al., 2008; Akpona et al., 2015). An exploratory survey in its occurrence areas in Benin shows that G. kola is found in home gardens and fields close to houses (Dah-Nouvlessounon et al., 2015). Assuming that land-use could also have an impact on G. kola, the current study aims at understanding how  in  its area of distribution, land-use types affect tree and fruit characteristics of G. kola.

Study area

In Benin, G. kola occurs in the Guinean ecological zone as defined by White (1983). This zone is located in the Southern part of Benin between latitude 6°25 N and 7°30 N. The present study was conducted in the three administrative regions (Atlantique, Ouémé and Plateau) where the species was identified in Southern Benin (Figure 2). Southern Benin has a subequatorial climate with two rainy seasons and two dry seasons of unequal length. The region is characterized by a rain fall gradient from 900 mm in the West to 1300 mm in the East (Adomou et al., 2007). The mean annual rainfall is 1200 mm. The annual average temperature ranges between 25 and 29°C and the relative humidity between 69 and 97%. The soils are either deep ferrallitic or rich in clay, humus and minerals (Adomou, 2005).

Data collection and processing

Characterization of G. kola was based on 17 morphological variables related to tree trunk and canopy (tree growth), fruits, leaves and seeds characteristics. Tree growth variables are stem diameter at 1.30 m above the ground (dbh), total height, first ramification height, crown height, and crown diameter. Variables related to leaves are leaf blade length, leaf width, petiole length, and petiole diameter. Fruits’ variables are fruit length, fruit thickness, fruit weight, fruit husk weight, and number of seeds per fruit while variables measured on seeds was seed length, seed thickness, and seed wet weight. The tree growth characteristics were accessed using the formula in Table 1. The leaf blade length and width, the petiole length were measured using a decimeter while stem diameter were measured using a pentadecameter. The length and the width of the fruits and the nuts were taken using a slide foot. Fruit weight, fruit husk weight and seed wet weight were measured using a balance of precision 0.01 with a maximum range of 500 g.

Statistical analyses

Mean values, and standard error of mean and coefficient of variation of each morphological traits were calculated for all the trees and trees within each land-use type. A two-independent samples t-test was performed to test for differences in morphological traits between land-use types. To identify the most important traits that discriminated trees regarding land-use types, a canonical discriminant analysis was applied to all morphological traits. The most discriminant traits were then used to plot trees according   to  their   land   use   types   in  the  canonical  axes. All analyses were run using R software version 3.4.3 (R Core Team, 2018).

Impact of land-use on morphological characteristics of bitter kola

Impact of land-use tree growth characteristics of bitter kola

Tree growth traits were relatively dispersed in both land-use types (CV >25%) except for total height and crown height which were less dispersed. The highest variation was found in first ramification height followed by stem diameter (dbh), and crown diameter (Table 2). There was no significant difference between land-use types for total height, first ramification height, and crown height. However, stem diameter was significantly different between land-use types. The largest trees were encountered in farmlands with a mean dbh of 51.53 cm; whereas the thinnest trees were observed in home gardens with mean dbh of 41 cm (Table 2).

Impact of land-use on leaf size characteristics of bitter kola

Compared to tree growth traits, most of the morphological descriptors of leaves were relatively less dispersed (CV < 25 %) in both land-use types (Table 3). The highest variation was recorded in petiole diameter in both home gardens and farmlands. There was no significant  difference  between  blade  leaf width, petiole length and petiole diameter and land-use types. Leaf blade length in home gardens and farms differed significantly (Table 3). The leaf blade was longer in farmlands (129.52 mm) than in home gardens (115.76 mm).

Impact of land-use on fruits and seed’s characteristics of bitter kola

In general, most fruit traits had relatively low variation (CV  < 25%)  except  for  husk   weight   and  number  of seeds. However, the fruit traits did not vary in the same way between land-use types. In home gardens, a relatively high variation was observed only in number of seeds while it was observed in husk weight and fruit weight in farmlands (Table 4). The highest overall trait variation as well as the highest variation in home gardens was observed in number of seeds. The highest variation was recorded in husk weight in farmlands. There  was   a  significant  difference  between  land-use types for fruit length, fruit width, fruit weight, and husk weight. No significant difference was observed between land-use types for the number of seeds (Table 4). In farmlands, fruit length (56.06 mm), fruit width (59.53 mm), fruit weight (109.97 mm) and husk weight (103.23 mm) were higher than fruit length (52.02 mm), fruit width (55.55 mm), fruit weight (88.50 mm) and husk weight (85.43 mm) in home gardens. All seed traits had relatively  low   variation  (CV <  25%)  in  both   land-use types. The highest overall trait variation as well as the highest variation in home gardens was observed in seed length followed by seed weight. The highest variation in farmlands was observed in seed weight followed by seed length (Table 5). Seed length (32.92 mm), seed width (18.58 mm) (7.15 mm) and seed weight were significantly higher in farmlands than seed length (27.59 mm),seed width (16.76 mm) and seed weight (6.04 mm) in home gardens (Table 5).

Discriminant morphological descriptors of G. kola according to land-use types

The canonical discriminant analysis indicated that  some morphological descriptors separated G. kola trees according to land-use types (P < 0.001). The first axis saved 100% of the information. Only 8 out of the 17 initial descriptors were identified as the most discriminant of trees according to land-use types. There were stem dbh, blade length, fruit length, fruit width, husk weight, seed length, seed width and seed weight (Figure 3). Trees in farmlands are bigger, with longer blades, longer and bigger fruits, and longer, bigger and heavier seeds.

This study assessed the variability of tree and fruit morphometry of G. kola and tested their differences between home garden and farmland. The observed variability in morphological traits highlights the potential for selection within the species. We also found significant difference between both land uses, with stem diameter, fruits and seeds of the trees being bigger in farmlands possibly due to difference in management practices. Experimental studies are needed to provide better understanding of the species response to different management practices. This morphological study should be completed with amolecular analysis of the intraspecific genetic variation. This would provide insights to define an effective conservation strategy for G. kola and would be very useful for the development of breeding programs in order to increase G. kola production in Benin.

The authors have not declared any conflict of interests.

This study was funded by the Pan African University- African Union Commission scholarship. We are thankful to the local people for allowing us to access their trees. We also thank Emile Ogugbe for his invaluable help during data collection in the field.