Co-Occurring Plant Species of the West African Critically Endangered Aubregrinia taïensis Heine, in Côte d’Ivoire ()
1. Introduction
Recent studies show that a third of the flora of tropical Africa is potentially threatened with extinction [1] . Overexploitation, loss of natural habitats, deforestation, and shifting agriculture, driven by human development, are all factors that explain the decline of species. The West African Upper Guinean Forest, which extends from eastern Senegal to western Togo (with a gap called the Dahomey gap between Ghana and Togo), is one of the world hotspots of biodiversity [2] . It therefore contains a rich biodiversity and a high rate of endemism. But this forest is subject to heavy deforestation. Despite their biological richness, a number of ongoing threats to biodiversity in the Upper Guinean forests have resulted in the loss of more than 85% of native vegetation cover [3] . For example, in Côte d’Ivoire, the forest which was estimated at 16 million hectares has been reduced to 2.7 million hectares at the end of the 20th century [4] . This has the corollary of the disappearance of ecosystems but also of certain plant species. On the scale of Côte d’Ivoire, more than 30 years ago, 77 plant species in the process of extinction of the flora of the country were listed [5] . In another publication which appeared a few years later, the author adds other species and emphasizes the need to take conservation measures in situ and ex situ, for species of Sapotaceae (including the one that makes the object of this study, Aubregrinia taïensis), in order to avoid their total disappearance from the flora [6] . The Sapotaceae constitute one of the botanical families comprising a high number of threatened trees in Africa and particularly in Côte d’Ivoire [6] [7] . The situation of this family is explained, at least in part, and in addition to the aforementioned factors, by the high endemicity of African species [6] and the “natural” rarity of many species of the family [7] . Moreover, among the 43 native Sapotaceae species of Côte d’Ivoire, 9 are endemic only to the Upper Guinean Forest. Among these species, there is A. taïensis which, in addition to being threatened, is endemic to two countries: Côte d’Ivoire and Ghana. It is a unique species in the genus Aubregrinia Heine and is classified as critically endangered by IUCN. Indeed, the species is threatened by logging and wood harvesting. As mentioned by Aubréville [7] , in the evergreen forest of West Africa, the notion of distribution ranges is difficult to be applied with some species because of the small number of known or reported individuals. A. taïensis is one of these rare species. Therefore, ex situ and in situ conservation strategies must be dedicated to this species once an individual is found. In the frame of an intensive search for A. taïensis conducted in and around the Tai National Park in Côte d’Ivoire, one individual was observed in a small fragment of private forest, therefore located outside the park. This individual did not bear flowers or fruits. The preservation of the individual and species needs not only the conservation of the biotope (the forest) but also the production of seedlings for introduction into botanical gardens or habitats favorable to its development. In such a context, the knowledge of the species associated (co-occurring species) with this individual is extremely important because it is the first level of evaluation of the ecology of the species. Indeed, highlighting co-occurrence patterns is important for understanding the factors that shape plant associations [8] [9] . This requires clear identification of the species present. The current study has been carried out in order to inventory woody plant species associated to the individual of A. taïensis.
2. Material and Methods
2.1. Study Area
A. taïensis is a Guineo-congolese species that can grow in evergreen moist or wet evergreen forests or in moister semideciduous forest zone especially on or near hills. The study was carried out in the village of Gahably located in the department of Taï not far from the Taï National Park (Figure 1). Indeed, to date, the investigations aimed at locating the individuals of Aubregrinia taïensis, in Côte d’Ivoire, have only made it possible to locate a single individual of the species, in this village, in a forest of about 3 hectares, belonging to a peasant. The area is located in the South-West of Côte d’Ivoire. The climate of this region is of the subequatorial type, characterized by heat and humidity during all the year. The average annual rainfall varies between 1400 and 2100 mm [10] . The average annual temperature remains close to 26˚C. There are 4 seasons. The long rainy season (March/April to July) is followed by the short dry season in August. The rain starts again with the small rainy season (September and October). The great dry season which lasts from November until February/March ends the year.
2.2. Studied Species
A. taïenis is a tall tree up to 50 m high and 95 cm in diameter. The bark is pale brown, exuding a white latex. The leaves are alternate, oblong, entire and leathery measuring 13 - 25 cm in length and 5 - 10 cm in width. They are narrowly to broadly obovate, glabrous with base mostly asymmetric, decurrent and arranged at the end of the branches. The venations are very fine and very tight. The fruits are berries containing about 8 seeds. The seeds have a hard, yellow to dark brown, shiny seed coat.
2.3. Inventory and Identification of Co-Occurring Plants
BA surface inventory has been applied and the dimensions used by Tom-Dery et al. [11] were adopted. Thus, with a measuring tape and wooden post, a circular plot of 30 m of diameter (i.e., 706.5 m2) was established around the individual of Aubregrinia taïensis (Figure 2) which was the center of the circle. All the species present in this circle were recorded. Three circular sub-plots of 5 m radius were
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(b)
Figure 2. Aubregrinia taïensis; (a) The individual found in the forest fragment of Gahably near Taï National Park (photo Doudjo Ouattara); (b) Leaves from an herbarium specimen of the CSRS (photo Doudjo Ouattara).
placed inside the plot to facilitate the inventory. Recognition of regeneration individuals (seedlings) was carried out with a specialist botanist (Mr. Téré Henri) from the region who has a great knowledge of the flora. The search for possible individuals of A. taïensis within this stage of development was carried out by focusing on plants that had latex (Moraceae, Apocynaceae, Sapotaceae, Clusiaceae, etc.). Herbarium samples were collected especially for plants that could not be identified in the field. All the plants were identified at the species level. The samples collected in the field were sent to the herbarium of the “Centre Suisse de Recherches Scientifiques en Côte d’Ivoire”. All the specimens are deposited in this herbarium (CSRS). The names of the inventoried species were checked in the online database (https://tnrs.biendata.org/). The nomenclature adopted is that of the phylogenetic classification [12] .
2.4. Floristic Analysis
A qualitative analysis of the diversity was carried out in this study. First, the taxonomic diversity was assessed through the number of species, genera and families Then the conservation status, of each species was checked on the IUCN website (https://www.iucnredlist.org/). The chorology of the specimens was determined at the national and regional (West Africa) levels according to the classification proposed by Aké Assi [12] . These are:
-GC: Guinean-Congolese species (represented in the forest areas located in the South of the country);
-SZ: Sudano-Zambesian species (represented in the Center and North of the country;
-GC-SZ: Common species for the Guinean-Congolese and Sudano-Zambesian phytogeographical regions;
-GCW: Endemic species of the forest area situated at the west of Togo, including Ghana, Côte d’Ivoire, Liberia, Sierra Leone, Guinea, Guinea Bissau, Gambia and Senegal. The life-form of each species has been determined according to Aké Assi [12] . Thus, the megaphanerophytes (MP) are trees over 30 m in height, mesophanerophytes (mP) are trees with heights between 8 and 30 m, microphanerophytes (me) are shrubs with heights between 2 and 8 m, nanophanerophytes (np) are shrubs with heights between 2.25 m and 2 m high. Woody lianas (L) have also been classified on the same model. All these parameters made it possible to acquire an overall idea of the richness and floristic composition of the studied forest. Threatened species and GCW were considered as special status species in this study due to their value for conservation [13] .
3. Results and Discussion
3.1. Taxonomic Diversity of Co-Occurring Plants of A. taïensis
A total of 130 woody plant species were inventoried in the site (Table 1). These species belong to 109 genera and 51 families. The most represented genera are Diospyros (4 species), Cola and Vitex (3 species each). The most representative families in number of species are those of the Euphorbiaceae (12 species, 9%), Rubiaceae (10 species, 8%), Annonaceae (7 species, 5%) (Figure 3, Table 1). These families are among the dominant ones identified by Bakayoko et al. [14] in forest fragments from the same area, in 2011.
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Figure 3. Proportions of most represented families of the co-occurring species of the individual of Aubregrinia taïensis.
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Table 1. Co-occurring species of Aubregrinia taïensis in a forest fragment of Gahably near Taï.
GC: Guinean-Congolese species; GC-SZ: Common species for the Guinean-Congolese and Sudano-Zambesian phytogeographical regions; GCW: Endemic species of the forest area situated at the west of Togo, including Ghana, Côte d’Ivoire, Liberia, Sierra Leone, Guinea, Guinea Bissau, Gambia and Senegal; mp: Microphanerophyte; np: Nanophanerophyte; mP: Mesophanerophyte; MP: Megaphanerophyte; Lmp: Liana microphanerophyte; LmP: Liana mesophanerophyte; LMP: Liana megaphanerophyte; Lnp: Liana nanophanerophyte.
3.2. Chorology and Life-Form of the Species
Regarding chorology, species from the Guineo-Congolese Region (GC) are the most abundant (72%). Also 19% of the species are endemic of the West African forest (GCW). Only 8% of the species are from the savannah-forest transition zone (GC-SZ). The different proportions are shown in the Figure 4. The species belong to eight life-forms but all phanerophytes. The dominant life-form is that of microphanerophytes (mp). As shown in Figure 5, more than 40 % of species
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Figure 4. Proportions of the species according to their chorology. GC: Guinean-Congolese species (represented in the forest areas located in the South of the country); GC-SZ: Common species for the Guinean-Congolese and Sudano-Zambesian phytogeographical regions; GCW: Endemic species of the forest area situated at the west of Togo, including Ghana, Côte d.’Ivoire, Liberia, Sierra Leone, GuiGuinea Bissau, Gambia and Senegal.
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Figure 5. Proportions of the eight life-forms observed in the site around Aubregrinia taïensis. mp: Microphanerophyte, np: Nanophanerophyte, mP: Mesophanerophyte, MP: Megaphanerophyte, Lmp: Liana microphanerophyte, LmP: Liana mesophanerophyte, LMP: Liana megaphanerophyte, Lnp: Liana nanophanerophyte.
belong to this life-form. It appears that 20% of the inventoried species are lianas. This relatively high proportion of lianas and the dominance of microphanerophytes show that the forest studied would have undergone degradation. Indeed, the studies of Bakayoko et al. [14] on forest fragments in western Côte d’Ivoire have shown that these forests are heavily disturbed by human activities, resulting in an increase of pioneers and lianas. Furthermore, Bongers et al. [15] indicated that the main reason for the abundance of lianas would be related to the disturbance. This suggests that Aubregrinia taïensis could thrive on anthropized sites if individuals are spared. Also, it appears that the research of the plant should not be oriented only in the National Park of Tai. Community forests, forest fragments, secondary forests and even old fallows should also be visited.
3.3. Special Status Species
The forest is home to 24 species with special status (GCW and/or VU) as indicated in Table 2. Three of these species are listed as threatened (all vulnerable)
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Table 2. List of special status species co-occurring with Aubregrinia taïensis.
EN: Endangered species; CR: Critical endangered species; LC: Least Concern species; NE: Not evaluated species; GC: Guinean-Congolese species; GCW: Endemic species of the forest area situated at the west of Togo, including Ghana, Côte d’Ivoire, Liberia, Sierra Leone, Guinea, Guinea Bissau, Gambia and Senegal.
according to the IUCN categories. These are: Campylospermum amplectens, Placodiscus boya and Trichoscypha cavalliensis. Ake Assi [15] consider Chrysophyllum taïensis as an endemic species of Côte d’Ivoire while other authors believe that it is a West African endemic species [16] [17] [18] . We adopted this opinion in the present study. However, for Hawthorne and Jongkind [19] Chrysophyllum taïensis could be a synonymous of Chrysophyllum subnudum with the level of variety. These authors suggested a detailed review study of the plant and related species across Africa. In Côte d’Ivoire the distribution of this species is limited to the west of the Sassandra River. Such species are called Sassandrian [16] . Among the 24 species, 16 (i.e., 66.66%) have not been assessed according to the IUCN threat categories. This lack of status is a risk of disappearance of these species. Indeed, without conservation status, species are often not among the priority ones in the frame of in situ or ex situ conservation projects such as in botanical gardens or agroforestry. Yet these species could be in severe threat status. Indeed, all these species are endemic to the Upper Guinea forest, one of the world’s hotspots of biodiversity [2] . But this area, is subject to a high rate of deforestation, habitats loss and degradation. The presence of 26 special status species around Aubregrinia taïensis (CR species) shows that this forest is important for the conservation [13] . Thus, the forest should be preserved. In addition, no fruit or flower has yet been observed on the individual Aubregrinia taïensis observed. Vegetation multiplication trials (cuttings, layering) should be carried out in order to obtain seedlings quickly. Studies of cuttings of Tieghemella heckelii (A. Chev.) Pierre ex Dubard, another tree species of the Sapotaceae growing in the same area with Aubregrinia taïensis, have given satisfactory results [20] .
4. Conclusion
This study made it possible to know the companion flora of an individual of Aubregrinia taïensis. Although the site is secondary forest, it is home to priority species for conservation due to their endemicity or conservation status. The floristic composition of the forest shows that Aubregrinia taïensis can grow in secondary and/or degraded forests. This fragment of forest, although belonging to a person, and therefore constituting private property, should be preserved. The possibility of accompanying the owner for its erection into a Voluntary Nature Reserve (RNV) is one of the options to be considered for its conservation. The lack of conservation status for the majority of species could favor the disappearance of many plants because they could not be taken into account in species safeguard projects.
Acknowledgements
This study was carried out during field missions financed by Botanical Garden Conservation International (BGCI) through the project “in situ conservation of Aubregrinia in Côte d’Ivoire”.