The diversity of the woody tree flora of Mont Sangbé National Park (west of the Ivory Coast)

The present study concerns the diversity of the woody tree flora of Mont Sangbé National Park (West of Côte d'Ivoire). The main objective was to analyze the diversity of the woody tree flora of this park impacted by human activities. Specifically, it involved (1) identifying the anthropogenic factors of the PNMS, sources of impact (2) assessing the impact of these activities on the diversity of the woody tree flora of the PNMS and (3) propose measures to mitigate biodiversity losses. To carry out the study, a device of 14 square blocks of 1 ha each was used on the impacted sites and the control sites in the different biotopes of the park. Each block was subdivided into 100 square plots of 100m2 each. The results show that anthropogenic activities have reduced the flora richness of the park, modified its floristic composition and caused the disappearance of some status species. It was proposed as a measure to step up surveillance of the park.


Introduction
The deforestation, with its multiple harmful consequences, is becoming more and more threatening for the survival of humanity. In recent decades, we have noted the drastic disappearance of huge areas of forests across the world. According to the FAO (2010), during the decade from 2000 to 2010, around 13 million hectares of forests were converted each year to other uses around the world. Regarding the intertropical zone, tropical Africa is the hardest hit by this environmental crisis. The plant formations in this region are degrading at an alarming rate. The net change in forest area in intertropical Africa is the largest of any region in the world (FAO, 2001). In Côte d'Ivoire, the fall in forest areas was spectacular from 1960 to 2012 due to the anarchic logging of forests and the successive political and military crises suffered by the neighboring populations. The natural ecosystems there have been severely damaged by various human activities (UNEP, 2015). For the protected areas in the west of the country that belong to the Guinean Forests of West Africa Hotspot, one of the 18 tropical zones of global importance for their biodiversity, the damage has been considerable. Almost all of them have suffered various looting of natural resources and even human occupation for more than a decade. This seriously damaged their integrity (Virginie et al, 2012). In this area, the case of Mont Sangbé National Park, the subject of our study, was particularly critical. We noted on this site a massive introduction of riparian populations during the period of crisis suffered by the country from 2002 to 2012. This resulted in the disappearance of large areas of forest for the benefit of cocoa plantations and food crops. (Anonymous, 2014). After these events, we have no idea about the floristic diversity of this site. The last floristic inventories carried out in this park date back more than 20 years and took place before the 2002 crisis during which the situation worsened. It is in order to contribute to a better knowledge of the diversity of the tree-like woody flora of the PNMS after its anthropization that the present study was initiated.

Study zone
The Mont Sangbé National Park (PNMS), with a total area of 97,000 ha, is located in the west of Côte d'Ivoire between 7 ° 51 'and 8 ° 10' north latitude and 7 ° 03 ' and 7 ° 23 'west longitude ( Figure 1). It straddles the departments of Biankouma (S / P de Biankouma and Gbonné), Touba (S / P de Foungbesso, Guintéguéla and Touba) and Sifie has 33.5 km of conventional limits, 102 km of natural limits made up of watercourses and a 5 km limit in tracks (Lauginie, 2007). According to Guillaumet and Adjanohoun (1971), the PNMS is located in the mesophilic sector of the Guinean domain, a transition zone between the dense humid evergreen forest of the rainforest domain, the open forests and the savannas of the sub-Sudanese sector. The predominant climax for the southern part is dense humid semi-deciduous forest and for the northern part, open forest. The soils are highly desaturated type ferralitic in the southwest and mountainous part of the park and moderately desaturated in the other parts of the park. The climate is a transition type between subequatorial and tropical climates with five to six months of dry season and seven months of rainy season with an annual rainfall of 1100 and 1600 mm of water. The rather rich hydrographic network is dominated by the Sassandra River in the east and its tributaries the Bafing rivers in the south and Baba in the north. The main activity of the populations is agriculture, which is strongly dominated by food crops (cassava, taro, rice and maize which occupy a prominent place). Perennial crops, on the other hand, are made up largely of coffee, cocoa, cashew and rubber plantations. Livestock farming in the neighboring villages is of the traditional type and involves cattle, sheep, goats and poultry. Figure 1 shows the geographical location of the PNMS.

Collection of data
The data was collected using three survey methods: surface surveys, traveling surveys and linear transepts surveys. The surface survey method consisted in delimiting on the selected sites square plots of one hectare in area subdivided into 100 square plots of 100 m² each (10 m x 10 m). Each of these plots was covered entirely by recording the names, diameters at breast height (DBH) and sizes of all the woody species encountered. These surface surveys have been associated with the traveling method, which consists of traversing the plots in all directions in order to identify the species that the plot method has not been able to identify. For the line transect method, it consisted in delineating transects 500 m long and 20 m wide in areas of riparian forest which are quite narrow. All the species that border the watercourses and which are located within these transects have been recorded. The identification of the species was made from the flora of Aké-Assi (1982) and the UJLOG herbarium.

Data analysis
The richness of the tree flora of the site was determined from the number of species inventoried on the different blocks. Concerning the floristic composition, the analysis of the biological types was made based on the classification method proposed by Raunkiaer (1934) and the classification of species according to their phytogeographic distribution from the work of Aké- Assi (2001;. Finally, the determination of species with special status, including endemic species, rare species and threatened species, was made from the IUCN red list (2019) and that of Aké-Assi (1998). The Shannon index (1848) was used to measure site specific diversity and the Piélou Equitability index (1966) to analyze the regularity of species distribution. Family Importance Values (VIF) and Species Importance Index (IVI) values were determined by summing the relative frequencies, relative dominances and relative densities (Mori et al. 1983). These indices are expressed as a percentage and are equal to 300 for all the families or species encountered on a studied area. The relative frequency of a family or a species is the ratio of the number of species found in that family to the total number of species found in a sample. The relative dominance and the relative abundance of a family or a species have the same definitions, at the taxonomic level. Finally, Sorensen's Coefficient of Similarity (1948) was used to discriminate study blocks. Its mathematical formula is: Cs = (2 c) / (a + b) x 100 where a and b respectively represent the numbers of species present on sites A and B and c the number of species common to the 2 sites. This coefficient (Cs) varies from 0 to 100% depending on whether the two sites have totally different (c = 0) or identical (a = b = c) plant compositions. For a similarity coefficient greater than or equal to 50%, the two sites concerned were considered to be floristically identical. The variability of the woody richness of the sites investigated was evaluated by the Kruskal-Wallis test. The Past software was used for performing the statistical tests.

Floristic wealth
A total of one hundred and eighty-seven (187) tree species have been identified, divided into one hundred twenty-five (125) genera and forty (40) botanical families. The riparian forest witness block (BFRT) with 56 species is the richest and the block impacted by agriculture in forest areas dense (BAFD) with 16 species is the least rich. The number of species in the other blocks are between these two extreme values. Table 1 presents the wealth in the blocks. In terms of genera, out of the one hundred and twenty-five (125) recorded, thirty-one (31) include at least two (2) species, i.e. 24.8% and 94, only one species, i.e. 75.2%. The majority of genera identified in the study area are therefore represented by a single species. Ficus and Cola are the most represented genera of the PNMS. Table 2 shows the detail of the richest genres in the blocks. Across the PNMS as a whole, the most represented families, in decreasing order of the percentage of species, are: Caesalpiniaceae (10 p.c.), Rubiaceae (9 p.c.) and Moraceae (7 p.c.). In the blocks located in the savannah zone, namely BF, BAS and BTFC, the richest families are the Caesalpiniaceae, Euphorbiaceae, Fabaceae, Rubiaceae and Mimosaceae. Bignoniaceae were noted in the BAS block.     At the level of species with special status ten (10) (17) vulnerable species (VU), two (2) near threatened (NT) and four (4) endangered (EN) have been identified in the park. . Table 3 presents the threatened species according to IUCN (2019).  Table 3 shows the various predominant families.

Predominant species
In PNMS, the top three most prominent species in descending order of their IVI are: Cola cordifolia, Uapaca togoensis and Hollarhena floribunda ( However Gymnostemon zaizou and Tricalysia coriacea are recorded only in the control block BTFD. Table 4 presents the lists of the first three predominant species of the park and of the blocks.

Specific diversity indices
The

Similarity between inventoried blocks
The greatest value of the similarity coefficient was observed between the BF and BTFC blocks. It is estimated at 64.28 p.c. the similarity between the pasture block (BP) and the control block (BPT) is estimated at 55 p.c. We also note a strong similarity on the one hand between the BD clearing block and the d block. Forest and agricultural exploitation BEAF estimated at 50 pc, and between the BD block and the agricultural block of dense forest (BAFD) estimated at 50 pc The lowest coefficient (5.40 pc) was recorded between blocks BF and BEF. Table 5 shows the different coefficients of similarity between the inventoried blocks.

Discussion
According to the results one hundred and eighty-seven (187) species of woody tree flora were inventoried in the PNMS against 206 species for previous studies (ENGREF, 1996;Poilecot, 1998). Furthermore, a comparison of our list with the old list, indicates that 129 species recorded by these authors registered on our list and 77 species are not found there. On the other hand, 44 species recorded by the present study are not mentioned in the old list. These data seem to reflect a loss of floristic richness and a modification of the floristic composition of the inventoried sites. If we take into account data from Pascal (1988), who estimates the tree richness of tropical rainforests between 120 and 200 species, it appears that the woody tree flora of the PNMS is moderately diverse. This richness is higher in the control block of riparian forest because of the watercourse that allows both the development of species of dense semi-deciduous and evergreen forests, as well as those that are directly in contact with the water (Lauginie, 2007). On the other hand, the BAFD and BAS blocks record the lowest wealth because of agricultural practices which have led to the massive destruction of woody plants. Indeed, according to Adjanohoun (1964), during clearing, cultivators often use ax and machete for felling trees and daba for establishing mounds and cleaning crops. When there is tree vegetation, it is completely felled and burned over the entire area. For the grazing site, the loss of floristic richness would be due to trampling of young plants by herds of oxen and to overgrazing which, according to Kessler & Geerling (2006), leads to a reduction in the natural regeneration of woody plants. For the bushfire site, this loss of wealth, according to Guillaumet and Adjanohoun (1971), comes from the impoverishment of the original flora and the reduction in soil fertility by recurrent fires. Regarding logging, Laugenie (2007) notes that certain operations such as setting up camps, creating access tracks the fall of trees and deforestation around the logs to be cut are the cause of the disappearance of some species. At the level of botanical families, the predominance of Caesalpiniaceae, Rubiaceae and Moraceae is not specific to PNMS, because the majority of tropical forests in West Africa (Etien and Traoré, 2005,) and Central Africa (Tchouto , 2004) is dominated by the same processions of families. Regarding the biological types, the predominance of microphanerophytes reflects the disturbance of the environment by human activities (Sangne, 2009). For the phytogeographic distribution, the strong dominance of Guinean species in the flora background of the PNMS constitutes, according to Sonké (1998), proof that this site does indeed belong to the Guinean-Congolese region of White (1986). Moreover, this percentage associated with those of endemic Ivorian (Gi), West African (GCW) and transitional (GC-SZ) species, gives a total of 82 p.c. of species. This means, according to White (1986), that the PNMS has a remarkably pure Guinean-Congolese type flora. The phytogeographic distribution of this flora has therefore not been significantly impacted by human activities. However, the high proportions of widely distributed species such as Afr-Tr, Am, Aa, Pant, noted in some blocks seem to reflect disturbances caused by anthropogenic activities. In fact, agricultural practices, clearing and harvesting of ligneous plants cause openings which favor the invasion of sites by species from other regions (Lubini, 1981). For species with special status, their abundance in the PNMS attests that this is indeed a conservation site belonging to the "Guinean Forests of West Africa Hotspot". In addition, the increase in the number of threatened species according to IUCN (2019) is due to the appearance of new species on degraded sites. Finally, according to Tchouto (2004) and Van Gemerden (2004), according to Tchouto (2004) and Van Gemerden (2004), the high number of species with special status recorded in undegraded forest areas compared to impacted sites where they are absent shows that these species are the most sensitive to the disturbances caused. by the man. Because, according to these authors, anthropogenic activities, the result of which is the destruction of the forest, do not promote the survival of these species which looking for a particular microclimate.
In addition, the preponderance of Bignoniaceae in the BAS, BDF, BD and BPL blocks reflects the degradation of these sites by human activities. Indeed, according to Aubreville (1959), the Bignoniaceae constitute a family which often invades clearing sites and whose introduction is due to the modifications made by man to the primitive environment. The predominant species found on these sites, such as Ceiba pentandra, Markhamia tomentosa and Malacantha alnifolia, corroborate this fact. Effect, according to Prota (2020), Ceiba pentandra is a pioneer species of secondary and rainforest that grows rapidly in openings in the canopy caused by disturbance. Malacantha alnifolia, on the other hand, is a pioneer species that tolerates fire and regenerates abundantly in regularly burned forests Prota (2020). Finally, Markhamia tomentosa is a species with winged seeds easily dispersed by the wind, on cleared sites. Regarding the Shannon diversity index, with the value of 4.26 it confirms that the flora of the PNMS is moderately diverse. At the level of the similarity coefficients, the floristic similarities between the blocks BTFC and BF, BP and BPT, BD and BDF would be linked to their belonging to the same type of plant formation which are respectively, open forest, shrub savannah and forest island. The greatest dissimilarities recorded between blocks BF and BEF, BEF and BAS and between BEF and BTFC could be explained by the difference in the types of vegetation to which they belong. Indeed, the BEF block is located in dense semi-deciduous forest, while the BF, BAS and BTFC blocks are in the savannah.

Conclusion
The study of the diversity of the tree-like woody flora of the PNMS has enabled us to list 187 species divided into 125 genera and 34 families. This moderately diversified flora has been strongly impacted by human activities which have caused a reduction in their richness of about 9 p.c. and a modification of the floristic composition by the appearance of new species in the degraded areas of the park. These more marked impacts on former farming sites have led to the disappearance of a number of special-status species that were abundant in the PNMS in the past. Invasive species, including Chromolonae odorata, have now invaded the former degraded sites of the park, preventing the regeneration of the forest species that were there. This modification of the landscape could be at the origin of the disappearance of certain animal species linked to these habitats. There is therefore an urgent need to combat the degradation of the park and the spread of invasive species by restoring degraded habitats and conserving the remaining special status species.