Characterization of the Woody Vegetation of Savannah Ecosystems of Mayo-Danay Division in the Sudano-Sahelian Zone of Cameroon

Phytodiversity is for man a vital element and a capital which provides for hismultiple needs. This study aims to characterize the floristic diversity and the structure of the woody vegetation of the Mayo-Danay division in the SudanoSahelian zone of Cameroon. The transect method was used for the floristic surveys. The experimental deviceconsists of four (04) treatments which are the sub-divisions and three (03) repetitions, or three (03) villages, per treatment. In the savannah ecosystem of each village, a transect (500 m x 20 m) was installed and along each transect, all woody trees with a dbh ≥ 10 cm at 1.30 m are inventoried, the diameter is measured as well as the height. The villages surveyed are as follows: Djelme, Gouma and Nouldaina in Gobo sub-division; Bangana, Kononaye and Tchalawa in Guere sub-division; Madalam, Sadamakou and Vagandja in kaïkaïsubdivision; Kalfou-center, Bougaye and Ngalaï in Kalfou subdivision. The analysis of variance permitted to realize the comparison between the different sites and the Fisher's LSD test allowed to make the comparison between the averages, and all was performed by Statgraphic + 5.0 Software. The total specific richness is 57 species belonging to 43 genera and 26 families out of 36 surveys that were carried out in our study area. The spescific diversity in Mayo-Danay varied according to the sites: the Shannon diversity index varies from 2.93 bits in the Bangana site to 4.82 bits in the center of Kalfou; the Pielou Evenness values varied from 0.45 in the Bangana site to 0.67 in the Kalfou-center site; the density was 14.89 ± 3.27 stems/ha in the Bangana site to 55.44 ± 10.49 stems/ha in the central Kalfou site. The basal area varied between 23.01 ± 13.17 m/ha in the Kononaye site and 109.64 ± 22.93 m/ha in the Vagandja site. The evaluation of the horizontal structure in the shape of an inverted J, and of the vertical in the shape of a bell shows a predominance of young subjects and especially of average individuals. The study made it possible to assess the structural characteristic of the woody settlement of the MayoDanay division. It thus reveals a regressive evolution of the woody stand with a very high proportion of species of small circumference and medium height. As part of a sustainable and rational management of natural resources in the SudanoSahelian environment, this study provides additional information on the current state of the woody populations of Mayo-Danay.


Introcuction
Life on earth began with phytodiversity [1]. Without the phytodiversity of our planet, which includes more than 30,000 plant species, animal life as we know it could never have existed [1]. Adapting continuously to the natural dynamics of their environment, phytodiversity quickly invaded almost all of the land that emerged to form the first forests. Phytodiverity has always been intimately linked to human evolution [2]. Numerous studies have revealed the importance of phytodiversity in human food in Africa [3,4]. Within the whole phytodiversity consumed, there are woody and herbaceous plants [5]. These phytodiversity resources which produce fruits, seeds, tubers, flowers, sap and other Mayo-Danay Division in the Sudano-Sahelian Zone of Cameroon edible products are as much used for human food as for livestock, in traditional medicine, in agroforestry as fuelwood as lumber and services. They contribute directly to food and play a vital role in the economy of rural populations [6]. In Burkina-Faso, authors noted that natural phytodiversity is for humanity a vital element and a capital which supports multiple needs of a population with a high growth rate [7]. In Cameroon, the average annual growth rate is estimated at 2.7% / year. In the Far North region this annual population growth rate is around 3% [8]. This demographic explosion is causing ever-increasing pressure on the phytodiversity of natural ecosystems [9,10].
However, many woody species are today threatened with extinction [11][12][13] due to over-exploitation of plant species, poor harvesting practices and their poor regeneration [12][13][14]. The effects of the degradation of phytodiversity include notably the progressive disappearance of a good part of plant and animal species, no longer making it possible to meet the needs of the population [15]. The immediate consequences are the scarcity of certain natural resources and plant species [16][17][18]. This rate of loss of phytodiversity and the threats of ecological extinction of the resources it abounds are at the heart of international concerns in terms of sustainable management [19]. The need to conserve and protect managing biodiversity on a planetary scale was established within the framework of the Rio Convention, adopted in 1992. Since then, the principle of maintaining phytodiversity has occupied a central place in the various conventions and the various principles of sustainable development.
In Cameroon and more particularly in the Mayo-Dany division, Far Northregion of Cameroon, despite the intervention of the State and Non-Governmental Organizations in the conservation of biodiversity, this pressure on phytodiversity is accentuating as urban centers grow and needs become more pressing. This problem becomes more urgent in the Sudano-Sahelian zone where species are becoming scarce and the pressure on phytodiversity resources is very strong [20]. The Mayo-Dany division which is recognized for its rich potential in phytodiversity [21,22] in recent years has become the site for the supply of wood energy in the Far North region [23] ; which creates a strong pressure on plant diversity, accentuated by the increase of demography in this division. In fact, in the Mayo-Danay division, important work related to the study of the dynamics and structure of phytodiversity on the influence of various factors has been carried out in protected areas and in savannah ecosystems, notably those [24][25][26][27][28]. Faced with this growing demography, it is important to know the current state of woody stands in Mayo-Danay divisionfor sustainable management, hence the main objective is to contribute to the knowledge of the floristic diversity of the Mayo-Danay division. It is more specifically a question of assessing the current state of the woody floristic composition and of analyzing the structural variability of the woody stand of twelve villages in four subdivisions of the Mayo-Danay division.

Study Area
Our study took place in the Mayo-Danay division located situated between the 10 th and 11 th degrees of latitude North and the 14 th and 15 th degrees of longitude East, in the Far North, Cameroon ( Figure 1). It is limited to the North, by the Logone and Chari division, to the North-east by the Diamaré division, to the West by the Mayo-Kani division and to the East and South by the Republic of Chad. It covers an area of 5.300 km 2 with a cosmopolitan population estimated at more than 600000 souls, with an average density of 142 inhabitants per km 2 . The climate is of the Sudano-Sahelian type, with a long dry season (September-May) and a short rainy season (June-August). Its relief consists of a vast savannah spread over a uniform plain. Mayo-Danay is practically the only division in Cameroon where no mountain rises on the horizon, offering you a panoramic view of a landscape of steppe and shrub savannah. Several ethnic groups live in peace and harmony and share the department. They are among others, the Massa, the Toupouri, the Musgum, the Mousey, the Peulhs, the Kanuri and the Kera.
There are four types of soil in the Mayo-Danay division [29]: -Clay soils (or vertisol), locally known as "Karals"; 1. Ferruginous soils (or sesquioxide soils) with little or no leaching; 2. Halomorphic soils these types of soils are generally uncultivated and known locally as "harde"; 3. Hydromorphic soils. The vegetation is dominated by species like Terminalia avicennoides, Guiera senegalensis, Detarium senegalensis, Faidherbia albida, Strychnos spinosa, Hyphaene thebaica, Prosopis africana and Parkia biglobosa. The type of vegetation is either a shrub savannah or a forest savannah, as in the Kalfou reserve. The sandy-clay bulges of Mayo-Guerléo (Kaïkaï district), Logone (Gobo district) and Northern Toupouri (Districts of Tchatibali, Doukoulou, Datchéka, and Kalfou) are occupied by a more or less shrubby savannah based on: Faidherbia albida, Balanites aegyptiaca, Ziziphus mauritiana, Hyphaene thebaica, Capparis corymbosa, Calotropis procera, Ziziphus spinachristi, Cissus quadrangularis. The grass carpet is most often formed by Aristida hordacea, Aristida adscensionis and Schoenefeldia gracilis. In the Kalfou reserve, we find in the interdunes, more or less flooded in the rainy season with a grass vegetation of Andropogoneae with frequently a tree layer of Terminalia macroptera, Bauhinia reticulata, Gardenia ternifolia, Combretum glutinosum and Mitragyna inermis, Prosopis africana. In the great clay plains of the division, there is often a tree savannah made from Acacia seyal. In the plains north of Yagoua, between Logone and Guerleo, and in the center-south of the department, where the flooding is very long, dominate grassland savannas with Hyparrhenia rufa, Cympobogon giganteus, Sporobolus pyramidalis and Echinochloa. In the north of Lake Fianga, in the plains that remain flooded even in the dry season, there is little more than less or less floating vegetation of Echinochloa stagnina [29].

Data Collection
Three villages were selected for each of the four districts, for a total of 12 villages. In the district of Gobo we have: Djelme, Gouma and Nouldaina. For the district of Guere the villages are: Bangana, Kononaye and Tchalawa. In the district of Kalfou we have as villages: central Kalfou, Bougaye and Ngalaï. The villages: Madalam, Sadamakou and Vagandja are in the district of Kaikai. The choice is justified by the presence of plant formations and the importance of anthropogenic activities linked especially to slash and burn agriculture.
The transect method [31] was used for the floristic surveys of our study area. We inventoried a total of 36 transects (i.e. 03 transects per site). The technique consists in delimiting a plot of 500 m of legnthand 20 m of width with an interval of 50 m between the transects. On each previously delimited transect, the floristic list was established and a few dendrometric parameters were measured. The measurements of these dendrometric parameters were brought to the heights of the trees and the diameters of the tassel were determined with a batten or a graduated pole 5 m long and the circumference (circumference at breast height 1.30 m) with a measuring tape [32]. For trees with a height less than 1.30 m, we measured at 0.3 m in soil height [33]. The identification of the species were done basing on the flora of West Africa [34].

Data Analysis
The data collected in the field from June to October 2019, were subjected to statistical processing. The Shannon index [35] expresses the relative importance of the species of a given environment. The index is minimum when all the individuals belong to the same species. It is maximum when each individual represents a distinct species [36]. It is given by the following relation: ISH = -Σ (n i / N) Log2 (n i / N), with n i = number of species i, N = number of all species; it is expressed in bits.
The regularity index or the Evenness of Pielou [37] is the ratio between the observed diversity and the maximum possible diversity of the number of species (N). It ranges from 0 to 1 and tends to 1 when each of the species is represented by the same number of individuals [38]. Its value is obtained using the following formula: The ecological importance of the species (IVI) [39] has been calculated. The ecological importance or importance value index (IVI) is given by the following relation: The centesimal relative density (Dre) corresponds to the proportion of individuals of one species (Ni) compared to individuals of all species (N). Relative dominance (De) is the area occupied by a species (by its basal area (Sti) or its overlap) compared to the area occupied by all the species in the sample (St). The relative frequency (Fr) designates the distribution of a species in relation to the distribution of all the species in the sample. It is given by the ratio between the centesimal frequency of presence of the species i and the sum of the centesimal frequencies of presence of all the species in the sample (F). Density (N) is the number of individuals per unit area. It is expressed in number of individuals / ha. It is obtained by the ratio of the total number of individuals in the sample (n) by the area sampled (S). N = where n = total number of individuals inventoried at each site ; S = total area sampled from the site in ha. The basal area (St) or basal covering, is the sum of the basal areas of all individuals whose basal circumference (C) is greater than or equal to 0.3 m. This basal area can be estimated by considering that the sections of the stems are circular. It is expressed per unit area (m 2 .ha -1 ) and is calculated as described by [40].

St = ∑
where St = basal area expressed in m².ha -1 ; C = circumference at 0.3 m from the ground of the individuals measured.
For the different plant formations identified, the diversity indices and dendrometric parameters such as density, basal area, diameter at breast height (Dbh) and height were subjected to analysis of variance (ANOVA) to test whether the differences between the plant formations of villages and districts are significant and Fisher's LSD test to compare the means.

Floristic Composition
The total specific richness is 57 species belonging to 43 genera and 26 families out of 36 surveys that were carried out in 12 villages in the 04 sub-divisions of the Mayo-Dany division ( Table 1). The village with the highest number of species is Kalfou center (30 species) and the village with the lowest number is Bangana (09 species) ( Table 1). The subdivision with the highest number of species is Kalfou (40 species) while Guere is the sub-division with the smallest population (19 species). The largest number of families is observed in the village of Nouldaina and the Gobo subdivision, on the other hand, the villages of Bangana and Kononaye and the Guere sub-division have the smallest number of families. Amongst the 57 species recorded, 28 species were scarce in the area (Table 2). The specific diversity in Mayo-Danay varies according to the sites. According to the villages, the Shannon diversity indices vary from 2.93 bits in the village of Bangana to 4.82 bits in the village of Kalfou-center ( Table 3). The Kalfou sub-division presents an index of Shannon diversity more high by all the sub-division, the Guere sub-divisionis the one with the lowest value in the Shannon diversity index ( Table  4). The analysis of variance reveals a non-significant difference between the villages (p = 0.1519 > 0.05) ( Table 3) ; on the other hand, there is a very highly significant difference between the sub-˂ divisions (p = 0.0004 0.05) ( Table 4).  (Table 3). The analysis of variance shows a non-significant difference between the villages (p = 0.8419 > 0.05) ( Table 3) and also between the sub-divisions (p = 0.5517 > 0.05) ( Table 4). For the entire woody stand in our study area, the Shannon index is 3.74 ± 0.29 and the Pielou's Evenness is 0.57 ± 0.02 (Table 4).  The most important species in terms of value of ecological importance (IVI) and which are found in all the sites of our study area were respectively Balanites aegyptiaca (60.21 from Sadamakou), Anogeissus leiocarpus (52.15 from Ngalai), Acacia seyal (49.59 from Bangana) ( Table 5). The importance value index varies from one site to other. It varies between 77.98 (Piliostigma reticulatum from Kononaye) to 10.44 (Terminalia avicennioides from Gouma) ( Table 5). The importance value indexaccording to the sub-divisions shows that in the Gobo sub-division these were the species like Terminalia macroptera (71.30) and Piliostigma reticulatum (63.89) from the village Gouma, and then Detarium microcarpum (63.37) from the village Djelme which are respectively the most ecologically important of the species unlike the Acacia seyal (10.44) from the village of Gouma with the lowest value (     Table 6).    Table 7 presents the density of the populations and basal area of the individual species according to the villages. It appears from this Table that, the density varies according to the villages (P = 0.0035) and ranges between 14.89 ± 3.27 ind./ha to 55.44 ± 10.49 ind./ha in average significantly higher density was observed inKalfou-center compared to the others villages The more weakly density was observed in Bangana (Table 7).

Dendrometric Characteristics
Depending on the density at the stand level, the Kalfou sub-divisionis the highest, moreover the Guere sub-division is the lowest ( Table 8). The results of the analysis of variance carried out for the density of the populations of the different sites reveal significant differences between the villages (P = ˂ 0.0035 0.05) ( Table 7) and between the sub-divisions (P = 0.0 ˂ 000 0.05) (  (Table 7). According to the stands, the Kaikai sub-division represents the highest value while the Guere sub-division shows the lowest value ( Table 8). The analysis of variances shows a non-significant difference between the villages (P = 0.4147> 0.05) ( Table 7) and neither between the sub-divisions (P = 0.0830> 0.05) ( Table 8).
The diametric structure of the trees generally has an "inverted J" shape ( Figure 2). The distribution of trees from different sites shows that the individuals with a diameter of less than 35 cm are the most numerous ( Figure 2). Above 70 cm, trees become rare (Figure 2). It is the same observation following all the sites.

Figure 2. Distribution of individuals by circumferences classes by villages and sub-divisions.
The vertical distribution of the individuals presents a shape which is in the form of a "bell" for the majority of the sites (Figure 3) except those of Bangana, Kononaye and the Guere sub-division which present a curve in the shape of an "inverted J" (Figure 3

Floristic diversity
The present study on the woody stands of the Mayo-Danay division reveals that out of all the surveys that have been carried out, 56 species distributed among 42 genera and 25 families. Our results are not similar to those obtained in Kalfou forest reserve which were 86 species, 58 genera and 28 families [25]. This could be explained by the strong demographic growth which leads to the enlargement of the cultural space in this study area. on the one hand, but also because of its study area which is a protected area, hence anthropogenic action is limited unlike our study area which is the natural ecosystem. Our results are similar to those obtained in a natural savannah ecosystem, subject to various anthropogenic disturbances in the Sudano-Sahelian zone North Cameroon (Laf) with 45 woody species [42]. However, our results were different from those obtained in Senegal precisely on Ferlo flora, with 20 species belonging to 18 genera and 12 families, and in Katane forest reserve in northern Ferlo with 23 species distributed between 18 genera and 13 families [33,43]. Compared to those results, our study site wass quite rich.
The Shannon index measured was on average higher for all the stands, but varied according to the sites. It was higher in central Kalfou, Nouldaina and weakly represented in Bangana and Kononaye. All the sites had an index greater than 03 bits except that of the Bangana site. The Shannon index values of this magnitude indicate that the diversity of vegetation on the site is important [44]. This confirms that the woody stand is more diverse in the villages of central Kalfou, Nouldaina and less diverse in the village of Bangana. Indeed, an author indicates that the Shannon index gradually increases until it is around 5 at the final stage of maturation [45]. Piélou's evenness index follows the same trend as that of Shannon. It is in the Bangana and Kononaye sites where the distribution of individuals across species is less equitable as stated in some studies: evenness tends to 0 when all the sampled individuals belong to the same species, it tends to 1 when individuals are fairly distributed across species [38]. This shows a particular dominance over the distribution of individuals of the different species of these sites, but also a greater stability of these same sites compared to the others.
The ecological significance of the species differs from one site to another. The high values of important ecological value of species like Balanites aegyptiaca, Anogeissus leiocarpus and Acacia seyal could be explained on the one hand by the fact that these are more adapted to the climatic conditions of this agro-ecological zone, but also by their frequency due to their importance for the local population who protect these species.
The predominance of families in importance of Combretaceae, Mimosaceae and Caesalpinoideae over other families is explained by the abundance of individuals of these families in the study area. This predominance could also be explained by the fact that these families are characteristic of African savannas [46][47][48]. These families are common in most of the wooded savannah mosaics in Africa and more typical of the Sudano-Sahelian area [41].

Dendrometric characteristics
The density of trees fluctuates from one site to another. It is of very high value in the villages of Kalfou-center, Vagandja and Nouldaina due to the fact that anthropogenic activities which seem less accentuated in these sites. On the other hand in the villages of Bangana, Kononaye and Gouma, they have the lowest values in density respectively, because of the anthropogenic activities which are more accentuated through the production of firewood, charcoal and logging at flush with woody species during clearing for agricultural plots.
The basal area varies according to the sites. The importance of the observed values of the basal area in the village of Vagandja, and Bougay, would be explained by the fact that the species which have very large diameters are dense in these sites while the basal area is a function of the diameter. These results are in agreement with those obtained in Ferlo [49]. Unlike the other villages, those of Kononaye and Bangana have a low value in basal area, this would be due to the activities of making charcoal and removing wood related to various other uses. These results are in agreement with those obtained the northern part of Ferlo [33,40].
The structure of the woody stand of the Mayo-Danay division established according to the diameter classes shows a predominance of the first classes for all sites and presents the form of "inverted J", reflecting a reduction in the number of stems when passing classes from small diameters to larger diameter classes. Such diametric distribution of woody individuals indicates a disturbed environment, in full reconstruction [50,51]. This scarcity of large-diameter species is explained by a disturbance in savannah ecosystems due to the overexploitation of woody species by humans. These results agree with those conducted in Yappo-Abbe forest classified, South Ivory Coast [52].
The structure of the stand according to the height classes shows that the individuals are more represented by the second class, whose height is between [10-20 m], on the other hand the lowest value is that of the last class [20-30 m]. Individuals whose height is less than or equal to 10 m is moderately represented. This confirms the high proportion of relatively more or less adult individuals, constituting a tree savannah. The stand-scale structure shows a very large distribution of individuals at the level of the second classes of height and circumference. This indicates that, the tree strata are predominant in the area. This is due to the fact that everywhere in the area, the species are highly exploited.

Conclusion
The study permitted to assess the floristic composition and structural characteristic of the woody vegetation in savannah ecosystems of Mayo-Danay Division. A total of 57 species grouped in 43 genera and 26 families was recorded. The Shannon diversity index varied from 2.93 to 4.82 bits. The analysis of variances revealed a non-significant difference between villages (ANOVA, p = 0.15; α = 0.05). The most important species in terms of importance value index (IVI) and which are found in all the sites of our study area were respectively Balanites aegyptiaca (60.21), Anogeissus leiocarpus (52.15) and Acacia seyal (49.59). Structural paramaters revealed a regressive evolution of the woody stand with a very high proportion of species of small circumference and medium height. The floristic composition and the dendrometric characteristics thus observed make it possible to affirm that the woody vegetation is more and more degraded under anthropogenic action and climate change. Results of this study will allow to consider reforestation as solution to the degradation of savannah ecosystems in Mayo-Danay Division.