African tropical rainforests are terrestrial ecosystems with one of the highest and most complex biological diversities on the planet [1] . It is the second largest block of dense rainforest after Amazonia, and its Central African forests extend over six countries: Cameroon, Central African Republic, Democratic Republic of Congo, Republic of Congo, Gabon and Equatorial Guinea. They play an important global role in preserving biodiversity and storing carbon [2] . Its forests account for around 70 percent of the African continent’s forest cover, nearly half of which are dense lowland forests [3] . Cameroon’s forests belong to the vast Central African forest massif and represent around 11% of this massif [4] .

Cameroon’s forest ecosystems cover around 46.27% or 22.5 million ha of the national territory [5] . Cameroon is one of the Central African countries with high forest potential, with 22 million ha of dense rainforest. Forest-savanna mosaics rank among the country’s most important forest categories, accounting for 9.4% of Cameroon’s forest cover with an area of around 2,537,718 hectares [6] .

Ecotones are transition zones between ecological communities, ecosystems or eco-logical regions [7] . Whatever the scale, transition zones generally cover vulnerable ecosystems that are relevant to the flow of organisms [8] , with high biodiversity and the presence of rare species [9] . Transitions between forest and savanna show a wide variety of vegetation types, with different characteristics, floristic composition and ecosystem functions [10] . Throughout the tropical region, forest and savanna largely coexist as a mosaic at the landscape scale [11] . These two vegetation types differ not only in terms of tree density and vegetation structure, but also in species composition, with few species present in both ecosystems [12] . Its rich biodiversity is an important natural resource for food, medicine, timber and firewood for local inhabitants [13] .

Knowledge on floristic diversity of an area can reflect the total resources and conservation status which have a key role for making conservation strategies and policies. Among the various ecological attributes of plant communities, floristic composition and ecological diversity are the most important ones that are influenced by a variety of biotic and abiotic factors [14] .

In the Ndjole ecological zone, observations have shown that dense semi-caducifolia forest has been continuously colonizing periforest savannas for several decades, and that this transgression movement is certainly not recent [15] . Transition zones often receive little attention in biodiversity conservation strategies, although they can generate adaptive responses to environmental change [16] . The forest-savanna mosaic provides local communities with arable land, wild fruits and honey, bushmeat, firewood, building materials, water and grazing areas, among other regulatory and cultural ecosystem services [17] .

Unfortunately, these forest-savanna mosaics have been severely fragmented, degraded and modified by human activities, such as slash-and-burn agriculture, unsustainable exploitation of wild resources and urbanization [18] [19] . Very few studies have been conducted on the characterization of forest-savanna mosaics in Cameroon, so it is therefore urgent to provide a good knowledge of the composition of the woody flora and other parameters. The goal is to highlight the floristic diversity, structure and ecology of vegetation in the forest-savanna transition zones in the locality of Ndjole.

The floristic richness of the Ndjole locality was estimated at 85 species belonging to 67 genera in 33 families, of which 68 species were found in semi-caducifolia forest, 45 in the savanna forest contact zone and 18 species in the savanna. This floristic richness is lower than that obtained by Koulibaly [38] who have225 species in the forest-savanna mosaics of the Lamto Reserve and Comoé National Park in Côte d’Ivoire. This result is also lower than those found by Soro et al. [39] in the forest-savanna ecotone of the Lamto reserve in Côte d’Ivoire (223 species) and this high species richness could be explained by the fact that the reserve enjoys protected area status and by Tiebre et al. [40] (349 species) in the forest-savanna transition zone of Foungbesso. This difference is due to the fact that these authors inventoried trees, shrubs, lianas, grasses and epiphytes. This difference in results could also be explained by sample size, sampling methods and other parameters such as climatic and edaphic conditions. The low number of species in savannas (18 species) is due to the fact that savanna environments are generally not very diverse [41] [42] . The loss of savanna diversity is also linked to the use of these biotopes for the establishment of certain perennial crops [43] . The specific diversity of families varies according to ecosystem. In the forest, we have the Urticaceae, Malvaceae, Fabaceae, Euphorbiaceae and Moraceae families. In contact zones, on the other hand, the Annonaceae families, followed by Fabaceae, Euphorbiaceae and Combretaceae were more diversified, and in savannas, the Combretaceae, Fabaceae, Annonaceae, Rubiaceae, Hypericaceae and Moraceae families. All these families, with the exception of Cannabaceae, are recognized for their high diversity in lowland Cameroonian dense forest [44] or in highland forest [35] [45] [46] . Indeed, according to Xiao-Tao and Jian-Wei [47] , Fabaceae, Euphorbiaceae, Annonaceae and Rubiaceae are common tropical forest families. Other studies on floristic diversity carried out in the Dja reserve (Cameroon) have shown that Fabaceae, Rubiaceae, Malvaceae, Euphorbiaceae, Sapotaceae, Annonaceae and Meliaceae are the most diverse families [48] . N’Dja et al. [49] in the Besso classified forest (Ivory Coast), Soro et al. [39] in the forest -savanna ecotone of the Lamto reserve in Ivory Coast and Tiebre et al. [40] in the forest-savanna transition zone at Foungbesso in Ivory Coast found virtually the same family procession. Indeed, several Ivorian forests are dominated by the same family procession [50] . The Fabaceae family is also considered to characterize old-growth forests, alongside the Burseraceae and Olacaceae [51] . According to Gonmadje et al. [52] , the high diversity of this family in an environment is indicative of environments belonging to the Guineo-Congolese domain.

The highest Shannon index value (H’) is obtained in the forest (2.37 bits), showing that it is more diverse than other ecosystems. The Shannon diversity index increases as specific diversity increases [29] . However, according to Kent and Coker [53] , forest communities are considered rich according to the Shannon index with a value of 3.5 bits. The value for the forest is below the norm and therefore reflects the low specific richness of this plant formation. This low species richness may be due to human activities, the main ones being agriculture, NTFP harvesting, energy/timber exploitation and road infrastructure, as well as environmental factors. This value is lower than that found by Kamga et al. [44] in Garcinia kola formations in the Central and Eastern Regions of Cameroon (2.98 bits and 2.13 bits respectively) and that obtained by Sunderland et al. [54] (3.92 bits) in the forests of the Monts de Cristal in Gabon. These authors concluded that this high value reflects high diversity and a good reconstitution of understory floristic diversity, no doubt due to favorable environmental conditions. These low values of the shannon diversity index (of which 2.37 bits is maximum and 1.7 bits is minimum) characterize the dominance of a species or group of species over the stand [30] - [55] . Indeed, these formations have recently undergone a great deal of anthropogenic disturbance, which is thought to be responsible for the reduction in their biodiversity. However, the Shannon index in these formations remains high compared with that of the gallery forests of Koupa Matapit (West Cameroon), which averages 1.28 bits [14] . The Pielou Equitability values (0.86 to 0.9) found in the present work tend towards 1, indicating that the species in these sites have the same abundance. These values fall within the range of values considered optimal (0.6 to 0.8) by Odum [56] . They reflect a good distribution of individuals within species. These values are comparable to those of Tiebre et al. [40] in the forest-savanna transition zone of Foungbesso (western Côte d’Ivoire) (0.60 and 0.95), these authors asserting that the high value of Pielou Equitability testifies to a good distribution of individuals within the stand. Simpson’s index, which is highly sensitive to the distribution of individuals between species, varies from (0.77 to 0.85). These high values of Simpson’s index in all plant formations express a weak organization of the ecological system and correspond to environmental conditions favorable to the installation of numerous species represented by a small number of individuals [30] and are higher than that obtained by Ngueguim [57] in the Mangombe forests (0.5); to that found in the Dja reserve in Cameroon, i.e. 0.50 [58] and to that of Wouokoue [59] , 0.12 at Mount Bambouto (Cameroon). According to these authors, the high value of Simpson’s index shows that the probability of two individuals taken at random belonging to the same species is high.

The species importance value index for individuals with DBH ≥ 10 cm varies according to plant communities. Important species in the Ndjole forest-savanna ecotones are (in order of importance): Terminalia superba, Myrianthus arboreus, Pycnanthus angolensis, Pentacletra macrophylla, Milicia excelsa and Musanga cecropioides (73.97% to 92.93%). In the contact zone, we have Xylopia aethiopica, Terminalia glaucescens, Piptadeniastrum africanum, Albizia ferruginea, Albizia adianthifolia and Nesogordonia papaverifera. Indeed, some of these species (including Musanga cecropioides, Pycnanthus angolensis, Myrianthus arboreus and Terminalia superba) are recognized by N’guessan and Kassi [60] as the most important species in the forests of the Agbo ecotone in Ivory Coast. According to Kamga et al. [44] , the latter include Musanga cecropioides and Myrianthus arboreus, which are pioneer species. This is confirmed by the high proportion of pioneer species in all the different ecosystems and exotics such as Mangifera indica, which shows that the environment has been disturbed by anthropogenic activities. In the savanna, we have Terminalia glaucescens, Nauclea diderrichii, Annona senegalensis, Piliostigma thonningii, Pittosporum sp. and Albizia zygia as the most important species, of which only Annona senegalensis has been in the periphery of the Mbam and Djerem National Park [61] . It should also be noted that the presence of certain savanna species found at Ndjole was reported by the same author and recognized as important by Youta et al. [15] in the valley of the confluence of the Mbam and Kim rivers in Central Cameroon; these are Hymenocardia acida, Lophira lanceolata, Combretum micranthum, Olax subscorpioidea, Vitex doniana, Crossopterix febrifuga and Albizia zygia. From the above, it is clear that the Albizia genus is represented in forest, contact zone and savanna. Youta [62] confirms that Albizia adianthifolia and Albizia zygia are part of the floristic procession with a wide distribution linked to their important ecological plasticity due to their important dispersal capacity. Kassi [63] asserts that the presence of Albizia adianthifolia in a stand is proof of its secondary origin.

This study highlights the dominance of mesophanerophytes followed by megaphanerophytes in forests and forest-savanna contact zones, and the dominance of microphanerophytes in savannas. This result is in partial agreement with the findings of Soro et al. [42] in forest relics in the Poro region of Côte d’Ivoire, who obtained a predominance of microphanerophytes (70%). The high representation of microphanerophytes in savanna shows a preponderance of shrub or low forest formations in the study environment [64] . Koffi et al. [65] , notes that in tropical forests, phanerophytes make up almost the entire plant community, while other biological types are poorly represented. Sarcochores are the most important type of diaspora in the study area. This dominance of sarcochores is in line with the work of Sonké (1998) [58] in Central Africa, with proportions in excess of 50%. The predominance of sarcochores over other types of diaspore is also highlighted in work by Yangakola et al. [66] and Nshimba [31] in intertropical Africa. The proportion of sarcochore species that correspond to zoochore species shows the importance of the role of animals in diaspora dissemination. Indeed, the main mode of spread in our surveys is zoochory (endozoochory). The importance of zoochory is noted in most studies of forests in Côte d’Ivoire [67] , as is the dominance of zoochorous species in the Congo Basin, with proportions generally exceeding 50% [58] [68] [69] . This dissemination mainly concerns birds, mammals (elephants) and frugivorous vertebrates (monkeys, rats, tiger cats, antelopes and squirrels). With regard to the distribution of phytogeographical types, it shows the overall dominance of Guineo-Congolese species in all plant formations. This result is similar to that of [38] , who also obtained the dominance of Guineo-Congolese species in the forest-savanna mosaics of the Lamto Reserve and Comoé National Park in Côte d’Ivoire. According to Sonké [58] , a high proportion of Guinean species in the floristic background of an area is proof that the area belongs to the Guinean-Congolian region. The high proportion of Guinean-Congolese species reflects the great maturity of these plant formations, which appear to be little or undisturbed by human activity [45] .

Vegetation structure

The distribution of individuals by diameter class in the various plant formations found in the different ecosystems is characterized by more individuals in the [10 - 20 cm[ diameter class. But from class [10 - 20 cm[ upwards, the number of individuals decreases from lower to higher classes. This is characteristic of tropical forests, where stand structure is generally inverted J-shaped and small trees (DBH < 10 cm) account for around 80% of species diversity [70] [71] [72] . In other words, they demonstrate that the number of individuals per diameter class decreases in much the same way as one moves successively from small-diameter classes to larger classes. This exponential downward trend confirms that the stand studied has a high regeneration potential. The density of individuals with DBH ≥ 10 cm for all plant communities studied is higher in the forest (235.78 stems/ha), than in the contact zone (217.18 stems/ha) and savanna (150.62 stems/ha). These values are lower than those found by Djuikouo et al. [73] in the Dja reserve (350 - 460.4 stems/ha). The low density observed in the Ndjole locality can be explained mainly by the strong anthropic pressure exerted on the trees.

In the present study, floristic diversity and plant composition types were analyzed for the vascular plants in Ndjole Forest-savanna ecotones and provided baseline information for further studies. The study has resulted in the documentation of 85 species belonging to 67 genera and 33 families. There were 68 species in semi-caducifolia forest, 45 in contact zone and 18 in savanna. In the forest, the Urticaceae, Combretaceae, Apocynaceae, Myristicaceae, Euphorbiaceae and Malvaceae families were the most diverse. In the contact zone, the Euphorbiaceae, Fabaceae, Annonaceae and Moraceae families were the most diverse, and in the savanna, we have the Combretaceae, Fabaceae and Annonaceae families. Diversity index values (Shannon’s index ranging from 1.7 to 2.37; Pielou’s equitability, 0.86 to 0.9; Simpson’s varies from 0.77 to 0.85) are generally low, however, forest is more diverse compared to contact zone and savanna. The degree of floristic similarity shows that the forest and contact zone (60.17%) belongs to the same plant community. The IVI values show variation in ecologically dominant species and have helped in understanding the ecological significance of the species in the forest-savanna mosaic. The diametric distribution of the different plant communities has a descending exponential trend, with a high regeneration potential. The flora is dominated by mesophanerophytes and megaphanerophytes in the forest and in the forest-savanna contact zone, and by microphanerophytes in the savanna. Sarcochores are the most important diaspores types, and the main mode of dissemination is zoochory (endozoochory). As for the distribution of phytogeographical types, it shows the overall dominance of Guineo-Congolese species in all plant communities. Plant diversity can often be increased at the local scale by changing or diversifying the management of a site. Local populations need to be made aware of the importance of biodiversity and the measures to be taken, such as protecting natural habitats, reducing logging, planting native species and promoting sustainable agricultural practices.

We thank the people of Ndjole for facilitating data collection. We are also grateful to the Department of Plant Biology for providing the laboratory facilities for completing the research activities.

The authors declare no conflicts of interest regarding the publication of this paper.

Tchaleu, D.V., Momo, S.M.C., Kambale, M.A., Tafen, N.C., Kamga, Y.B. and Nguetsop, V.F. (2024) Floristic Diversity, Stand Structure and Plant Life Traits in the Forest-Savanna Mosaic at Ndjole (Centre Cameroon). Open Journal of Ecology, 14, 309-330. https://doi.org/10.4236/oje.2024.144019