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The impact of wood cutting and of bush fires on the dynamics of the herbal strata of Sudano-Guinean Savanna

Few studies were carried out on the impact of wood cutting and bush fires on the dynamics of the herbal strata in savannahs of Adamawa, Cameroon. This study has as objectives to evaluate: 1. the recovery of the vegetation of shrub and grass savanna, 2. the evolution of the number of herbs with regards to time, 3. the floristic and specific contribution of the herbal specie. The inventories of herbaceous plants were carried out through quadra method in 9 sub-plots of 10 m × 10 m for each vegetal formation. The experimental design was split-plot with two (02) factors (savanna trees and shrubs), three (3) treatments (felling without fire, with fire and witness) and three (3) rehearsals/repetitions (portions). The analysis of variance (ANOVA) was used to compare means between treatments by using STATGRAPHIC XVII, and XLSAT to obtain result shown that 103 herbaceous plants divided in 73 genres and 23 families. The rate of recovery of savanna vegetation the herbs are more numerous 60% in the CSF, 50% in the CAF and 45% in witness site (wood savanna), Adversely, 30% in a witness site, 40% in the CSF and 50% in the CAF (shrub savanna). The number the species have grown from 3 to 45 in the tree’s savanna and 2 to 34 for the shrub savanna. The specific contribution of herbaceous varies according to the sites and the species.


Introduction
Increase of population and overuse of natural resources are main cause environmental degradation. This degradation is manifested by the erosion of the biodiversity, fragmentation of huge forests, the decreased of soils fertility [1,[2][3]. The vegetation of sub-Saharan Africa in general and those of Cameroon in particular is confronted with deforestation which is manifested by the unforeseen increase in the research for natural resources [4]. Environment in all its diversity is at the same time a type of space of life necessary and delicate for the long-lasting existence of human beings. This anthrophic pressure of the plants cover is responsible for the degradation of soils and for climatic change [5] thus; space will be in danger considering that its exploitation is at the same time illegal and unlawful. Beyond international preoccupations in matters of sustainable development [6], the questions on the degradation of these forests and of ecological extinction of resources constitute the interest of our study. The savannas are fragile ecosystems reposing on the coexistence between the herbal strata and the ligneous strata, they constitute the dynamic systems who's the evolution depends on the intensity of environmental factors (felling/cutting, drying, fire, rearing and agriculture). The conditions of their persistence are very complex and remained poorly known. They have constituted the topic of numerous researches. [7,[8][9]] that compete to conclude the majority is maintained in its state by fires and as a result, are considered as non-climatic. This situation coupled with the climatic change of these last ten years extreme poverty in the rural populations [10]. As concerns the felling of trees and bushfires causing deforestation, these two concepts have a relation of cause and effect and being capable of provoking climate change. Their pressure on ecosystems in the Sudano-guinean zone increases progressively with the strong increase in the population, this demographic explosion leads to the increasing pressure on the natural ecosystems [11,[12][13].
The savannas are particularly exposed to important risks of degradation in so far as that the major part is located in the arid and semi-arid zones and shelters an important part of the world population [14]. A part from their importance from the agricultural, ecological, and environmental point of view the question of durability represents a major stake [15,[16][17] which is worth studying. In situations of reasoning, the role of bushfires in the dynamics of the regeneration of the vegetation of savannas has not yet been well established especially as it concerns the method of use of these fires. Bushfires can cause the upsetting of bio-geographical cycles of the vegetation, that can lead to negative and or positive impacts on the production of natural ecosystems. These fire impacts were known since a long time by the scientific community [18].
In this Sub division, tree felling and bushfires remains a real threat to the protection of natural resources. These savannas undergo strong anthropic pressures such as deforestation, the over pasturing and above all bushfires like in the high Guinean savannas of Cameroon [19].
Relying on this observation, we question the modalities of the sustainable use and management of the vegetation coverage of the ecosystems. The study of the correlation between 'felling, fire, dynamics of regeneration of the vegetation permits to say that even if the fire is recognized as a factor of explanation of the dynamics of the vegetation of savannas, its importance and its role are not always well defined [9].
Besides, the reflexion begun in the logic of this research is a rich investigation which causes so many scientific domains to intervene. It is for that reason that so many research works (thesis, texts, dissertations, articles, reviews) are consecrated to it. It is a question for instance of studies carried out by [20] on the study of the dynamics of a shrub savanna in the Sudano-guinean zone of north Cameroon under the combined effects of pasture, from fire and of the felling of trees; [21] on the regeneration of trees, dynamics of the vegetation and the maintenance of biodiversity in Mount Cameroon; [22] in Senegal on the dynamics of the natural regeneration of the vegetation and [19] on the impact of tree felling on the natural vegetation on the peri-urban zone of Ngaoundere. However, no work has been carried out on the impact of the complex felling of trees and bushfires on the dynamics of the regeneration of the vegetation of savannas in the locality of Bini and Dori who are strongly damaged by the uncontrollable felling of trees.
The main aim of this work is to study the impact of felling of trees and bushfires on the dynamics of the herbal strata. It is specifically a question of; 1) evaluate the recovery of the vegetation of shrubs and tree savannas, 2) know the evolution of the number of herbs according to the time, 3) do the grouping of herbal species in the different vegetation formations and at last 4) determine the floristic composition, the centesimal frequency and the specific contribution of herbal species.

Geographical location
Situated in central Africa and in the north of Cameroon, the Cameroonian Adamawa is a region located between 6 th and 8 th degree latitude North and between 10 th and 16 th degrees longitude East. It spreads on an area of about 62000 km 2 , on altitudes made up of between 900 and 1500m. Its climate is the Sudano-guinean type with an annual pluviometer/rain gauge of between 1600 and 1800 mm, spread on 7 to 8 months. The annual average temperature is 23°C [23,24]. The High lands of Adamawa are the transit zones between sub-Saharan, north and the forest south. The Adamawa Region is one of the ten regions in Cameroon. The Adamawa is a frontier to the Central African Republic to the East and to Nigeria to the west this mountainous zone marks the frontier between the forest south Cameroon and the savanna north. Of a surface area of 64000km 2 , Adamawa is the 3th region of Cameroon as concerns the size. It is spread on about 400km between Nigeria and the Central African Republic and culminates/reaches its peak at 2460m of altitude at the summit of Tchabal Mbabo [25]. Adamawa geographically belong to Sudanese land but the altitude brings in a mountainous difference/variable. That creates conditions of a n original Sudano-gunean climate characterized by important precipitations, oscillating between 1400 and 2000mm per year, low and invariable temperatures (annual average temperature of 23°C), and a strong opposition of two seasons [26] (Figure 1).

Choice and the presentation of the study area
The Adamawa region (Cameroon) is subdivided administratively into five (5) divisions: Djerem, Faro and Deo, Mayo-Banyo, Mbere and Vina. Our study has been carried out in the Vina Division, specifically in the Ngaoundere III th subdivision in Bini, in two types of savanna (shrub savanna in the vicinity of Deanship of the University of Ngaoundere and in the tree savanna in Dori situated at 5 or 6 km from Deanship.

Method of floristic readings
The floristic inventory has been carried out by using method of quadra applied with success by [28] and [19] in the Guinean savannas of Adamawa. This method consists in delimiting a surface of 10m×10m with the help of a decameter within which one carries out measures of dendrometric parameters (height of trees and shrubs, diameters of the tuft, circumference of 0,30m and 1,30m) and systematic counting of individuals living and death of ligneous in the trees savannas of BINI and in the shrub savanna of Dori of the Ngaoundere III th subdivision. For each of the two vegetations forms, three (3) treatments (felling without fire (CSF), felling with fire (CAF), witness site (T)) have been retained. In each of the treatments, three observations or inventory were done on the potions of 10m×10m, that is a total of eighteen observations accounting for the entire works. These observation or inventories were done/carried out for twelve months on the forms established before. For the herbs, the presence of species has been noted in five layers of 2m×10m (2 meters wide and 10 meters long). The identification of herbs has been carried out systematically on the ground by a botanist. The unidentified species on the ground have been brought back to herbal area of Wakwa for identification.

Methods of calculating parameters
The different methods of calculating parameters leads to the intervention of the notions of presence, of centesimal frequency of a specie, of specific contribution and of global recovery [29].

Frequency of ligneous species
The absolute frequency of the specie (i) is the number of observations accounting this specie. According to [30], relative frequency is the proportion expressed in percentage between the number of observations accounting in this specie and the total number of observations accounting per 100. This method allows determining the accidental, accessory, more frequent, frequent and very frequent species.

Centesimal frequency (CF) of a given specie
report of the number of the presence of the specie (NPi) on the total number of points sampled (PE) in percentage. It is calculated following the formula:

Specific contribution (CSi)
Report of centesimal frequency (FCi) on the sum of centesimal frequency (FCn) of all the sampled species. It is calculated according to the formula:

Data Analysis
Data has been collected, treated and analyzed with the help of the software of Statgraphic plus 17.1.08 software which allows for the comparing of data by site. The parameters such as height, size, presence, enrolment, and number have been subjects analysis of variance (ANOVA). Excel has been used to achieve graphs and histograms, and to calculated percentages, frequency and specific contributions of herbal species.

Rate of recovery of vegetation according to biological types
CSFSab = felling without fire tree savanna; CAFSab = felling with fire savanna tree; TSab = witness savanna tree; CSFSAB = felling without fire savanna shrubs; CAFSAB = felling with fire savanna shrubs; TSAB = witness savanna shrubs. The rate of recovery of savanna vegetation's is characterized by a very strong presence of ligneous in the shrub savanna (70% in a witness site, 60% in the CSF and 50% in the CAF), but does not differ significantly between treatments (p ˃ 0.05). Adversely, in the wood savanna, the herbs are more numerous (60% in the CSF, 50% in the CAF and 45% in witness site). This difference is explained by the fact that in the shrub savanna, there is a domination of trees and shrubs whereas in the shrub savanna the herbs dominate followed by shrubs (figure 2). Our results are different from those of [31] on evaluation of the floristic diversity in herbaceous plants in the Manda National Park in Chad who obtained 99.73% as rate of herbaceous this difference may be due to the difference ecological zone.

Evolution of the number of herbs per site and per month
The evolution of the number of species varies according to months and sites (figure 3). Before the felling and fire (December), we have respectively in the tree savanna 12, 15, 11 (CSF, CAF and witness). In the shrub savanna, in the savanna shrub we counted on the contrary, 13, 14 and 16 (CSF, CAF and witness). It shows that there is a significant difference (p < 0.001) treatment in the both savannahs.
In the month of January, the number of specie has reduced in relation to the number before the fire and the wood cutting (3, 5 and 4) in the tree savannas of 2, 2 and 5 respectively in the sites CSF, CAF and witness of shrub trees. As from January until in August, the number of species increase progressively with the time. This may be explained by the rain (started in February in our study area) that stimulated the growth of the plants during the rainy. The season remains an important factor as it characterizes the bush fire regime and its impact on the environment as demonstrated by [32]. The number varies respectively from 3 to 35, from 5 to 43 and from 4 to 32 species in the sites CSF, CAF and witness for the tree savanna. Meanwhile in the shrub savanna the number of species varies respectively from 2 to 31, from 2 to 34 and from 5 to 22 for the CSF, CAF and witness. Globally the number of species is more elevated in the site CAF. This could already be the action of bushfires which has fostered the propagation of certain herbs. Contrary result has been obtained by [33] in the vegetation in the Sudano-Sahelian zone who had recorded 46 species. CSFSab = felling without savanna tree fire; CAFSab = felling with savanna tree fire; TSab = witness savanna tree; CSFSAB =felling without shrub savanna fire; CAFSAB = felling with savanna fire shrub; TSAB =witness savanna shrub. that is 20.41% for each of them, followed by Fabaceae with 07 species per family. That is 14.29% for a total of 55.11% as compared to 44.89% for families poorly represented having 01 to 04 species. The tree savanna presents 20 families for 68 species, the Asteraceae and the Poaceae which dominate with 10 species that is 18.18% followed by Fabaceae that have 07 species per family pour 14.55%. The dominating families have a percentagee of 50.91% slightly superior to families that are poorly represented having 01 to 03 species for 1.82% to 5.45% that is 49.09%.
The high proportion of Poaceae and Asteraceae in the study area may be due respectively to their competition for light in the open environment and to their ease of dissemination and regrowth. [34] also recorded that Poaceae, Asteraceae and Fabaceae are the the most represented families in the savannah's of Ndikinimeki, Cameroon. Ours result is also lead with those of [35] who found that Poaceae was also the most dominant family in their study area.

Floristic composition, centesimal frequency and specific contribution of herbaceous species
In the two types of savanna (shrub and tree), the floristic composition does not differ significantly between treatments. However, there is a clear relation between density, frequency, the dominance of ligneous, the centesimal frequency and the specific contribution of the herbaceous strata. The more the ligneous, the more the specific contribution of herbaceous dominates. The regular the of herbaceous allow recolonization the vegetation.  [31] has also obtained in the Manda National Park in Chad 39.47% as specific contribution from abundant species. The specific contribution elevated of species for certain sites can be due to the recolonization of the vegetation by the herbaceous stratas.  Figure 6 shows that in the tree savanna the accidental species are more represented more by the very frequent species.  figure 6a: These results corroborate that of [37] who found that the dominant species to be: accidental, accessories, more frequent and frequent. Meanwhile, in the shrub savanna the species having a representative frequency are the accidental (Cornea sp. Indigofera mumelarifolia, Oldenlandia corymbosa) followed by more frequent species (Borreria verticilata, Pteridium aquilinum, Centela asiatica) in the CAF. The accessories species are Hypoetes cancelata, Loudetia arundinacea and Biophytum petersionum. For frequent species we have Stylosanthes guanensis, Vernonia cinera, Trifolium sp and for very frequent we: Asparagus schweinfurthii; Chromoleana odorata; Vernonia colorata ( Figure 6b). The disappearance or the rareness of certain herbal species in an area is due to bush fires which burn diaspores and the grains in the soil. [34] also the regression of species in the burning savannah's Ndikinimeki due to the presence of bush fire which limits the growth and development of certain species. These result means that bush fire reduces the specific richness.

Conclusion
The research study on the impact of the complex wood felling and bushfire on the dynamics of the regeneration herbaceous of the sudano-guinean vegetations has evaluated the recovery of the vegetation of shrub and grass savanna, knowed the evolution of the number of herbs according to time, done the regroupings of herbal species in the different vegetation formations and at last and determined the floristic composition, the centesimal frequency and the specific contribution of the herbal specie. The result showed that the shrub savanna has 103 herbal species sampled in 73 types belonging to 23 families, 58 species for 18 families in the shrub savanna and 68 species for 20 families in the tree savanna. The floristic richness of herbs in the savannas is dominated by certain families: Poaceae (10 species); Fabaceae (10 species); Asteraceae (10 species); followed by Malvaceae and Rubiaceae (4 species). The specific contribution elevated of species for certain sites can be due to the recolonization of the vegetation by the herbacea stratas. It is advisable to use the controlled method of felling trees without fire for a natural regeneration and a lasting management of resources while avoiding the repeated felling of trees every year on the same potion of land.