Relationship between Tropical Forest Distribution and Soils on Different Types of Mother Rocks in the Republic of Congo

The factors influencing the distribution of forests and their development are important in order to better understand the bio-functioning of tropicals ecosystems forests. The Republic of the Congo has an important forest area of 23.5 million ha subdivided into three large massifs with different forest units from the north until the south of the country. The present study proposes to highlight the relationship between the edaphic and pedological factors and the distribution of the floristic species of some tropical forests of the Congo. To achieve this aim, a principal component analysis (PCA) was to identify similarities or oppositions between variables and to locate the most correlated variables. Also, the indices of biodiversities were used to assess the biodiversity between forest plot and forest sites. A total of 238 species distributed in 46 families were counted. We noted a CS similarity between Mbomo-Kellé and FMU Mokabi-Dzanga of 50%. However, there was considerable variability between the forests of the Impfondo-Dongou axis and of the forest of other localities. The main component analysis carried out showed that the distribution of floristic species in the studied forests is determined by the edaphic factors.


Introduction
Sustainable management and conservation of forest natural resources have been Yet, the distribution of tropical forests on a global scale, within continents and countries, was not randomly. Some authors claim that if, at the biogeographic scale, forest communities are linked to geological, historical and climatic causes (Ricklefs, 2006;Gaston, 2000), the factors like the relief, the substrate and the abiotic conditions are most likely to explain the coexistence of species on a regional or local scale (Condit et al., 2012). At the global level, some studies have been conducted to see the links between vegetation and soils on different types of parent rocks (Verheye, 1972;Laura di Feo, 2011;Sayre, 2014). These studies showed that in many cases there was a strong link between the different types of vegetation cover and the soil type following the source rock. Boudy, (1948) stated "The forest soil is of little importance and that because of the action of the roots is the the rock or less altered, mother rock, the geological soil in one word, that plays the main role from the edaphic point of view, fires, runoff most often destroyed the forest floor.".
However, it is important to note that this relationship is not always obvious to elucidate.
In the Republic of Congo, despite these efforts to know the floristic composition of different forests, it is fitting to indicate that very few comparison studies between the forest ecosystems of the North and those of the South have been carried out in Congo. On both sides, there are no in-depth studies to understand what are the different factors that explain the distributions forest throughout the entire national territory.
It is with this in mind that the overall objective of this study is to highlight the impact of edaphic and geological factors on the distribution of floristic species between four localities located in North Congo (Mokabi-Dzanga, area Impfondo-Doungou), central-western Congo (Mbomo-Kellé) and southwestern Congo (Gouongo), with a view to improve knowledge of the parameters that ensure the functioning of Congo's forest ecosystems. Specifically; It aims to: 1) study the distribution of floristic species in the study areas; 2) highlight the determinants of this distribution of floristic species.

Introducing Study Areas
The floristic data from this study were obtained from four sites with different  With regard to the botanical inventory in the forests of the Impfondo-Dongou axis, plots of 50 m × 50 m have been delimited between Impfondo and Dongou.

Methods for Collecting Field Data
To allow for a better inventory, these plots were then subdivided into 25 m × 25 m plots in which all species of DHP-20 cm were identified ( Figure 3). DBHs and the number of tree individuals were noted. For trees with buttresses or stilt roots, DBH was measured at 30 cm above them.   survey. This analysis is based on pre-established software. In the case of this study, the main component analysis was conducted using CANOCO 4.5 software.

Methods for Assessing Biodiversity
In order to determine the floristic composition of this study, several parameters and indices of floristic diversity will be used, including: Specific wealth; The goal and weighted spectra; The Shannon Index expresses diversity by taking into account the number of species and the abundance of individuals within each of these species. Thus, a community dominated by a single species will have a lower coefficient than a community with all species co-dominant. It is represented by a real positive number often between 0 and 5. The Shannon Index ( H ′ ) is calculated using the following equation: where H ′ is the Shannon index, p i -ni/N is the probability of counting species i, nor the number of individuals of species i and N the total number of individuals.

 Equitability or Pielou Index
The equitability index is the Shannon Maximum Diversity Index report. It marks the level of diversity achieved by the system relative to the maximum possible .
is the logarithm of the total number of species (S). The values of the Piélou index are between 0 and 1. When they tend to 0, a dominance phenomenon is revealed and when they tend to 1, the index marks a regular compensation of individuals among species .

Similarity Indices
This category of indices is intended to compare objects on the basis of presence-absence of species. To calculate the similarity coefficients of our samples, we used two binary coefficients excluding double zeros; this is the Jaccard coefficient (S) and the coefficient of Surensen (K). The best known is the Jaccard coefficient. The coefficient of Sarensen gives a weight twice as high to the double presence, one can consider the presence of a species more informative than its absence. We used similarity indices in our study to verify and confirm the results of the main component analysis

Floristic Diversity
The floristic survey carried out in the various localities: FMU Mbomo-Kellé,

PCA Results on Attendance-Absence
The main plan for PCA based on a species absence table in each locality explains more than 77% of the species distribution in the surveys conducted Indeed

PCA Results from Tree Numbers per Survey
The PCA's results from the abundance
In addition, the values of the Sorensen similarity coefficient showed a florist affinity between the floristic surveys of FEU Gouongo and FMU Mbomo-Kellé These results on the PCA showed that there is a floristic affinity between the Mbomo-Kellé and Mokabi-Dzanga groups because these floristic surveys were carried out on exonerated soils, while those of the Impfondo-Dongou and Gouongo groups were carried out on flooded soils flood-flooded. This proves that ecology plays a key role in the distribution of plant species on a given.
These PCA have also shown that the distribution of species in the various forest ecosystems studied is determined by edaphic factors, including the geological substrate. This action of the substrate is reinforced locally by climatic factors including rainfall. This result is in line with that of Koubouana (2010) obtained in the Niari Valley. In addition, the role of edaphic factors as determinants of tree distribution is well documented in tropical forest (Webb & Peart, 2000;Itoh et al., 2003).
However, we find that our PCA results on the basis of tree numbers appear independent of the edaphic and climatic variables studied in considering axis 2 of this main plan. This study may not have incorporated some critical factors.
Indeed, the chemical composition of soils would have influenced not only the number of individuals per species but also their presence or absence. Numerous research in tropical forests in Africa and the Amazon has shown links between soil nutrient levels and the presence of certain tree species (Svenning et al., 2004;Mirkka et al., 2005).

Conclusion
The study on the relationship between forest vegetation and soils on different