Its expansion is favored by its single-walled spore germination (Leptosporangie), allelopathy mechanisms and aggressive rhizomes \cite{PichiSermoli_1985, Habiyaremye_1997, VanDerBurg_2004, Mangambu_2013, Mangambu_2016}.

\begin{figure}
\centering
\includegraphics{figure_1.pdf}
\caption{General View of \textit{Pteridiaie} in the Forest Edge of Mugaba Station (2305 M Altitude) On the Middle Horizon of the KBNP Mountain Stage (Photo Taken By Mangambu, 2/06/2012).}
\label{figure_1}
\end{figure}

The vegetation formation called "forest Edge" is a limit, a boundary portion or a specific type of boundary between two adjacent habitats that differ in one or more aspects \cite{Forman_1995, Alignier_2010}. While "Chablis" is a disturbance that is one of the natural and natural stages of the forest cycle as it unfolds in nature, which locally translates into a light gap favorable to the natural regeneration of the forest; for both grasses and young trees \cite{DeCaceres_Legendre_2009, Alignier_2010}. Edges related to forest ecology are the areas where two or more different ecological features come together, often associated with variations in the convergence of plant species. Edges are important to wildlife and offer a mix of native plants that are oftentimes found nowhere else throughout the ecosystem \cite{Whitney_1989, Alignier_2010}. For this study, we will study the grouping with \textit{Pteridium aquilinum} which dominates in these two plant formations in the medium and high altitude zones of this part of the Park and also to seek to know its role, either in dynamics or in regression vegetation. We will also examine the architecture, the physiognomy and the structure of the grouping as well as the salient physiognomic characteristics that allow the group to evolve towards a forest formation. Other points that will be addressed in this work include floristic traits, the relationship between environmental variables and the components of \textit{Pteridium aquilinum}. The latter will be evaluated by the indices of diversity to determine the characteristic species that one thinks, will play a role in the floristic dynamism to replace this fougeraie.

\section{Material and methods}

\subsection{Study area}

Our study is carried out in the Kahuzi-Biega National Park, with an area of 6000 km$^2$ (Fig. \ref{figure_2}), in its middle and high altitude floors, situated in the Albertine Rift or Africa's Western Rift Valley is one of the most biodiversity regions of the African continent \cite{Mangambu_2017, Mangambu_VanDiggelen_2017}. This Park was established in 1970 to protect eastern lowland gorillas (\textit{Gorilla beringei graueri} Matschie) and their habitat \cite{Fischer_1996}. It includes two main peaks that are non-active volcanoes (extinct), the Kahuzi at an altitude of 3308 m and the Biega of 2790 m, and is located in the southern part of the Kivu-Ruwenzori mountain range, NNE-SSW axis, skirting, in the west, the Albertine Rift \cite{Webster_1995}, between the two centers of endemic Guineo-Congolese and Afro-Mountainous \cite{Mangambu_2013}.

According to their physiognomy and their floristic composition, according to the altitude \cite{Mangambu_2016}, the forests of the PNKB are differentiated in planetary stages (678--1250 m), sub-montana (1250--1700 m), mountainous (1700--2600 m) and afro-subalpine (2600--3326 m). As a whole, the study area benefits from mountain climates (such as Cf K\"oppen), with abundant rains oscillating between 1750--2000 millimeters per year \cite{Mangambu_2016}. Mount Kahuzi, with nocturnal frosts, has an Afro-Alpine climate. Atmospheric humidity is constantly elevated in parallel with the change in cloud cover. The duration of the dry season does not exceed two months \cite{Fischer_1996}. The temperature varies according to the altitude, and the soil is superficial and acidic \cite{MangambuEtAl_2013}.

\begin{figure}
\centering
\includegraphics{figure_2.pdf}
\caption{Map of Kahuzi-Biega National Park (On the Right, Mangambu Et Al. 2010, Amended), and the Location of the Surveys Made in the High Altitude Part (Medium and High Altitude on the Left).}
\label{figure_2}
\end{figure}

\subsection{Methodology}

\subsubsection{Sampling layout}

The sampling device consists of 10 phytosociological surveys at altitudes ranging from 1531 m to 2452 m altitude (Table 1). In the field, the directly perceptible ecological factors were mentioned as well as the geographic coordinates provided by the GPS for each survey site. The locations of the survey sites were established using GPS using the UTM system (see Table 1). In an attempt to confirm the later dynamics of the vegetation in the clusters and to indicate the scarcity of trees in the surveys, all dbh "Diameter at breast height" trees and shrubs $\geq$10 cm were measured based on measurements of circumference taken 130 cm above the ground.

\begin{table}
\centering
\caption{Location of the Readings and Survey Area}
\label{table_1}
\begin{tabular}{c l c c c c c}
\hline
N$^{\circ}$ & Toponyme & Lat. & Long. & Elevation (m) & Area (m$^2$) & Slopes (degree) \\
\hline
1 & Kamosso: in a north-east direction, next to the river & 643933 & 9714720 & 1531 & 324 & 5 \\
2 & Kabona following the south-west orientation in the middle of the forest & 655051 & 9716110 & 1601 & 324 & 2 \\
3 & Ihembe following the south-west orientation on the mainland in the middle of the forest & 669384 & 9716751 & 1672 & 250 & 0 \\
4 & Kanoiso & 680398 & 9724234 & 1478 & 250 & 0 \\
5 & Tshivanga: in a south-east direction near Musisi & 690110 & 97477 & 2189 & 324 & 3 \\
\hline
\end{tabular}
\end{table}