On peut utiliser `anchor=north` :
\documentclass{beamer}
\usepackage{tikz}
\begin{document}
\begin{frame}
\frametitle{Occupation du site de Bibracte}
\begin{tikzpicture}[
scale=.68,
every node/.style={scale=.68},
quadri/.style={rectangle,draw,fill=white},
case/.style={text width=3cm,text centered,anchor=north,font=\large\bfseries},
myarrow/.style={-stealth,line width=5pt,blue}
]
% Cases
\node[case] (Ed) at (0,4) {Éduens};
\node[case] (Opp) at (0,0) {Oppidum de Bibracte};
\node[case] (Acc) at (-6,-4) {Voies d'accès};
\node[case] (Gis) at (-2,-4) {Gisements de minerais};
\node[case] (Eau) at (2,-4) {Présence d'eau};
\node[case] (Def) at (6,-4) {Altitude du site: défense naturelle};
% Flèches
\draw[myarrow] (Ed) to (Opp);
\draw[myarrow] (Opp) to (Acc);
\draw[myarrow] (Opp) to (Gis);
\draw[myarrow] (Opp) to (Eau);
\draw[myarrow] (Opp) to (Def);
\end{tikzpicture}
\end{frame}
\end{document}
![alt text][1]
---
Une version automatique :
\documentclass{beamer}
\usepackage{tikz}
\begin{document}
\begin{frame}
\frametitle{Occupation du site de Bibracte}
\centering
\begin{tikzpicture}[
scale=.68,
every node/.style={scale=.68,text width=3.5cm,text centered,anchor=north,font=\large\bfseries,color=black},
level/.style={sibling distance=3.5cm,level distance=3cm,-stealth,line width=5pt,blue},
]
\node {Éduens}
child {node {Oppidum de Bibracte}
child {node {Voies d'accès}}
child {node {Gisements de minerais}}
child {node {Présence d'eau}}
child {node {Altitude du site: défense naturelle}}
};
\end{tikzpicture}
\end{frame}
\end{document}
![alt text][2]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-05-28_at_21.54.25.pnghttps://texnique.fr/osqa/upfiles/Screen_Shot_2019-05-28_at_21.54.25.png
[2]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-05-28_at_23.11.57.png
