答案1
根据您拥有的数量,此解决方案可能会满足您的要求。调整括号及其标签的位置需要反复试验。我不知道是否有办法使这个过程更加自动化。
\documentclass{article}
\usepackage{forest}
\usetikzlibrary{decorations.pathreplacing,calc}
\forestset{solid nodes/.style={for tree={circle,draw,inner sep=1,fill=black}},
dir/.style={for tree={grow=#1}},
leaf/.style={label=$#1$},
mytree/.style={solid nodes, for tree={grow=north,s sep=1cm}}}
\begin{document}
\begin{forest}mytree
[[
[,dir=north east
[,leaf=\mu,name=X] [,leaf=\mu,name=Y] [,leaf=S_2]
]
[,dir=north west
[,dir=north east
[,leaf=\mu_2] [,leaf=\mu_1]
]
[,leaf=S_1]
]
]]
\draw[dotted] (Y) -- (X);
\draw[decorate,decoration={brace,raise=5pt}] ($(Y)+(0,.35)$) -- node [pos=0.8,above=2.5ex] {$e$} ($(X)+(0,.35)$) ;
\end{forest}
\end{document}