Rooted trees with labelled leaves

Depending on how many of these you have, this solution may do what you want. Adjusting the position of the brace and its label is a bit of trial and error. I don't know if there's a way to make this more automatic.

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}