\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{fit}
\usepackage{forest}

\begin{document}
\begin{minipage}{0.3\textwidth}
\begin{forest}
[a
  [b
    [c]
    [d]
  ]
   [e,draw,dashed]
]
\end{forest}
\end{minipage}%
\begin{minipage}{0.3\textwidth}
\begin{forest}
[f
  [g]
  [,phantom]
  [h,tikz={\node [draw,dashed,remember picture,fit=() (!1) (!l)] {};}
    [i]
    [j]
  ]
]
\end{forest}
\end{minipage}
\end{document}

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{fit}
\usepackage{forest}

\begin{document}
\begin{minipage}{0.3\textwidth}
\begin{forest}
[,phantom,s sep=1in
[a
  [b
    [c]
    [d]
  ]
   [e,draw,dashed,name=e]
]
[f
  [g]
  [,phantom]
  [h,name=h
    [i,name=i]
    [j,name=j]
  ]
]
]
\node (H) [draw,dashed,fit=(h) (i) (j)] {};
\draw (e.east) -- (H.west);
\end{forest}
\end{minipage}
\end{document}