TikZ forest - Drawing upon a subtree

Question

I would like to add a rectangle upon a subtree like in the following picture. How to do that ?

Here is the forest code.

\documentclass{article}
\usepackage{forest}
\useforestlibrary{linguistics}

\begin{document}
\pagestyle{empty}

\begin{forest}
    for tree = {
        sn edges,
        grow'=0,
        l=2.5cm,
        s sep=0.2cm,
        anchor=west,
        child anchor=west}
  [
    [O
      [R
        [T]
      ]
      [T
        [R]
      ]
    ]
    [R
      [O
        [T]
      ]
      [T
        [O]
      ]
    ]
    [T
      [O
        [R]
      ]
      [R
        [O]
      ]
    ]
  ]
\end{forest}

\end{document}

Answer 1

You can assign names to nodes in forest, so you can use this together with the Tikz fit library to draw a node around the nodes you have chosen.

If you don't want to use a node, you can also just do (doesn't require any library):

\draw[red,thick] (bottomright.south east) rectangle (left.west|-topright.north);

Output

Code

\documentclass[margin=10pt]{standalone}
\usepackage{forest}
\useforestlibrary{linguistics}
\usetikzlibrary{fit}

\begin{document}
\begin{forest}
    for tree = {
        sn edges,
        grow'=0,
        l=2.5cm,
        s sep=0.2cm,
        anchor=west,
        child anchor=west}
  [
    [O,name=left
      [R
        [T,name=topright]
      ]
      [T
        [R,name=bottomright]
      ]
    ]
    [R
      [O
        [T]
      ]
      [T
        [O]
      ]
    ]
    [T
      [O
        [R]
      ]
      [R
        [O]
      ]
    ]
  ]
\node[draw=red,thick,fit=(left)(topright)(bottomright)] {};
\end{forest}
\end{document}