\documentclass{article}
\usepackage[linguistics]{forest}
\forestset{barrier/.style=
{tikz={\node[draw=red,circle,thick,fit=(),inner sep=0pt,outer sep=0pt]{};}},
default preamble={for tree={inner sep=0pt,s sep=.25in}}}
\begin{document}
\begin{forest}
[AP [BP [B ]] [CP,barrier [C ]]]
\end{forest}
\end{document}