\documentclass{article}
\usepackage[linguistics]{forest}
\forestset{%
barrier 1/.style={%
tikz={%
\node[draw=red, circle, thick, fit=(), inner sep=0pt, outer sep=0pt] {};
},
},
barrier 2/.style={%
before drawing tree={%
draw=blue,
circle,
typeset node,
},
},
default preamble={%
for tree={inner sep=0pt, s sep=.25in},
},
}
\begin{document}
\begin{forest}
[AP [BP [B ]] [CP, barrier 1 [C ]]]
\end{forest}
\begin{forest}
[AP [BP [B ]] [CP, barrier 2 [C ]]]