How to draw a curve to indicate syntactic domain in the qtree?

Question

How to draw a curved line in the qtree and add a description like the following picture?

\Tree [.n,v,aP [.n,v,a ] [.$\sqrt{P}$ [.$\sqrt{\alpha}$ ] [.$\sqrt{\beta}$ ] ] ] 

Thanks!

Answer 1

Using forest or tikz-qtree is definitely the way to go for trees. I would recommend forest since it allows you to name nodes very simply which then allows all sorts of annotations of the sort you need to be made quite simply. tikz-qtree can do the same sort of thing, but you need to encode the nodes specifically in the tree, which makes the code much more clunky.

Here's both solutions, using the answer here:

• In forest (tikz) for syntax trees, how do I create an arc above one node separating it off from the rest of the tree?

And here's another couple of solutions, also using forest:

• Regarding drawing phase boundaries on syntactic trees

\documentclass{article}
\usepackage{tikz-qtree}
\usepackage[linguistics]{forest}
\usetikzlibrary{calc,positioning}
\begin{document}
\begin{tikzpicture}
\Tree [.n,v,aP [.n,v,a ] [.$\sqrt{P}$ [.\node (A){$\sqrt{\alpha}$};] [.$\sqrt{\beta}$ ] ] ] 
\draw 
  ([xshift=-14pt]A) arc[start angle=180,end angle=60,radius=2cm] node[below] {Spellout domain};
\end{tikzpicture}

\begin{forest}
[{n,v,aP} [{n,v,a} ] [$\sqrt{P}$ [$\sqrt{\alpha}$,name=A] [$\sqrt{\beta}$ ] ] ]
\draw 
  ([xshift=-14pt]A) arc[start angle=180,end angle=60,radius=2cm] node[below] {Spellout domain};
\end{forest}
\end{document}