reducing white space below tikz-qtree tree

Question

I am including syntactic trees in my dissertation. Below each tree there is a tremendous amount of white space, which will cause the university's editorial office to scream and howl and moan, then ultimately to reject my dissertation until I fix it. How do I remove the superfluous white space below the tree?

MWE:

\documentclass{article}
\usepackage{tikz-qtree,tikz-qtree-compat}
\usepackage{gb4e}
\tikzset{every tree node/.style={align=center, anchor=north}}
\begin{document}
The relevant structure in \ref{tree:opt-marker-verb-move} shows the necessary movement for the verb to precede the suffix. XP refers to some unknown structure to which the verb is drawn.

\begin{exe}
\ex
\leavevmode\vadjust{\vspace{-\baselineskip}}\newline %this command makes the example number line up with the top of the tree, rather than the bottom
\begin{tikzpicture}
\tikzset{every tree node/.style={align=center,anchor=north}}
\Tree [.XP \node(XP){}; 
[.\textsc{FocusP} =em 
[.TP
[.vP {} 
[.VP {} 
[.V \node(VERB){puv}; ] 
] ] ] ] ]
\draw[->] (VERB) [in=-90,out=-90,looseness=1.5] to (XP);
\end{tikzpicture}\label{tree:opt-marker-verb-move}
\end{exe}

It is difficult to justify this verbal movement. First, we would have to identify a higher structure in a suitable relationship with the verb to motivate verbal movement. Then we would have to identify features to check that exist on both the new structure and the verb.
\end{document}

Answer 1

One option is to use pgfinterruptboundingbox to interrupt the bounding box for the curved path; some manual spacing then will have to be added (or, see below for avoiding manual intervention):

\documentclass{article}
\usepackage{tikz-qtree,tikz-qtree-compat}
\usepackage{gb4e}
\tikzset{every tree node/.style={align=center, anchor=north}}
\begin{document}
The relevant structure in \ref{tree:opt-marker-verb-move} shows the necessary movement for the verb to precede the suffix. XP refers to some unknown structure to which the verb is drawn.


\begin{exe}
\ex
\leavevmode\vadjust{\vspace{-\baselineskip}}\newline %this command makes the example number line up with the top of the tree, rather than the bottom
\begin{tikzpicture}
\tikzset{every tree node/.style={align=center,anchor=north}}
\Tree [.XP \node(XP){}; 
[.\textsc{FocusP} =em 
[.TP
[.vP {} 
[.VP {} 
[.V \node(VERB){puv}; ] 
] ] ] ] ]
\begin{pgfinterruptboundingbox}
\draw[->] (VERB) [in=-90,out=-90,looseness=1.5] to (XP);
\end{pgfinterruptboundingbox}
\end{tikzpicture}\vspace{30pt}
\label{tree:opt-marker-verb-move}
\end{exe}

It is difficult to justify this verbal movement. First, we would have to identify a higher structure in a suitable relationship with the verb to motivate verbal movement. Then we would have to identify features to check that exist on both the new structure and the verb.

\end{document}

To avoid having to add spacing by hand, simply place anything sensible (an empty node, for example) below the lowest one after the bounding box has been interrupted:

\documentclass{article}
\usepackage{tikz-qtree,tikz-qtree-compat}
\usetikzlibrary{positioning}
\usepackage{gb4e}
\tikzset{every tree node/.style={align=center, anchor=north}}
\begin{document}
The relevant structure in \ref{tree:opt-marker-verb-move} shows the necessary movement for the verb to precede the suffix. XP refers to some unknown structure to which the verb is drawn.


\begin{exe}
\ex
\leavevmode\vadjust{\vspace{-\baselineskip}}\newline %this command makes the example number line up with the top of the tree, rather than the bottom
\begin{tikzpicture}
\tikzset{every tree node/.style={align=center,anchor=north}}
\Tree [.XP \node(XP){}; 
[.\textsc{FocusP} =em 
[.TP
[.vP {} 
[.VP {} 
[.V \node(VERB){puv}; ] 
] ] ] ] ]
\begin{pgfinterruptboundingbox}
\draw[->] (VERB) [in=-90,out=-90,looseness=1.5] to (XP);
\end{pgfinterruptboundingbox}
\node[below=of VERB] {};
\end{tikzpicture}
\label{tree:opt-marker-verb-move}
\end{exe}

It is difficult to justify this verbal movement. First, we would have to identify a higher structure in a suitable relationship with the verb to motivate verbal movement. Then we would have to identify features to check that exist on both the new structure and the verb.

\end{document}

Answer 2

You could use \useasboundingbox before the \draw command, to avoid it influencing the bounding box. Some extra space below must be added for the arrow, hence the [yshift=-1.2cm] (found by trial and error).

\documentclass{article}
\usepackage{tikz-qtree,tikz-qtree-compat}
\usepackage{gb4e}
\tikzset{every tree node/.style={align=center, anchor=north}}
\begin{document}
The relevant structure in \ref{tree:opt-marker-verb-move} shows the necessary movement for the verb to precede the suffix. XP refers to some unknown structure to which the verb is drawn.

\begin{exe}
\ex
\leavevmode\vadjust{\vspace{-\baselineskip}}\newline %this command makes the example number line up with the top of the tree, rather than the bottom
\begin{tikzpicture}
\tikzset{every tree node/.style={align=center,anchor=north}}
\Tree [.XP \node(XP){}; 
[.\textsc{FocusP} =em 
[.TP
[.vP {} 
[.VP {} 
[.V \node(VERB){puv}; ] 
] ] ] ] ]
\useasboundingbox (current bounding box.north west) rectangle ([yshift=-1.2cm]current bounding box.south east);

\draw[->] (VERB) [in=-90,out=-90,looseness=1.5] to (XP);
\end{tikzpicture}\label{tree:opt-marker-verb-move}
\end{exe}

It is difficult to justify this verbal movement. First, we would have to identify a higher structure in a suitable relationship with the verb to motivate verbal movement. Then we would have to identify features to check that exist on both the new structure and the verb.
\end{document}

Answer 3

I was able to eliminate most of the white space by fiddling with the out value in the draw command. I changed it from -90 to -180, and achieved the desired spacing.