Just realized, you can actually put a tree in a \node. One important thing to note is that you must have a space between the last part of the tree and the closing }; of the node (perhaps obvious to you, but it caught me at first).
\documentclass{article} \usepackage{linguex} \usepackage{qtree} \usepackage{multicol} \usepackage{tikz} \begin{document} \begin{multicols}{3} \ex. \small \a. \begin{tikzpicture} \node (a) {\Tree [.DP [.NP ]. NP ].DP }; % space before }; is necessary \draw (a.south) -- +(0,-0.5) node[below] {[bla]}; \end{tikzpicture} \b. \begin{tikzpicture} \node (a) {\Tree [.DP [.NP ]. NP ].DP }; \draw (a.south) -- +(0,-0.5) node[below] {[blu]}; \end{tikzpicture} \c. \begin{tikzpicture} \node (a) {\Tree [.DP [.NP ]. NP ].DP }; \draw (a.south) -- +(0,-0.5) node[below] {[bli]}; \end{tikzpicture} % an empty line before \end{multicols} is necessary \end{multicols} \end{document}

Original answer
If switching to tikz-qtree is an option, you can draw the arrow in the same tikzpicture as the \Tree:

\documentclass{article} \usepackage{linguex} \usepackage{tikz-qtree} \usepackage{multicol} \usepackage{tikz} \begin{document} \begin{multicols}{3} \ex. \small \a. \begin{tikzpicture}[baseline] \Tree [.DP [.NP ] [.NP ] ] \draw [->] (current bounding box.south) -- +(0,-0.5) node[below] {[bla]}; \end{tikzpicture} \b. \begin{tikzpicture}[baseline] \Tree [.DP [.NP ] [.NP ] ] \draw [->] (current bounding box.south) -- +(0,-0.5) node[below] {[bla]}; \end{tikzpicture} \c. \begin{tikzpicture}[baseline] \Tree [.DP [.NP ] [.NP ] ] \draw [->] (current bounding box.south) -- +(0,-0.5) node[below] {[bla]}; \end{tikzpicture} \end{multicols} \end{document}
tikz. :)picture, I think. At least, my hack usespictureand I started fromqtree's code, so it certainly usedpicture4 years ago.