Positioning two forest trees side by side as numbered gb4e subexamples

Question

I am adjusting the tabularx solution to position two syntax trees side by side, as numbered subexamples.

The problem I am having is too much whitespace between the second-level label ("a.") and the tree, particularly the first one. This space only appears if I add the tabularx environment from the solution, I do not have that issue with the forest and the gb4e packages on their own and in combination. I seem to have enough space on the page for both tree examples to fit nicely; is there a way I could do that with these packages? As it is, the positioning is weird.

MWE:

\documentclass[11pt]{article}
\usepackage{fontspec}
\usepackage[margin=1truein]{geometry}

% The below is a standard package setup for syntax trees.
\usepackage{forest}
\useforestlibrary{linguistics}
\forestapplylibrarydefaults{linguistics}

% This package is for numbered linguistic examples.
\usepackage{gb4e}

% The following three packages allow me to position trees side by side and as numbered examples.
\usepackage{tabularx}
\usepackage{array}
\usepackage{environ}

% The following code is later used to to position trees side by side and as numbered examples.
\newcolumntype{T}{>{\refstepcounter{tabenum}}X}
\newcounter{tabenum}[xnumi]
\renewcommand{\thetabenum}{\thexnumi\alph{tabenum}}
\newcommand*{\tabex}{\relax\alph{tabenum}.\quad}
\NewEnviron{multiexe}{%
    \setlength{\extrarowheight}{1ex}
    \begin{tabularx}{\linewidth}[t]{@{}TTT@{}}
       \BODY
    \end{tabularx}
}

\begin{document}

\begin{exe}
\ex
 \begin{multiexe}
 \tabex 
\begin{forest}
[S[NP[D\\A][N\\cat]][VP[VP\\slept][PP[P\\on][NP[D\\the][N\\mat]]]]]
\end{forest} 
\label{tree1} &
\tabex 
\begin{forest}
[S[NP[D\\A][N\\cat]][VP\\slept]]
\end{forest} 
\label{tree2}
 \end{multiexe}
\end{exe}


\end{document}