How can I use this math mode pictogram (tree + leaf)?

In an article, I saw an equation with a pictogram where one might typically find a Latin letter: How did they do it? How can I replicate this equation in my own document?

If you want to use other symbols in order not copy the one from your PDF, you may get some inspiration here. There are many symbols included in the Unicode 6.0.

% arara: lualatex

\documentclass{article}
\usepackage{fontspec}
\usepackage{fontawesome}
\def\faTree{\FA\symbol{"F1BB}}

\begin{document}
\faLeaf
\faTree % not yet part of the package
\setmainfont{symbola.ttf}
\symbol{"1F331}
\symbol{"1F332}
\symbol{"1F333}
\symbol{"1F334}
\symbol{"1F335}

\symbol{"1F33F}
\symbol{"1F341}
\symbol{"1F342}
\symbol{"1F343}
\symbol{"1F384}
\symbol{"2E19}
\end{document} % arara: lualatex

\documentclass{article}
\usepackage{fontspec}
\usepackage{fontawesome}
\usepackage{mathtools}
\usepackage[bold-style=ISO]{unicode-math}
\usepackage{graphicx}
\newcommand*{\twig}{\reflectbox{\rotatebox{35}{\setmainfont{symbola.ttf}\symbol{"1F33F}}}}
\newcommand*{\leaf}{\ensuremath{\text{\faLeaf}}}

\begin{document}
In the following formula I will use a \twig{} and a \leaf{} combined to $\twig^\leaf$:
$\mbfitY=f(\mbfitX)+\mbfscrE\approx\twig^\leaf_1(\mbfitX)+\twig^\leaf_2(\mbfitX)+\ldots+\twig^\leaf_m(\mbfitX)+\mbfscrE,\quad\mbfscrE\sim\mscrN_n(\mathbf{0}, \sigma^2 \mbfitI_n)$
\end{document} • Excellent, thank you. I actually don't like their particular choice of symbol because I feel they're too "chunky" and "round" and break up the flow of the equation. However I really like the idea of using symbols aside from the Greek/Latin alphabets, so a general solution is even better. I could also probably mess around with font sizes and macros to make these less conspicuous. – shadowtalker Sep 17 '14 at 11:56

The one symbol can be found in the package textcomp as \textleaf. In your preamble, just enter

\usepackage{textcomp}

and in the body of the document write \textleaf.

The other symbol can be found in the package phaistos as \PHplaneTree.

\documentclass{article}
\usepackage{textcomp}
\usepackage{phaistos}
\newcommand\myleaf{\mbox{\textleaf}}
\newcommand\mytree{\mbox{\PHplaneTree}}
\pagestyle{empty}
\begin{document}

$\mytree^{\myleaf}$

\end{document} I found that I had to put the two objects in their own boxes to get these symbols to work in math mode.

From the command line you can type

texdoc symbols-a4

to get a master document showing a variety of different symbols and the packages which provide them.