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?
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?
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.
As addition to A.Elletts perfect answer:
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}
