I had to draw rooted (upward oriented) trees (with leaves). I found the package forest useful, although most likely the work-arounds I employed are not really optimal in any proper sense. An example of such a tree may be the following.
\documentclass[a4paper,10pt]{article}
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{forest}
%opening
\title{}
\author{}
\date{}
\begin{document}
\begin{center}
\begin{forest}
for tree={grow'=north, circle, draw, minimum width=0.7cm,anchor=center,fit=rectangle,
edge={>=stealth,-,shorten >=1pt,shorten <=1pt,line width=0.5pt},
scale=0.75,font=\sffamily\small\bfseries}
%
[, node options'
[$l$, edge label={node[midway,right,font=\scriptsize]{a}}
[$l'$, edge label={node[midway,left,font=\scriptsize]{b}}
[, node options', edge label={node[midway,left,font=\scriptsize]{e}}]
[, node options', no edge
]
[, node options', edge label={node[midway,right,font=\scriptsize]{f}}]
]
[, node options', no edge
]
[, node options', edge label={node[midway,left,font=\scriptsize]{c}}
]
[, node options', no edge
]
[$l''$, edge label={node[midway,right,font=\scriptsize]{d}}
]
]
]
\end{forest}\end{center}
\end{document}
Question. How can I draw an arrow from one rooted tree to another one, as it is done (using xypic) at page 7 of this paper [1]?