XML is playing an increasing role in data exchanges and the volume of available resources is thus growing dramatically. As they are heterogeneous, these resources must be translated into a {\em mediator} schema to be queried. For this purpose, automatic tools are required. These tools must allow the extraction of common data structures from the tree-like XML data. In this paper, we present a novel approach based on a low memory-consuming representation which can be improved by considering a binary representation. We show that these representations have many properties to enhance subtree mining algorithms, especially when considering soft tree embedding constraints. Experiments highlight the interest of our proposition.