We define a separation logic (BI-Loc) that is an extension of the Bunched Implications (BI) logic with a modality for locations. Moreover, we propose a general data structure, called resource tree, that is a node-labelled tree in which nodes contain resources that belong to a partial monoid. We also define a resource tree model for this logic that allows to reason and prove properties on resource trees. We study the decidability by model checking of the satisfaction and the validity in this separation logic. This model is extended with quantifications on locations and paths and (un)decidability results are given for such extensions. Moreover, we relate the logics and the models to the specification and verification of properties of XML and semi-structured data.