As XML documents are increasingly being used in a wide variety of applications and often people work in teams distributed across space and time, it is very important that users are supported for editing collaboratively XML documents. Existing tools do not offer appropriate support for the management of conflicting changes performed in parallel on XML documents. In this paper we propose a merging mechanism that offers users the possibility to define conflict nodes prevented from integration of concurrent changes. Changes referring to non-conflict nodes are automatically merged, while users are assisted to manually merge changes referring to conflict nodes. Changes are tracked by means of operations associated to the nodes they target and merging relies on an operation-transformation mechanism adapted for hierarchical structures.