So I find out that rich:tree has an attribute "selection". It has to be bound to Collection<org.richfaces.model.SequenceRowKey>. I still can't find any documentation for class org.richfaces.model.SequenceRowKey. So I still don't know, if I know which tree nodes have to be selected and no idea of their sequence row keys, how to add them to this collection?
You can define an empty Set<SequenceRowKey> in your bean, and make listenerSelectionChanged to output it content to see how it is filled.
A SequenceRowKey must be filled with keys of nodes from the top node to the leaf, that need be marked as selected.