Quasi-actions whose quasi-orbits are quasi-isometric to trees

We give necessary and sufficient conditions under which a quasi-action of any group on an arbitrary metric space can be reduced to a cobounded isometric action on some bounded valence tree, following a result of Mosher, Sageev and Whyte. Moreover if the quasi-action is metrically proper and quasi-orbits are quasi-isometric to trees then the group is virtually free.