The film makers CGI managed quite well. Treebeard the Ent looked quite convincing but then he didn't have a spreading canopy of leaves.
I think you would have to postulate quite a neat set of branches otherwise your ent would be unstable and the gait would be ridiculous. The trunk would be the heaviest and lowest part of the Ent apart from the mobile roots. One thing that could change is the interaction of roots and earth, if you postulate variable adhesion the the biomechanics becomes simpler (wouldn't like to attempt it now though).