Attached is what I've managed thus far for the Fangorn style Ancient Forests. I couldn't get your mesh to split into the 4 pieces I needed using what I know in NifSkope, so I just swiped the DDS and applied it to some resize/relocated basic trees. The overall feel seems the same to me, but I haven't spent long with the base Fangorn to say that with any real clout. End result though is that the forests are very distinct from their neighbors and behave properly with coasts, rivers, roads and most improvements.
The trees look great but they are to tall and thick. You can't see the units in them. Is it possible to less blocking version?