Not the best tree I admit though, it's a billboard.
I tried something different with trees to test what possibilities might exist to improve the rendered output from billboard trees!
1. Exported Onyx tree model
2. Imported to Maxwell, applied bark an leaf maps,
3. Rendered high res, low render time, sky dome,
4. Exported rendered image, alpha map, depth map (z buffer)
5. Created MXM (maxwell material) colour map, clip map, depth map as displacement,
6. Created SketchUp billboard with tree mapped and linked to MXM
7. Exported render.
Here is the result, it actually created I think far better depth than just flats.
The nice thing is the shadow cast on areas of the tree itself and thus the correct response to sun direction as opposed to normal billboards. And best of all low cost in sketchup.