This is really nice, Love the attention to detail on the foiliage!
I think maybe the main tree in the scene may benefit from a darker tone of brown for the diffuse texture? but that might just be me.
But i think you should definatley lower the amount of cracks and imperfections on the root section of the tree, as roots are usually smoother than the trunk
Also, maybe some sections of moss growing over the roots and lower sections of the trunk could be a good addition to this scene, It would make the focus point more interesting, and also make the tree look like its been stood there for a good while
Fantastic work though man, this is much better than anything i have ever tried to do with vegetation!