HollowTree.jpg
If I wanted to create what I think you want to create, this is what I would do:
1) Create two fractal polygons, identical in shape, with the inner polygon about 95% the size of the outer polygon.
2) Fill the outer polygon with a bark texture.
3) Fill the inner polygon with a wood texture.
The graphic above is a quick-and-dirty (5 minutes) sample made with FM8.