The bump on the right tree is great (though a bit too much perhaps), but on the left tree it's a bit repetitive. You could try adding another image map shader between part and default shader, calling another bark type (or knotholes, damaged bark, moss, whatever), set to UV as well, set a little bump also, repeated (don't know by heart if that's necessary), and mask that by a world space PF (experiment with contrast , noise and size). Gives nice variation.
