Presumably, the bark is less pure cellulose than the wood, cellulose
ought to burn with no ash, going to CO2, H2O. The bark probably has
lots and lots of compounds in it, I don't know what myself, but
anything with Na, K, Ca, Mg would presumably leave ash.

P.S. For some of those who needed to make wisecracks about a
more-or-less reasonable question, you are a much worse contributor to
dreck on this board than Archimedes. I'll take this question over
babbling about RELATiViTY that doesn't have anything to do with
chemistry anyways, any day of the year.