We used to feed "willow" to our beaver. Not sure if it is the same kind since neither of us have a Latin name for our "willow", but if your kind is the same kind as my kind, then it seems to me that it would not be toxic. Having said that, I have no experience using that as a substrate under a tortoise, so I can't recommend for it or against it.
So I got my response on tom post about fir bark. Next I’ll have to find cypress mulch I guess, I think fir bark might be more availlable on the west coast?