I would leave it alone and monitor it - I think it will just callous over and be fine, but obviously you want to make sure that is what is happening. If you start seeing this spot growing bigger it may be worth applying some anti fungal/antibacterial/insecticide, but right now it does not look like it will need it.
I had to move a much smaller A. dichotoma this spring and the base got damaged quite a bit worse. I won't know till the Fall if the move will be successful - it is trying to survive through the summer while not quite rooted in very well, but if it does not make it, I am pretty sure it will not be because of the damage to the trunk.
