I will make a try:
The branch is like one of his arms, but also a part of plant. So every plant may recover itself even iy cut with:
Time:
I guess time will heal all wounds, and for Bob time doesn´t matter at all. He might be old, ancient or maybe just a sapling in his tree-age. But if his wounds are healed, time is a important key of a tree.
Syntrophy:
The ashes from Bob will give a very good fertilizer, you know, from ashes to ashes, from ashes to tree. Bob will be part of his self, for eternity.
Cyclicity:
Every beeing has to died, otherwise there will be not space and not "fertilizer" for another ones. Like Bob´s branch is broken, there no might be enough space for 2 new branches, even if they are smaller as the beginning.
Imagination:
Believe of Bob´s "incarnation", without believe, this is only a tree - but if you have faith then call him Bob! This is like the spiritual energy for the recovery.
Balance:
Even Bob will be balancing something in this world, his pain is someone elses pleasure and vice versa.
Spells:
I guess a spell might call him, but if he response...?!
So all together there should be a bunch of believe and positive energy into the branch, fertilized with his own ashes, and then given an protected while aging. And if you if this branch comes back to life again, you might have a clue...