Answer: Bark splitting is usually the result of winter sun directly hitting frozen tissue. The sun expands the inner tissue and the bark splits because of the expansion. There's not much you can do once it has happened - healthy trees will callous over the wound all on their own, but you can try to prevent it by using a tree trunk wrap (usually perforated hard plastic that you wrap loosely around the trunk). All you can do at this point is wait and watch. I hope you see some improvement this summer as the tree attempts to close the injury.
Best wishes with your ash tree!
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