Posted: 05/11/2013 at 18:12
I had a similar problem with a young apple tree and a tree guard - the ants built a nest behind it and the mud caused the bark to rot. However, once it was exposed to the air it righted itself and healed over.
One tip I learnt from a neighbour is to wrap strips of black bin bag round the trunk over the damaged bark - like a bandage. We get deer, who occasionally decide to strip the bark from branches. The black plastic acts like a second skin and the tree keeps going (I have two magnolia stellatas which have been like that for three years now - each year I retie the bandage over a slightly smaller portion of the wound to let a bit more heal).