Mystery Acer Problem

[Mike Green]

Having received an Acer Katsura recently as a present I was disappointed to find that there appears to be a mysterious problem with its stem/trunk (see photo).
After endless research I have not identified what is issue is or how to cure it (if possible!). Any help would be appreciated!

 

[Raintree]

Rodent wounding, see little teeth marks, approx. year old. Leave it alone & expose the root collar.

 

[Jace]

And a mesh guard or something to protect it from the rodents