I've lost many trees before to bark stripping, but never expected this from my Alma.
Earlier this year around January, while the tree was dormant, I saw the extensive damage and was unable to use the opportunity to make an air layer and let it go. I didn't expect this.
These photos are taken today.

A vole will do that.

I lost my black Madeira and my jack lily this year to this. What causes this to happen? They were in ground other in pot

Trying to figure out of why this tree recovered while others didn't.

Wild rabbits will strip bark.I have caught them in the act.They will do it in the summer and winter.

A cheap and easily solution that works great for me for my outside overwintered plants is a piece of tin foil wrapped around the bottom 8". Its easy to form around branches. I had quite extensive mice/vole damage in my greenhouse one spring and as soon as I started using the tin foil it stopped.

The sapwood must have been thicker than the others. It will give out eventually since no new sapwood or bark is being made, but the tree will live on through that sucker.

Hi
I see some space to add dirt in the pot.
I would add some dirt, let it mais roots and then cut it.
For the critters, no other choice but fight. Coule be a rabbit or a vole or ...
If the damage is done in milder days or nights the trees can cope with it better.
But nos that is a weak point... So I would try and do something...

Strap grafts could also help to repair the damaged area.

https://www.google.com/search?q=tree+trunk+repair+grafts&client=ms-android-sprint-us&source=lnms&biw=360&bih=512&prmd=sivn&tbm=isch&sa=X&ved=0ahUKEwiqwPzf8P_NAhUD8CYKHbWTDZAQ_AUICCgC