Mountain Ash - Completely girdled but doing ok?

Hi All - I have a Mountain Ash that I am not too fond of but given that it is a mature 25 ft tree with some decorative appeal, I am resisting cutting it down. That being said, if it is damaged and soon to be in decline, it would give me a good excuse to get rid of it.

I was digging around the base a few days ago and noticed what looks like pretty significant trunk damage. The previous homeowers have planted a number of the trees in the yard much too deeply which might have contributed to the damage. The tree, however, seems to be doing ok, though its growth is fairly slow, its bloom mediocre this year, and its fruit production seemingly less robust than in previous years. To me, it looks like the trunk damage has completely girdled the tree. What do you think? I've attached a pic.

 

Looks like it was filled around after being there for some time. Is the affected area soft?

 

Hi Ron, yes, it is soft, and yes, I'm pretty sure the previous homeowners filled in around it to level out the yard (I have a cherry now planted 2 feet too deep!).

 

If the crown has declined markedly you should probably cut it down.