Girdled Ash tree
Can you see what is girdling the tree? Often it is a wire so band of some sort. Whatever it is, it much be removed.
Nothing visible on the surface as you can see. I'm concerned about digging down and around the tree... Any advice on how to approach finding the problem below the surface? I'm just a homeowner, never dealt with a problem like this. Thanks!
Dig around the trunk to see if there is anything underground that is causing the problem. Don't go any deeper than you have to.