Could you give us some more information about the dog?
How old?
Male or female?
Neutered?

Do the trees have mulch around them--the reason I ask, is that we just had our yard done this year, and they loved the mulch--there's something in it they can't resist!!

What kind of dog is it?

Is he bored? Does he have other toys to play with in the yard instead of digging up trees?