Q: I have a 100 year old white oak. It has a few dead limbs, but most limbs are healthy and have leaves. To slow the demise of this old oak should I fill in the hollow with dirt or leave it open? Anything I can do to help prolong its life?
A: You can’t do much to prolong the life of the tree. The missing bark once covered the system of tubes (phloem) that carried sugars from the leaves down to the roots. With them now gone, that side of the tree will suffer greatly.
Furthermore, the hollow under the tree makes me suspect there is considerable hidden rot. If that is the case, the tree is a hazard and should be removed.