The best thing to do would be to have a certified arborist come out to assess the health of your tree on site. They can check the extensiveness of the decay you see at the base of the trunk, and they can also assess the overall health of the crown growth. With an on-site evaluation, they can give you options for treatment, or assistance to take the tree down safely, if needed.

Arborists are certified by the International Society of Arboriculture and have knowledge of tree health care including insect and diseases. You can search for an arborist in your area by using the following link: https://www.treesaregood.org/

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