" As oaks age they typically lose much of their inner xylem tissue, creating large hollow spaces within their trunks that serve as home to countless creatures, from rare fungi to raccoons, opossums, squirrels, bats, bobcats, and even black bears. We have been led to think that once there are hollow spaces created by rot within a tree trunk, that tree must come down. Not so! Such “rot” is normal and does not affect the living cambium that lies just under the bark of your oak nor the functional strength of the trunk. Hollow trunks are just one feature of ancient oaks that makes them such valuable ecological additions to our landscapes. Beating "

― Douglas W. Tallamy , The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees