Millions of you have seen it in the movie, or read about it in Harper Lee's book: the big old tree in To Kill a Mockingbird, where reclusive "Boo" Radley left presents for Scout in a hole in the trunk.
While I'm a strong proponent of growing the healthiest trees possible, there can be magic in the imperfections of old trees. Besides providing hidey-holes for stashing treasures, hollow trees are prime nesting sites for woodpeckers and many other species of birds, as well as squirrels and other small mammals.
At least one rotten tree I knew went well beyond nesting-site status. It was merely a six-foot, one-sided snag of cambium layer and bark, with a single short, gnarly branch. For more than thirty years, this remnant of a once large specimen produced enormous blackjack oak leaves—proof that trees really do want to live, given half a chance. (Sadly, the snag was finally clipped by a truck and toppled.)
I admit, I do have a soft spot for needy plants. But there is a limit...and that limit comes up hard against safety. A hollow tree that might fall on a neighbor's property, or otherwise threaten property or lives, is a serious candidate for removal. And if a tree is downright ramshackle, with major dead and dying limbs that make an arborist shake his head, then it needs to come down. (I recently spotted one tree in my community with a hollow that went all the way through the trunk at the base—large enough for a small child to crawl through.)
[Note: for information on the legal liabilities involved with ramshackle trees, refer to the book, Weatherproofing Your Landscape: A Homeowner's Guide to Protecting and Rescuing Your Plants.]
Of course, a hollow tree that you've chosen to keep as long as possible needs extra care and attention. An annual inspection by a certified arborist will help assure that your tree isn't infested, diseased or unstable. While he's at it, the arborist can prune away weak limbs and keep the tree as sturdy as possible. Even so, I certainly wouldn't park my favorite birdbath—much less, my vehicle—beneath the limbs of a hollow tree, especially when bad weather threatens.
All that said, a hollow but still-stable tree is both a treasure and a delicate balancing act—in a way, a fitting symbol of our own life condition. In an age where perfection is the ideal, I rather fancy the idea of celebrating imperfection. And so, apparently, does Nature.
These days, I can stand at my door from spring through early summer and watch birds that are in a family way come and go from a hollow tree in my front yard. So far, the old oak tree remains robust—last spring, it withstood 60 to 90 mph winds without losing so much as a twig.
Of course, with their protective shield of bark compromised, such trees are always vulnerable to invasive organisms and other maladies that can bring them crashing down. But if the old post-oak snag was any indication, this tree could be around for a long, long time.