With some trees, the knotholes are the last things to go. You can find them staring up from the ground, eye sockets that never belonged to a skull.
It makes sense that trees would grow their hardest wood around the weakest points in their architecture. This is called the branch collar, and it is knit with wood first from the branch overlapping onto the trunk, and then from the trunk overlapping onto the branch.
Behind the collar, in the parent trunk or limb, the branch core forms. As the Wikipedia entry on branch collars puts it,
The accretion of layers of wood behind the branch collar is a conical decay-resistant structure called the branch core. The knot found in lumber is this branch core.
When woody plants naturally shed branches because they are nonproductive, usually from lack of light, these branches die back to the branch collar. Insects and fungi decompose the dead branch, and it eventually falls off, leaving the exposed branch core. The branch core resists the spread of decay organisms into the parent branch or trunk during the time it takes for the woundwood, or callus, to seal over the wound.
There are intergrown and encased knots, loose and sound and pin knots, red knots and black knots. Whatever you call them, though, they can’t be untied. So they are not really knots, but heads — what else goes through a collar? — dense, convoluted, and all too easy to lose.
For the Festival of the Trees.