A knotty solution

Knots show where the tree lost a branch and tried to seal the wound. All kinds of events can cause the tree to lose branches, from high winds to ice storms and city tree trimmers. But the tree can even do it to itself, as it does some self-pruning.

The larger and broader a tree becomes, the more the upper levels of foliage shade the leaves farther down and keep them from doing their job. So it sheds the upper leaves. And what about the poor, naked branch, which uses resources without contributing anything? It falls off, too. A cork plug will replace the useless branch. Eventually, all that will remain is a knot, marking the place where a proud branch once grew.