In Harper Lee's To Kill a Mockingbird, what ends the knothole gifts?

In To Kill a Mockingbird, by Harper
Lee, the children have been finding mysterious gifts left in the hole in the tree near
the Radley's property. It puzzles and perhaps frightens them a little at first. (Jem
gets worried when Scout eats a piece of gum left there, thinking it might be poisoned.)
Then they think it might be someone's hiding place. Leaving a ball of grey string in the
hole for a couple of days find it umoved: so Jem doesn't think it's a hiding place. From
then on, the kids believe that whatever is left there is their
property.

They find some more gum (an entire pack), two
well-carved (or so says Jem) figures of them—made from soap—and even a broken watch and
chain. The tree is something they look forward to checking each day coming home from
school.

The kids decide to leave a note for the mysterious
"gift giver," but when they approach the tree, they see the whole has been filled with
cement. Jem soothes Scout, telling her not to cry. When they see Mr. Radley and ask
about the tree, he says that the tree was dying. The children ask Atticus about the
tree's health and he says it looks fine, but hearing Mr. Radley explanation, he decides
that Mr. Radley knows best about the health of his trees, and the discussion is
dropped.