Jem turns "stark white" when he first discovers that the once secret knothole in the Radley oak has been sealed with cement. Jem waits patiently for Boo's brother, Nathan, to pass by, and when he sees him, he questions him about the tree. According to Nathan,
"Tree's dyin'. You plug 'em with cement when they're sick. You ought to know that, Jem." (Chapter 7)
But Jem soon finds that Boo's brother isn't being truthful with him. When Jem asks Atticus about the tree, his father points out that
"... the leaves, they're all green and full, no brown patches anywhere..."
"It ain't even sick?"
"That tree's as healthy as you are, Jem." (Chapter 7)
It becomes obvious that Nathan Radley has discovered Boo's hiding place--he must have witnessed either Boo or the children placing or retrieving objects from the knothole--and he decides to eliminate the tree from any further usage for contact between them. Why Nathan finds this so important is unknown, but he may think that only trouble can come from Boo and the children trading gifts--and possibly becoming friends. It is an act that leaves Jem in tears, and one can only hope that Nathan's decision is meant to protect Boo and the children, and not just an act of cruelty.
he did that because he knew that it was a way that Boo communicated with Scott and Jem