"They do," Bran said with sudden certainty. "They dream tree dreams. I dream of a tree sometimes. A weirwood, like the one in the godswood. It calls to me. The wolf dreams are better. I smell things, and sometimes I can taste the blood."

A few pages later (p. 58), Osha suggests that the gods are trying to talk back to him in his dreams. Certainly, it doesn't seem unlikely that something or someone -- perhaps the Children of the Forest -- are trying to contact Bran through his dreams, and perhaps through the weirwood as well.