Glyn Burgess suggests the branch is merely a signal Tristan has already told Iseult about in an earlier message; the poet indicates that Iseult would be on the lookout for the branch, "for this had all happened before".
Glyn Burgess implies that the branch is likely something Tristan has already shared with Iseult in a previous conversation; she will be vigilant about it since similar events have transpired previously.