Based in my reading of the books (Clash of Kings, Chapter 28 Bran IV) not my understanding of the show (which I rarely understand, and which never took the time to explain how it all works). Maybe, as I suggested later in the piece, my problem is in trying to make sense of the show at all. Here is the passage:

“The First Men believed that the greenseers could see through the eyes of the wierwoods. That was why they cut down the trees whenever they warred upon the children.”

Greenseers might also been thought to be able to have Green Dreams which allow them to see (in dreams) through the eyes of other people the events of the past (it is not physically being present in the reality of the moment — more like VR in a dream state).