The tree always existed. They weren't self-aware about it or their history until they were anchored to the present. Imaginary Scramble explains how a 'real' person entering the fiction makes it real, or something similar. It's been a while but when the chapter dropped, we discussed if that was what happened with the Crypters and Lostbelts. But in LB6, PHH Morgan traveled back which was another reality to fiction invasion to spread self awareness in the former script. Witches seem to have some connection with fantasy, with the ploys.
Perhaps the existence of Archetypes in lb7 also changed this.