The Tree didn't get a lot of discussion in the first three books, by which I mean Agent of Change, Conflict of Honors, Carpe Diem.
The first published mention of the Tree as a physical presence is in Local Custom, and in Scout's Progress, we begin to understand that it's actually communicating.
But! Steve and I had taken a couple whacks at what eventually became the Crystal books back around the time the first three books were published -- we have a lot of partials from that period, like Shan and Priscilla Ride Again -- and Jela came with a Tree. So, we did know that there had been a Tree, and it amused me to suppose that there still was a Tree in the Dark Times, when Custom and Progress were written.
Re: the reality of the Tree
The Tree didn't get a lot of discussion in the first three books, by which I mean Agent of Change, Conflict of Honors, Carpe Diem.
The first published mention of the Tree as a physical presence is in Local Custom, and in Scout's Progress, we begin to understand that it's actually communicating.
But! Steve and I had taken a couple whacks at what eventually became the Crystal books back around the time the first three books were published -- we have a lot of partials from that period, like Shan and Priscilla Ride Again -- and Jela came with a Tree. So, we did know that there had been a Tree, and it amused me to suppose that there still was a Tree in the Dark Times, when Custom and Progress were written.