I don't think the trees made the choice. The way I see things having gone down, he Forest was being decimated and someone (who later became a Forestal) stepped forward to rally the Forest to preserve itself. Spoiler
It's like the Ranhyn. They didn't choose the Ramen to serve and protect them. They were being destroyed by kresh and what not when the Ramen saw them and decided to do something about it. Afterwards, the Ranhyn decided to keep them as protectors...it was, afterall, in their best interests.
'Tis dream to think that Reason can
Govern the reasoning creature, man.
- Herman Melville
I am Lazarus, come from the dead,
Come back to tell you all, I shall tell you all!
"All creation is a huge, ornate, imaginary, and unintended fiction; if it could be deciphered it would yield a single shocking word."
-John Crowley