Personally I feel like this might end with Yoru coming back, in part because of what Sei said about her own happiness, and the two of them will get together, though the relationship will of course be kinda messed up. And Aya will end up with her best friend. That's what I hope for anyway.
As upsetting as it is that Sei went along and gave in to Yoru at the end, I am glad to see her yelling at Yoru like that first. I was worried she'd just give in right away.
I kinda feel like the title and description are kinda misleading. She isn't really so much becoming a tree as she is part tree. I mean we see her as a kid in that one panel and if anything she was actually more tree back then.