It's been said before, but I want to say it again because it's pretty and this is something I only learned relatively recently when someone else pointed it out.
This tree is the one that would have been over Neighbor's Room. The trees stump would have looked like this if it wasn't cut down. Neighbor's Room is the treehouse, so this tree being cut down gives it connection to both Sunny's favorite tree and that tree said treehouse was put in. And now its been brought back here.
>>531869096
Even after everything, it's Mari's words that come back to save him when he's overwrought by these fears. She's still saving him even now. She loved him and he killed her. Even if he does get better after the ending, that day Mari saved his life at the lake is going to haunt him every now and then.