I have taken as long as humanly possible to write this review. Because this is one of the most intense novels I've ever read, and deserved rumination.
At its core, this novel is The Great American Tree Novel. What the author tries to do here is write the definitive novel about trees. And I mean that quite literally. The characters he's created, what he has them do over the course of the novel, and how they interact to tell a complete story, describe everything you will ever need to know about trees.
That sounds flippant. And as if it's unnecessary. It isn't. Everyone should know all these things about trees. What it means to plant a new forest. What makes a tree activist. What trees look, smell, feel and taste like. What their history with humans has meant and foretells. And, last but certainly not least, how trees talk to each other over the aeons.
Regarding the structure of the novel, I don't want to give too much away. But be forewarned - he starts the entire tale by setting the scene for each character, almost like a set of short stories. About 1/3 of the way through, he shifts the focus to describe their interactions, and that carries for the rest of the book. Try not to be distracted or upset by that shift in focus. It's deliberate and justifiable.
I will also say that I may have disagreed with the path he took for a couple of the characters - and his reveal on one that was vastly more understated than it should have been - but I still fully understand why he took all those paths. This is a tour de force. My tiny quibbles are barely worth noting.