As Pyren wanders away from the camp, looking around as if search for a way out, his eyes slide across one of the trees, trailing it from the trunk. Here's where we see the descriptions of the knuckle-like branches fusing with each other and the adjacent trees.
Serinth (stepping in from behind): They're fused together.
Pyren immediately turns back to her in fear but there's no emotion from Serinth as she continues speaking casually, as if describing the weather.
Serinth (cont'd): They call this place the 'Deadwood', but it's all one big organism.
Pyren: What does it want from us?
Serinth: Why does it have to 'want' anything?
Yeah I ripped that line off from Solaris. Anyway, the scene continues as Pyren accuses Serinth of knowing where she was taking them, of knowing that Shuik was going to die, and not doing anything about it.
Pyren specifically tells her she could have brought the others to the basin, she could have been ready. She simply replies, no, she couldn't have.
In this scene when Pyren is first describing the fused branches to the audience, we describe the hundreds of little twigs reaching out and connecting to each other as like fibers in a nervous system, with woody 'polyps', like knuckles (we specifically use that word) connecting them.