This story contains spoilers for Season 2 of Netflix's The OA.
By the time the new season of Netflix’s The OA gets to the giant telepathic octopus, you think you've seen it all. And then, just an episode later, the main character of the show falls through the ground and has a conversation with the roots of a giant tree. The tree, and its nearest tree brethren, tell O.A. (co-creator and star Brit Marling) that they've been calling to her for years, and she hasn't answered, and then they give her a warning about her future. This conversation, like many of the ideas in the show, may seem like wild fantasy, but (also like many things on the show) it’s actually based on real scientific theory.
We can't actually talk to trees, but trees certainly do talk to each other -- all the time. This has been pretty common knowledge amongst naturalists for a number of years, who have studied forests' interconnected underground networks and found out that trees communicate through their roots using the mycelium of a fungus: intertwining with each other, checking each other's growth, feeding their comrades, sending warnings to their friends, creating a web of information not unlike the sparking neurons of the human brain.
When a tree dies, often it will release the nutrients it won't use anymore into its root network, so that the trees nearby can benefit. If a pine is being attacked by bark-eating beetles, it sends signals to the other trees around it so they can marshal their defenses. Some species will steal nutrients out of the network from other trees nearby so that they can grow bigger, taller. If an overconfident sapling is growing too fast, its larger, older mother will seep away some of its food so that it slows down, keeping it from breaking through the canopy too early, which would kill it for any number of reasons (snapping, overexposure to sunlight, etc.).