Some trees are wise, but sometimes they aren't smart enough. At least, alone. With a forest, these trees can act as a sort of neural network, creating a collective consciousness. An arboreal brain. Plants have roots, and roots kind of look like neurons, so perhaps that comparison is why the idea of a sentient forest is a thing.
These neural roots can be considered both a Genius Loci and Hive Mind, being both a sentient location and a meld of hundreds or thousands of individual minds. It might explain why the jungle or forest is trying to kill you; the whole thing's an intelligent environment and doesn't take kindly to trespassers. If it's just a single intelligent tree, its a Wise Tree.
- Avatar: The Ewya of Pandora is described as a vast network of fibre-optic-like branches and roots. Na'vi are able to hook themselves into this network and upload or download information.
- Alan Dean Foster:
- In Midworld and Mid-Flinx, the green world with no name is effectively this trope, all of its vegetation being linked as a gestalt consciousness. They-Who-Keep, a type of tree in which the human inhabitants entomb their dead, form this global "nervous system's" most crucial "ganglia". It's later shown in a short story that the plants don't even need to be in physical contact when the forest has a conversation with the AI on Flinx's ship about the ethics of forcing Flinx to go save the universe via mind control vs just trying to nudge him into making that decision on his own through the plants he was given as gifts before leaving the world. It Makes Just As Much Sense In Context.
- Not necessarily intelligent, but there's a communal plant-creature in The Hour of the Gate that moves slowly through the Swordsward, through the coordinated movements of hundreds of interwoven plants. Definitely some sort of neural-analogue communication at work there.
- Double Vision, by Tricia Sullivan, has the Grid, a seemingly sentient fungal clonal colony on an alien planet. It turns out that it's the main character's subconscious attempting to interpret TV marketing.
- Federation of the Hub: In "Balanced Ecology", the diamondwood forest on Wrake is a closely integrated ecology with, as it turns out, its own plans for how to cope with the human colonists on its world.
- Hainish: Vaster Than Empires and More Slow features a group of astronauts coming upon a planet devoid of animal life but covered by a global forest, from which intense fear can be felt emanating. The astronauts eventually realize that the forest itself functions like a tremendously vast mind, and the fear they felt was the world-forest's apprehension at encountering other thinking beings for the first time after a long, long life in isolation.
- In the kids' book Ki Kiwi, the forest is said to be able to think, but the trees themselves seem like normal trees.
- The Lord of the Rings:
- Tom Bombadil's "Old Forest" seems to be something like this, and alongside Fangorn Forest is a remnant of a massive, ancient forest that once covered most of Western Middle-earth in the First and Second Ages. The trees by themselves don't appear to be completely sentient (although Old Man Willow comes closest), but they all behave together to funnel intruders deeper into the forest. They are also described as having their own strange thoughts and emotions, as well as something of a hierarchy, with the oldest trees influencing the others.
- Fangorn Forest on the other side of the Misty Mountains is, like the Old Forest, a remnant of a massive ancient forest, and is populated by a combination of Ents, regular trees, Ents that have become tree-like, and Huorns — sentient, mobile trees that the Ents have woken up. Old Man Willow itself is strongly implied by Treebeard to be a malevolent Huorn.
- Speaker for the Dead: The trees on Lusitania, although each one is intelligent, are host to a more powerful entity collectively.
- Star Wars Expanded Universe: Bafforr trees used roots to create a surface wide forest brain on Ithor.
- The True Game, by Sheri S. Tepper, has sentient forests.
- The Vorrh: The trilogy's eponymous Genius Loci is an ambiguously benevolent one. It was supposedly created by God at the beginning of time and houses the Garden of Eden, and spending too long inside can turn one into a mindless pseudo-zombie called a "limboia".
- In Magic: The Gathering, Dominarian forests have maro-spirits, entities who are empathically and spiritually linked with every tree and living creature in the forest and act as a living representation of the forest's soul.
- Warhammer: Athel Loren, the forest where the Wood Elves live, is so thoroughly steeped in magic that it's a living, aware thing in its own right, and conscious of everything that goes on beneath its canopy. While the Wood Elves are themselves fully capable of defending their home from intrusions, Athel Loren itself actively fights back against attacks, twisting paths, moving trees and directing dryads, treemen and forest dragons to its defense.
- Mass Effect: The Thorian in the first game is a sentient plant-like thing. As researchers in-game commented, no-one really knows what to classify it as. At first they thought there were a bunch of plants along the surface of the planet, but later they realized it was just one big plant.
- In Sid Meiers Alpha Centauri, the Xenofungus serves as a hindrance to terraforming and a hiding place for the rapacious Mind Worms. It also turns out to be a vestigial neural network housing a fledgling sentience, one which has achieved near-godhood on multiple occasions in Planet's history only to inadvertently trigger mass extinctions which resulted in its inevitable collapse. The human factions can not only aid the Planetmind in its ascension, but use it to achieve their own transhuman evolution.
- Stellaris: The Ancient Relics DLC has the Baol, an ancient race of plantoid Precursors. At their height, the Baol were spread out over much of the galaxy. Each individual world counted as an Intelligent Forest in and of itself, and all of them were linked together into one vast hive consciousness.
- In Kevin & Kell, the Tree that Lindesfarne and Fenton live in has mentioned that all plant life can communicate via an interconnected root system. And supposedly, it somehow works for potted plants as well.
- In Avatar: The Last Airbender, there's The Swamp, a huge habitat reportedly made from one central tree. The "forest" itself seems to have a mind of its own, too, though whether it's sentient or not is mysterious.
- Totally Spies!: In "Nature Nightmare", the spies have to fight Mad Scientist Lasputin Zero, who has created trees with intelligence in order to protect themselves from humanity, creating an entire forest of them. They go out of control however, and plan to attack innocent people. The Spies eventually stop the trees by destroying the queen tree.