What do trees dream about?
Is it gentle sunlight, caressing breezes, and fancies of bucolic harmony?
That is a product of the imagination of short-sighted humans, who see a forest and believe it to be still and peaceful. It's slower-paced, granted, but no less red in root and branch than animals' share of nature.
Trees will race to the top so they can starve out those below them. They'll poison the soil to deny it to their fellows. Trees can be arsonists that'll burn themselves and everything around them to the fucking ground for a competitive edge. They'll take over entire regions with their own immortal clones. They're vicious sons of birches living a war of all against all is what I'm getting at.
It should be no wonder that their dreams reflect this.
When animals dream we lie still, become vegetative, withdraw into ourselves. For trees it's the opposite. Trunks that stood straight for millenia twist and writhe, and fantasies nigh-incomprehensible to fleshy minds blur into reality.
Fortunately for everything else, trees don't fall asleep easily. The rapid activity of us beasts is enough to keep them awake. Trees on the edge of a forest, those near grazing flocks and villages hungry for timber, they're downright insomniac. However, far from grazing deer and the woodsman's axe, in the deepest, densest, darkest part of the woods, the trees begin to dream, and the world shudders.
Let's return to the beginning: what do trees dream about?
Firstly, they dream of a world before animals, without fleshy, mayfly-lived things to wake them and eat them and cut them and burn them. That's the first sign of a dreaming tree: near it there will be nothing alive which makes noise, not a single bug buzzing or bird singing. They will all have fled, or been erased. The human ego is stronger, thankfully, and can't be unimagined as easily.
Next, they'll dream of a world where they're winning. The tree will grow taller than any other in the forest, and then wider, expanding its trunk to envelop and suffocate other trees or else transmute them into copies of itself. They'll remake the world to their design, starting with the resilient animals that stick around the dream like sand in their craw. Dryads, treants, mossmen, woodwoses, they've all got the same origin: they're the dreaming tree's improvement on us, things that look like us but think like them.
Lastly, after they've filled their dream to its edge with themself, the tree will dream that it's everything, with no more need for sun or earth or anything but itself. Its leaves will glow with golden light, soil and stone will be replaced by an endless tangle of roots feeding off roots, and rain and rivers will flow with sap instead of water. It will be a self-contained, solipsistic universe.
The first stage is what rangers exist for. People that go further into the woods than anyone else to plant a flag for the animal kingdom, that know them well enough to recognize the subtlest signs of them going wrong. They're the ones who wake trees before they get out of control, when all that's needed to wake them is a slash from a machete or some burned bark.
The second stage tends to provoke something between a gold rush and a witch hunt. A tree which reshapes the world and creates life is blasphemous, but the creations of trees are a precious resource to mages, artists, speculators, and the like. If discovered in this stage, things will get violent, many people will die in the tree's dream for their greed (likely more to each other than the tree), but the most likely end will be that the tree is awakened and stripped to its last ring.
A tree in the final stage of its dream cannot be woken. The only way to stop it is a crusade against the green, a mass-mobilization of mobs wielding axes, machetes, and sickles against the old growth and its mocking spawn. Pray it never comes to that. The trees have been waging war for billions of years before the first human bashed the second's head in with a rock, and mastered its lessons long ago.