In this story I read long ago, the trees of each forest are all connected (through their roots ? by some psychic power ? I forgot). Each tree is thus just a "body part" of the forest he belongs to, and it is the forests that are sentient, not the individual trees.
I don't remember any other living beings on this world but forests, let alone other sentients.
Forests fight for space. The fight viciously. One forest is in danger of being totally destroyed by one or several of its neighbours.
The endangered forest discovers uranium ore in its territory. And some very old memory comes to its mind as how to use it. Marshalling all the resources it can still afford, it pushes some kind of "vanguard" as deep as possible into the territory of its enemy, and its root system brings enough uranium there to reach critical mass. Of course the "vanguard" is annihilated, but enough of its enemy is destroyed that the forest can easily conquer a large area.
I think I remember that after that other forests also start remembering the use they can make of uranium ore and a generalised nuclear war takes place, but I am not sure of the ending. Maybe the first to think of uranium dominates the world ?