Why do trees seem to respect each others personal space?

I know that trees with have branches in each others area at the periphery but they do not seem to become intertwined and an impenetrable mess. Why not? What is it that keeps them from entangling so that in a forest you could not cut down a tree because it would not fall? Why would the limbs from one tree not go all the way through to the other side of an adjacent tree and vice-a-versa?