Read Christopher Alexander's piece, A City Is Not A Tree. He means the abstract structure of a tree, the sort of top down triangular thing that is often used to show leadership structure. This is a bit heavy, but it's vitally important:
It is not merely the overlap which makes the distinction between the two important. Still more important is the fact that the semilattice is potentially a much more complex and subtle structure than a tree. We may see just how much more complex a semilattice can be than a tree in the following fact: a tree based on 20 elements can contain at most 19 further subsets of the 20, while a semilattice based on the same 20 elements can contain more than 1,000,000 different subsets.
Central planners may be able to handle 19, but go for a million and even the best of you will get it wrong. This is why literal city building is daft; cities emerge. If one tries to design, plan, and build a city from scratch it is inevitably inhuman. Do you merely fit people into a room, or do you actually make a room for a person? And then, how many rooms are there in a city?
For the human mind, the tree is the easiest vehicle for complex thoughts. But the city is not, cannot and must not be a tree. The city is a receptacle for life. If the receptacle severs the overlap of the strands of life within it, because it is a tree, it will be like a bowl full of razor blades on edge, ready to cut up whatever is entrusted to it. In such a receptacle life will be cut to pieces. If we make cities which are trees, they will cut our life within to pieces.