A set of nodes is simultaneously contained in multiple different tree structures. A node might be a leaf in one tree and an internal in another tree. The same node might have different children and a different parent in different trees. Trees may have different root nodes. Any given tree does not necessarily contain all the nodes.
Every node is in at least one tree.
You should consider the effect of large number of nodes and large number of trees. Topics might be organized by different types of data spaces, interaction strategies, or views, or even by calendar. These can be hand-drawn with pencil and paper. The intent here is low-fidelity rapid prototyping, not development. 1 page of description.