A problem while working with 2D graphics, in the style of big concept maps or other kind of diagrams, is that when the matter is complex, they can become a problem to navigate and edit them like in the next very cluttered map:
This is unavoidable if you need to show (or print) the map as single unit.
However, with the help of software, you can divide the map in hierarchically contained sub-maps, spanning multiple levels. Consider the next diagram, made with ThinkComposer, representing a simple aircraft design as an advanced concept map:
With this multilevel capability you can “compose” (or decompose) your ideas to any extent into a single document. Plus, notice that you can still connect nodes, not within the same view (map/diagram), by using Shortcuts that you can point/click to easily navigate to the original source.
Although not exactly a tree (i.e.: born from a root), a Genealogy Tree or Family Tree is an specialized graph of nodes depicting family lineage.
The next is an example made with ThinkComposer (click to enlarge):
A common need is the graphical showing of hierarchies, containment, origination or classification, which can be represented as one-to-many relationships in a Tree structure. Tree in the sense of a root with many branches, usually shown downward for reading purposes. Examples of these are organizational charts, containment diagrams or navigational controls in software applications. A special application of trees is the representation of space or usage as areas such these shown in Treemaps. A very good explanation of trees and treemaps can be found here.
Also there is the radial representation and navigation in the so called Hyperbolic Trees, either in 2D or 3D views.
Hyperbolic tree samples
The problem with this structure is that each node can have just one parent or dominant, making it not always suitable for reflecting reality albeit simple to understand and affordable for implementation.