I am about to do some research with coverings of trees, and it was only natural to look at Baez-Dolan metatrees and the corresponding notion of opetopes. The paper contains a famous 5-minute definition and it is a brain teaser worth reading. They introduce trees and nestings (actually two sides of the same coin) and their superposition, called a constellation, which has to follow some rules, but drawing spheres is a creative process.
Then there are zooms which connect constellations as long as the left nesting and the right tree are morally identical.
My lambda graphs are basically search trees and a nesting would add the operational notion of evaluation. Since we can freely choose our evaluation strategy (confluence?), the latter corresponds to a nesting. It remains to find out what the degenerate zooms are under this aspect.
I am just reading the "Polynomial functors and opetopes" paper (http://arxiv.org/pdf/0706.1033.pdf) and it asserts that it's starting constellation is a one leafed tree: