Adjective for "composed by trees" (mathematics)

"summation over a tree" or "tree-folding formulas" is what you are looking for.

In mathematics we have things like "optimization over a graph", meaning the optimization problem is defined in graph-like structures. Similarly, one can say that he is doing "summation over a tree", the reader will immediately understand the phrase. If you want an adjective, "tree-folding" is a good candidate. To fold something means to make something into a compact form by iterating over the indices of its elements in a pre-defined fashion. A "tree-folding formula" is a formula that is derived by folding trees (tree-like mathematical structures).