Adjective for "composed by trees" (mathematics)

I suggest treed. The word is an adjective to be found in some dictionaries, it has sporadic use shown in Google ngram, and it has been used in mathematical articles, for example see

Canadian Journal of Mathematics

I'd consider using arboreous, but I don't know if anyone uses that for math papers.

Adjective: arboreous aa(r)'bo-ree-us

1. Inhabiting or frequenting trees

   "arboreous apes";

   • arboreal, tree-living

2. Abounding in trees

   "an arboreous landscape";

   • arboraceous, woodsy, woody

3. Resembling a tree in form and branching structure

"arboreous coral found off the coast of Bermuda";

• arboreal, arborescent, arboresque, arboriform, dendriform, dendroid, dendroidal, treelike, tree-shaped

See also: branchy, wooded

https://www.wordwebonline.com/search.pl?w=arboreous

Not all usages can be specified by single words - otherwise language would be reduced to single words. I would suggest that your phrase:

can be graphically represented by trees

describes quite well what you are attempting.

"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).