[pessimizer]

Everything in programming is trees. If you write a program that calls functions, that call other functions, that's a tree.

[dalke]

I believe continuation-passing style is an alternative to the direct style you mention. Return-oriented programming and threaded programming may be specific counter-examples.

[cjhopman]

Unlikely. Unless a single function corresponds to a possibly infinite set of nodes, most programs are not trees.

[pessimizer]

A single node is a tree. main() is a tree.