Well, Nightmare, when I saw tautology there this morning, I was perfectly happy with it.
In Fowler's Modern English Usage, tautology is defined as "the repetition in the immediate context of the same word or phrase, or of the same idea or statement in other words." It comes from the Greek word meaning 'the repeating of what has been said'
In the same source, pleonasms are defined as "grammatical constructions that contain an element or elements of redundancy." It comes from the Greek word meaning 'added superfluously'.
Both seem to fit pretty well with 'He lit the branches on fire." It's my view that, in circumstances like these, there is no significant difference; indeed, they are virtually synonymous. Perhaps an expert in the conventions of classical rhetoric might be able to separate them, but I really can't!
Either might be used deliberately, of course, for effect. Think of Coleridge's ancient mariner...
"Alone, alone, all, all alone,
Alone on a wide, wide sea!"
Wonderfully tautological...or pleonastic, of course!