Ash.

Ash bark and leaf preparations carry tannins and hydroxycinnamic acids that bind non-heme iron in the gut and lower how much of it crosses the intestinal wall. This is a general property of tannin-rich plant material rather than something particular to ash. The usual formulation answer is to space an iron dose a couple of hours away from a tannin-rich botanical. This is chemistry in the lumen, not a measured clinical outcome.

The phenolic compounds in ash donate an electron and are left as phenoxyl radicals. Ascorbate reduces phenoxyl radicals back to the parent phenol in solution, which is why the two are so often formulated together. Read this as mechanistic rather than clinical: nothing here says a person notices anything.

Ash bark and willow bark appear together in old European herbal formulas aimed at joint comfort and mobility. Willow contributes salicin-type chemistry, ash contributes secoiridoid glucosides and phenolics, so the two are not doing the same thing. The pairing rests on tradition and formulation habit, not on a head-to-head study.

Both are put into blends aimed at joint comfort during activity, and they come from unrelated chemical families, so neither crowds the other out. There is no combination trial for the pair. Count it as formulation convention.

Curcuminoids and ash phenolics are both electron-donating plant polyphenols, and blends often stack them for breadth of chemistry rather than for a known interaction. No study has tested the pair. The rationale is chemical family overlap, which is a reason to expect no conflict, not a reason to expect more effect.