Do Robert Graves's tree pairings in the thirteen sign Celtic tree calendar, and the placing of rowan at this position, correspond to anything beyond his own argument in The White Goddess?

Graves built the thirteen sign sequence in The White Goddess, 1948, by pairing his own reading of the Beth-Luis-Nion tree names to a thirteen month lunar calendar he reconstructed himself. That the wider tree calendar is his modern invention rather than an attested ancient system is not in dispute, and the tree names match because he took them from Ogham. What nobody has examined is the step after that: whether placing a given tree at a given calendar position reflects anything in the surviving Ogham material, or only his argument. No Celtic studies scholar has tested his month by month assignments either way, which is why this is an unfilled gap rather than a disagreement between camps. One question covers all thirteen signs, because the pairing is one act rather than thirteen.