Modern popular writing on ogham often treats Aicme Forfeda as a set of tree letters on the same footing as the original twenty, each assigned a plant and a divinatory keyword. The medieval sources do not support reading them that way. The tree associations attached to the core aicmi come from the kennings preserved in the ogham tracts themselves, word riddles built around each letter name, and even there scholars disagree on how far the tree scheme was a real mnemonic system versus a later gloss. For the forfeda, the tree names are thinner still: several, including Or and Ifin, carry meanings tied to metal or plant terms that shift between manuscripts, and Emancholl is explicitly a doubled form of Coll, hazel, rather than an independent tree.
What the medieval grammarians clearly did use the forfeda for is filling gaps in a working alphabet applied to a changing language. Old Irish had sounds and diphthongs, and eventually loanwords, that a twenty letter stone alphabet designed for an earlier stage of the language could not spell cleanly. The forfeda are best read as a scribe's solution to that problem, occurring in manuscript ogham and marginal glosses rather than in landscape inscriptions, which is also why their exact roster and order stayed unsettled long after the four stone aicmi had become fixed.
Sources Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.) with A Guide to OgamDamian McManus