The Sindarin word for “elm” appearing in notes from 1959, derived from the root √ALAB of similar meaning (PE17/153).
Conceptual Development: Tolkien used similar “elm” words for much of his life. The Gnomish Lexicon of the 1910s had G. lalm or larm “an elm, elm-wood” along with a more elaborate form G. {lalmin >>} lalmir “an elm tree” (GL/52). These were clearly cognates to ᴱQ. alalme “elm (tree)” under the early root ᴱ√ALA “spread” (QL/29). Early Noldorin Word-lists of the 1920s had an unrelated form ᴱN. {aulin >>} ólin “elm” (PE13/151). In The Etymologies of the 1930s Tolkien reverted to N. lalf or N. lalven “elm-tree” under the root ᴹ√ÁLAM of the same meaning (Ety/ÁLAM). This root had a variant ᴹ√LÁLAM, under which Tolkien had N. lhalwen or lhalorn “elm-tree” (Ety/LÁLAM).
Neo-Sindarin: In theory the 1930s “elm” words might be used with some adaptations like ᴺS. lalorn for N. lhalorn, but I’d simply stick to the 1959 “elm” word alaf for purposes of Neo-Sindarin.
_ n. Bot. _elm. Q. albe.