The basic Quenya word for “tree” (LotR/1113), derived from primitive ✶galadā and very well attested. This word dates all the way back to the Qenya Lexicon of the 1910s where ᴱQ. alda “tree” appeared under the early root ᴱ√ALA “spread” (QL/29). Tolkien seems to have switched its derivation to ✱galadā in The Etymologies of the 1930s, where ᴹQ. alda “tree” appeared under the root ᴹ√GALAD of the same meaning (Ety/GALAD). See also ornë “(tall) tree” for a discussion of another similar word.
Conceptual Development: There were a few instances where the word alda had a different meaning. In Early Qenya Word-lists of the 1920s, alda was glossed “branch” (PE16/139). In notes from 1959 Tolkien said “✱galadā, originally only large flourishing plant, as tree, and especially one that flowered, Q alda, S galað; the general word for ‘tree’ was Q orne ‘upstanding plant’ (PE17/153)”. But in its numerous appearance elsewhere, alda was simply a general word for “tree”.
alda noun "tree" (GALAD, GÁLAD, SA, Nam, RGEO:66, LR:41, SD:302, LT1:249, LT2:340, VT39:7), also name of tengwa #28 (Appendix E). Pl. aldar in Narqelion; gen. pl. aldaron "of trees" in Namárië. Etymology of alda, see Letters:426 and UT:266-7. The latter source states that primitive ¤galadā, whence Quenya alda, originally applied to stouter and more spreading trees such as oaks or beeches, while straighter and more slender trees such as birches were called ¤ornē, Quenya ornë - but this distinction was not always observed in Quenya, and it seems that alda became the general word. According to PE17:25, primitive galada (sic) referred to "a plant (large) and was a general term". Place-name Aldalómë ""tree-night" or "tree-shade-night" (LotR2:III ch. 4, translated in PE17:82); Aldarion masc. name, *"Son of (the) Trees" (Appendix A), Tar-Aldarion a Númenorean King (UT:210). Aldaron a name of Oromë (Silm); aldinga "tree-top" (VT47:28), aldarembina (pl. aldarembinë attested) adj. "tree-tangled", the cognate of Sindarin galadhremmin**(PM:17:26).Aldúya fourth day of the Eldarin six-day week, dedicated to the Trees (Appendix D). The word seems to include Aldu, a dual form referring to the Two Trees. The Númenóreans altered the name to Aldëa (presumably < aldajā), referring to one tree (the White) only. The dual Aldu seems to occur also in Aldudénië** "Lament for the Two Trees" (a strange word, since Quenya does not permit intervocalic d as in this word perhaps the Vanyarin dialect of Quenya did) (Silm)