This invertible root had a long history in Tolkien’s writings. Its earliest iteration was as a pair of roots in the Qenya Lexicon of the 1910s: ᴱ√OŘO [OÐO] with derivatives having to do with the “dawn”, which it was much confused with ᴱ√ORO whose derivatives were rising things (QL/70). The latter had derivatives like ᴱQ. orme “summit, crest, hilltop” and ᴱQ. orto- “raise” (QL/70), and Tolkien mentioned an inverted variant ᴱ√RŌ or ᴱ√ROHO with derivatives like ᴱQ. róna- “arise, rise, ascend” (QL/80). The contemporaneous Gnomish Lexicon also gave it as in an invertible root ᴱ√rō-, oro with derivatives like G. oros “rising” and G. ront “high, steep” (GL/63, 66).
The root reappeared in The Etymologies of the 1930s as ᴹ√ORO “up, rise, high” and ᴹ√RŌ “rise” (Ety/ORO, RŌ). The root was mentioned very frequently in his writings from the 1930s, 40s, 50s and 60s, generally glossed “rise” or “up(wards)”. Thus √RŌ/ORO “rise” was very well established in Tolkien’s mind, but distinct ᴱ√OŘO “✱dawn” seems to have been abandoned very early.
This root first appeared as ᴹ√RIS “slash, rip” in The Etymologies of the 1930s with the derivative N. risto “rend, rip” (Ety/RIS¹). Tolkien then created a new entry for ᴹ√RIS without deleting the prior entry, with derivatives like ᴹQ. rista/N. rhest “a cut” and ᴹQ. rista-/N. rhista- “cut” (Ety/RIS²); this seems to reflect a conceptual shift of “slash, rip” >> “cut”. Indeed, the root √RIS reappeared in Tolkien’s later writings with the gloss “cut” (PE17/87). Christopher Tolkien gave this root the gloss “cleave” in the Silmarillion Appendix (SA/ris).