The Quenya word for “willow” appearing as both tasar (PE17/81) and tasare (SA/tathar), derived from the root √TATHAR. This form of the word dates back to The Etymologies of the 1930s where ᴹQ. tasar, tasare “willow” appeared under the root ᴹ√TATHAR (Ety/TATHAR).
Conceptual Development: The Qenya Lexicon of the 1910s had a slightly different word ᴱQ. tasarin (tasarind-) “willow” under the early root ᴱ√TASA, though Tolkien marked it with a “?” (QL/89). This became tassarin “willow” with a double-s in Early Qenya Word-lists of the 1920s (PE16/139) before Tolkien adopted the form tasar(e) [þ] in the 1930s, as noted above.
tasarin noun "willow" (LT2:346; in Tolkien's later Quenya tasar, tasarë)