An Etymological Dictionary of the German Language/Annotated/Pappel
Pappel (1.), feminine, from the equivalent Middle High German papel, papele, feminine, Old High German (Middle Latin) papula, 1, ‘mallow.’ Of obscure origin; probably cognate originally with Pappel (2).
Pappel (2.), feminine, from the equivalent Middle High German papel, popel, feminine, ‘poplar,’ Latin pôpulus (French peuplier), with the Middle Latin variant papulus; from the French form (Old French poplier) are derived the equivalent Dutch populier and English poplar (Middle English popler). In the Romance classical Latin populus was changed in many curious ways in most of the dialects; compare Italian pioppo (from *ploppus for populus). Since the High German form is closely connected with the Latin, it must have been introduced by scholars, probably in the Middle High German period.