Fl. von Mitteleuropa, ed. 3, 3(1): 36, 24 (1981) where it is stated that W.Wettst. published the name Populus nigra subsp. betulifolia (Pursh) W. Wettstein (1952).
Comments
W.Wettst. did not use the rank 'subsp.' in the 1952 publication Schriftenreihe Osterr. Ges. Holzf., 5. On page 30 he refers to Populus nigra betulifolia Torr. and on page 45 Populus nigra var. betulifolia Torr.