This name is a synonym of Populus caspica (Bornm.) Bornm..
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 100375139) with original publication details: Bull. Herb. Boissier, sér. 2, 8: 724 1908.
This name record (from Tropicos) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Populus alba in version 1 of The Plant List.