This name is a synonym of Populus purdomii var. rockii (Rehder) C.F. Fang & H.L. Yang.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 28300966) with original publication details: J. Arnold Arbor. 13(4): 386–387 1932.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:50922401-1.
This name was in version 1 of The Plant List, record tro-28300966, and has not been changed.