In article , Gene Newcomb
writes
The Oregon Nursery Guide (www.nurseryguide.com) lists it as Betula alba a
synonym
of B. pendula according to other sources. The Amer. Hort. Soc. Encyclopedia
dodges
the question by giving it a listing separate from all species. Not much help I'm
afraid.

The RHS Plantfinder also doesn't assign it to a species, but it does
come up on a search for Betula pendula.

URL:http://www.rhs.org.uk/rhsplantfinder...?crit=Betula%2
0and%20Trost&Genus=Betula

URL:http://www.rhs.org.uk/rhsplantfinder...?crit=Betula%2
0and%20pendula&Genus=Betula

IIRC, Betula alba included both Betula pendula and Betula pubescens

Govaerts, R. 1996. (1261) Proposal to reject the name _Betula alba_
(Betulaceae) Taxon 45: 697-8, may clarify this.
--
Stewart Robert Hinsley