The name Betula alba is a good marker for an obsolete book or site.

Iris Cohen schreef
I realize that the correct name is B. pendula, but that doesn't help. Some
authorities list 'Trost's Dwarf' as a cultivar of B. pendula, others simply
as a Betula cultivar. According to the botanist at Hillier Gardens, 'Trost's
Dwarf' has downy shoots, which makes him think it is not B. pendula. They
don't have the plant any more, so he has no further information.
Iris,

+ + +
Are you sure you want to go there?
As I understand it the taxonomy of Betula is something else.
Just the difference between Betula pendula and Betula pubescens is enough to
fill up any amount of time (provided you can read Russian; if not, you can
add a few years to learn that)
PvR