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

Good grief! Ask a simple question & look what you get. Somebody pointed me at
the Westonbirt Arboretum. I'll see if they know anything.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)