In message , David in
Normandy writes
Are there different varieties?

I've just been looking out the window at the rain and wind
and noticed two of our mature silver birch trees look a bit
different. One has fine twigs that cascade downwards from
the branches and the other has erect twigs. Both trees are
multi-stem and around 50 feet tall.

In Britain there are three species of Birch, the Silver Birch (Betula
pendula), the Downy Birch (Betula pubescens) and the Dwarf Birch (Betula
nana). (Betula nana is not found in the lowlands.)

There might be more species present in Europe, but most of the published
names represent synonyms and subspecies of B. pendula and B. pubescens.

Hybrids among the native birches are common, and it is hard to draw a
line between the two species.

It is possible that you've got a Betula pendula (with downward cascading
twigs) and a Betula pubescens, but even after looking at the trees in
life it might not be possible to say for certain.
--
Stewart Robert Hinsley