"gardengal" wrote :

Clump birches are often "created" by the grower - they take multiple
saplings and plant them together in the same container or planting
hole, so they grow as if they were a multi-trunked tree. It's quite
common, as multi-trunked trees do not occur that way naturally with
great frequency.

That's just not true. The number of trunks of clump birch is created by
selective pruning, not by planting multiple trees (could be done but no
reputible nursery would, that would be cheating), planting a number of birch
close together does not a clump birch make... usually only the most dominant
will survive, the rest will eventually die. Many trees develop multiple
trunks naturally... I have a number of such trees growing on my property,
too many birch to count... I have a huge maple that naturally grows
multi-trunked, forest pansy redbud does also. All one need do is search
multi trunk trees.