Although Plant of the World Online lists Alnus viridis as a synonym of Alnus alnobetula subsp. alnobetula, that is a European subspecies which does not occur in North America.
The North American subspecies are crispa, fruticosa, and sinuata. Alnus alnobetula subsp. crispa occurs in the eastern US, much of Canada, Alaska, and souhern Greenland. The subspecies fruticosa occurs in northwest US and western Canada, as well as subarctic Asia. And the subspecies sinuata occurs in mountainous areas of the northwestern US, Alaska, and Canada.
The basionym of Alnus alnobetula (Betula alnobetula) was published 2 years earlier than that for Alnus viridis (Betula viridis).
Kew Science is a dreadful final authority for plant names. When they changed the name of Rubus parviflorus - Thimbleberry to R. nutkanus for no reason other than someone finding there was a conflict in naming over 100 years ago, rather than changing it because it was genetically too close or too far to be the a different species from the next one, or the same species as one it had been lumped with. That conflict in naming Rubus parviflorus 100 years ago formed no conflict today. They only made the name change to be the authority making a name change. We already have too many name changes today due to genetics, we don't need more name changes just to boost the egos of those making them. Kew is a botanical garden. Their specialty isn't taxonomy.
Unintended disagreements occur when a parent (B) is
thinned by swapping a child (E) to another part of the
taxonomic tree, resulting in existing IDs of the parent being interpreted
as disagreements with existing IDs of the swapped child.
Identification
ID 2 of taxon E will be an unintended disagreement with ID 1 of taxon B after the taxon swap
If thinning a parent results in more than 10 unintended disagreements, you
should split the parent after swapping the child to replace existing IDs
of the parent (B) with IDs that don't disagree.
Kew Science is a dreadful final authority for plant names. When they changed the name of Rubus parviflorus - Thimbleberry to R. nutkanus for no reason other than someone finding there was a conflict in naming over 100 years ago, rather than changing it because it was genetically too close or too far to be the a different species from the next one, or the same species as one it had been lumped with. That conflict in naming Rubus parviflorus 100 years ago formed no conflict today. They only made the name change to be the authority making a name change. We already have too many name changes today due to genetics, we don't need more name changes just to boost the egos of those making them. Kew is a botanical garden. Their specialty isn't taxonomy.