hunting burl

Ive been very sucessfull over the years finding more than I would sell but
often when you crack open your treasure its of less quality than expected.
Question is are the burl in diffrent parts of the country diffrent on the
same species? Birch burl around here are very diapointing however the
spalt and mineral stain on birch is exceptional. Mabe its the climate We
also have 2nd home in NH where we also cut, collect and sell craft wood
and burl. Seemes to be far less in the north than here in the midwest?
Burl here is abundant in some areas and void in others anyone know of a
soil or enviormental condition that would effect this? Just trying to be a
more efficent hunter Im getting good at selecting spalted trees but burl is
still a mystery. Thank you