At
http://www.skalnicky.cz/Hepatica.php there are good maps showing the natural range of the various species and sub-specific forms of
Hepatica. Regrettably, I do not read Czech, and the only online system I can find that handles Czech-to-English translation produces results that are worthless.
The map of Japan is of particular interest, as it shows that
Hepatica japonica f.
magna, the progenitor of most of the Japanese doubles, occurs along the shore of the Sea of Japan on Honshu. If the map is accurate, it explains why the Japanese double hepaticas are slightly tender, for they grow not far from the sea and its moderating influence.