Morphological, chemical, and biogeographical analyses of a hybrid zone involving Juniperus virginiana and J. horizontalis in Wisconsin

Natural hybridization between Juniperus virginiana (L.) and J. horizontalis (Moench.) has been reported in the Driftless Area in Wisconsin. We applied multivariate statistical techniques to morphological and terpene data both to document the phenomenon of hybridization and to determine more accurately the distribution of hybrids and parentals in the Driftless Area. Analysis of morphological, terpene, and electrophoretic data is consistent with the hypothesis that hybridization is occurring beyond the F1 generation. The biogeography of hybridization resembles an archipelago of hybrid populations arrayed along the Driftless boundary. The situation is unusual in that while most hybrid populations are in contact with extensive J. virginiana populations, none appear to be in contact with J. horizontalis. The one-sided structure of the hybrid zone suggests that hybrids are favored by selection along the eastern boundary of the Driftless Area.