Circumscription and Biogeographic Patterns in the Eastern North American-East Asian Genus Stewartia (Theaceae: Stewartieae): Insight from Chloroplast and Nuclear DNA Sequence Data

Stewartia is a genus of approximately 26 species which are frequently divided into two subgenera or genera based on the leaf duration. All evergreen species (= Hartia) are found in the Old World. The deciduous species (= Stewartia sensu stricto) are dominant in the Old World but there are two representatives in the southeastern United States, Stewartia ovata and S. malacodendron. Maximum parsimony and likelihood data analyses of molecular DNA sequence data from both the nuclear and chloroplast genomes produce similar estimates of phylogeny for the group. All evergreen species sampled are more closely related to each other than to any of the deciduous species. The two New World deciduous species are more closely related to the evergreen species than to the Old World deciduous species. These findings complicate earlier vicariance biogeography hypotheses for the genus and challenge the recognition of Hartia, the latter of which disagrees with many published classification systems.