Phylogeographical structure and temporal complexity in American sweetgum (Liquidambar styraciflua; Altingiaceae).

Eastern North American plant biogeography has traditionally focused on two primary issues: (i) the location of temperate Pleistocene refugia and their proximity to the southern margin of the ice sheet during the last glacial maximum, and (ii) the origin of the temperate element of northern Latin America. While numerous population genetic and phylogeographical studies have focused on the first issue, few (if any) have considered the second. We addressed these issues by surveying 117 individuals from 24 populations of Liquidambar styraciflua (American sweetgum; Altingiaceae) across the southeastern USA, eastern Mexico, and Guatemala, using more than 2200 bp of chloroplast DNA sequence data. To specifically address the issue of timing, we estimated intraspecific divergence times on the basis of multiple fossil-based calibration points, using taxa from Altingiaceae (Liquidambar and Altingia) and Hammamelidaceae (Hamamelis) as outgroups. More than half of the sampled localities exhibited multiple haplotypes. Remarkably, the greatest variation was observed within the USA, with Mexico and Guatemala sharing widespread haplotypes with Texas, Mississippi, Kentucky, Ohio, and northern Virginia. This lack of differentiation suggests shared ancestral polymorphisms, and that the genetic signal we observed is older than the disjunction itself. Our data provide support for previously proposed hypotheses of Pleistocene refugia in peninsular Florida and along the eastern Atlantic, but also for deeper divergences (approximately 8 million years ago) within the USA. These patterns reflect a dynamic biogeographical history for eastern North American trees, and emphasize the importance of the inclusion of a temporal component in any phylogeographical study.