Systematics and Phylogeography of the Genus Tilia in North America

Tilia americana, American basswood, is a species of tree found in mesic forests throughout eastern North America and the cloud forests of Mexico. Despite being common and widespread, little is known about basswood’s evolutionary and geographic history. In this study, the phylogeny and phylogeography of basswood were investigated using chloroplast DNA sequences, morphological measurements, and species distribution modeling. The North American basswoods were found to be a monophyletic group containing two distinct clades, one primarily in the United States and one primarily in Mexico. The majority of the genetic diversity resides within the United States, particularly in the southern Appalachian region. Diversity is reduced in the northern latitudes, a pattern consistent with post-glacial range expansion. Diversity is also low within Mexico. The most frequent haplotypes in Mexico appear to be closely related to populations in the southern Appalachians and Piedmont region, suggesting a U.S. origin for those Mexican populations. Other Mexican haplotypes are closely related to Texas populations; the Mexican distribution of basswood might therefore be the result of two independent colonizations by U.S. trees. Historically, the U.S. basswoods have been subdivided into three or more species based on morphological characters. In this study, such subdivisions are not supported by the presence of genetic lineages, and while there are regional differences in morphology, it is impossible to unambiguously delineate species within the U.S. On the other hand, despite the presence of a genetic division between the U.S. and Mexico, the basswood trees of the two countries cannot be distinguished based on morphology. Using a species distribution modeling approach, the U.S. and Mexico lineages can, however, be distinguished based on their ecological tolerances and geographic isolation. Using a combination of DNA sequences and ecological evidence, it is concluded that Tilia americana could be separated into two species, one in the U.S. and one in Mexico.