Morphological Variation in North American Tilia and its Value in Species Delineation

McCarthy, Diane; Mason-Gamer, Roberta

ABSTRACT: Premise of research. Tree species typically have high levels of genetic and morphological variation that make the detection of species boundaries difficult. The genus Tilia in eastern North America andMexico is known for its morphological variation and uncertain species boundaries. We summarized the extent of morphological variation in North American Tilia and assessed previously proposed two- and four-species classifications. Methodology. Samples were collected from 566 individual trees, and 12 morphological characters were measured or calculated. Descriptive statistics were generated range-wide and by morphospecies, geographic range, and chloroplast DNA (cpDNA) lineage. Correlations among morphological variables and with latitude and longitude were calculated. Principal coordinates analysis was performed to detect morphological clusters and their correspondence to defined subgroups. In addition, the degree of variation within individuals was assessed. Pivotal results. Morphology is highly variable across the range of Tilia. Morphological characters do not delineate the four morphospecies identified by past workers, supporting the lack of species boundaries within North American Tilia. Two cpDNA lineages, in Mexico and in the United States, also cannot be separated by any morphological character. Twig and leaf trichome density are moderately negatively correlated with latitude. There is a smaller degree of intraindividual morphological variation than expected. Conclusions. Morphological data do not provide support for the existence of two or four species of Tilia within North America. The high degree of morphological variation without distinct gaps in characters supports the recognition of a single species, Tilia americana s.l.