Inferring long‐distance dispersal and topographic barriers during post‐glacial colonization from the genetic structure of red maple ( Acer rubrum L.) in New England

Aim  This study aims to assess the role of long‐distance seed dispersal and topographic barriers in the post‐glacial colonization of red maple ( Acer rubrum L.) using chloroplast DNA (cpDNA) variation, and to understand whether this explains the relatively higher northern diversity found in eastern North American tree species compared with that in Europe. Location  North‐eastern United States. Methods  The distribution of intraspecific cpDNA variation in temperate tree populations has been used to identify aspects of post‐glacial population spread, including topographic barriers to population expansion and spread by long‐distance seed dispersal. We sequenced c.  370 cpDNA base pairs from 221 individuals in 100 populations throughout the north‐eastern United States, and analysed spatial patterns of diversity and differentiation. Results  Red maple has high genetic diversity near its northern range limit, but this diversity is not partitioned by topographic barriers, suggesting that the northern Appalachian Mountains were not a barrier to the colonization of red maple. We also found no evidence of the patchy genetic structure that has been associated with spread by rare long‐distance seed dispersal in previous studies. Main conclusions  Constraints on post‐glacial colonization in eastern North America seem to have been less stringent than those in northern Europe, where bottlenecks arising from long‐distance colonization and topographic barriers appear to have strongly reduced genetic diversity. In eastern North America, high northern genetic diversity may have been maintained by a combination of frequent long‐distance dispersal, minor topographic obstacles and diffuse northern refugia near the ice sheet.