Chloroplast haplotypes suggest preglacial differentiation and separate postglacial migration paths for the threatened North American forest tree Juglans cinerea L.

Abstract

Postglacial migration paths for most of the tree species of Eastern North America remain unknown. The presence of no-analogue forest communities prior to the last glacial advance suggests that individual trees species in Eastern North America may respond differently as climate changes and human impacts increase. In this study, we examined chloroplast haplotypes from natural populations of Juglans cinerea L., a North American forest tree, to infer postglacial migration patterns. Sequences from eight different regions of the chloroplast genome in 197 trees distributed across the range revealed 10 haplotypes. A minimum spanning network, phylogenetic analysis and haplotype distributions revealed that the three most common haplotypes were geographically disjunct and not closely related. Haplotype 6 (73 trees) occurred only in western populations, haplotype 10 (83 trees) occurred only in eastern populations and haplotype 7 (21 trees) occurred only at the southern edge of the native range. The southernmost population contained the most haplotype diversity but included no eastern haplotypes. Haplotype phylogeography suggested geographical differentiation prior to the last glacial advance in eastern populations and separate postglacial migration paths for eastern and western populations. As migration of J. cinerea to Atlantic Canada from southern refugia does not appear possible given known seed dispersal mechanisms, the possibility of northern refugia or dispersal by extinct megafauna merits serious consideration. Differences among species in preglacial history, ecological niche preferences and seed dispersal mechanisms suggest that response to long-term climate change and acute human disturbance may be highly species specific.