Biogeographical Distribution of Chloroplast Diversity in Northern Red Oak (Quercus rubra L.)

We mapped the biogeographical distribution of chloroplast haplotypes in northern red oak (Quercus rubra L.) to test the hypotheses that the founder effects during postglacial migration will result in a latitudinal gradient of haplotype diversity with exhaustion of haplotype richness at the extreme northern edge of the range. PCR-RFLP markers for 23 populations of Q. rubra in old growth and minimally disturbed forests across 13,000 km2 of the contemporary range revealed only four of the five haplotypes detected previously at 12 sites in Indiana. The four northernmost populations were fixed for either haplotype I or haplotype II, the two most common haplotypes. Haplotype richness declined poleward, but the biogeographical pattern of 829 haplotypes across the 35 sites did not clearly indicate postglacial migration routes along latitudinal gradients from the southern edges of the study area. Although Q. rubra and the European white oak Q. robur L. are both early successional species that recolonized vast areas as the glaciers retreated, the data indicate that Q. rubra has lower chloroplast haplotype richness, suggesting that the population dynamics of Q. rubra prior to or during Pleistocene climate fluctuations may have differed from the population dynamics of the European white oak taxa.