Ecological genetic differentiation in American ginseng (Panax quinquefolius L.) across an elevational gradient

Background/Question/Methods American ginseng is a wide-ranging, but sparsely distributed understory species of the eastern deciduous forest. Its breeding system favors reproductive assurance via selfing and molecular genetic studies have suggested that a high degree of between-population genetic differentiation should be found in this species. In this study, we used a reciprocal transplant approach to determine the degree of ecologically relevant genetic differentiation between two populations found at contrasting elevations (760 m and 270 m; here designated H for high elevation and L for low elevation). Within each transplant site, populations were planted in replicated unmixed plots to prevent crossing among populations and to allow demographic characteristics to be determined in home and away sites.

Results/Conclusions Phenological, morphological, and allocation patterns exhibited several consistent population differences or population x site interactions after two full years in place. The H population allocated more biomass to reproduction, as indicated by greater bud and seed numbers per unit leaf area, and greater peduncle lengths. Phenologically, the H population showed delayed flowering and seed maturation, relative to the L population, regardless of site. The L population showed greater morphological plasticity (e.g., in leaf length and leaf area per unit height) in response to the elevational difference between sites. Although three field seasons were not sufficient to judge fitness differences in this long-lived perennial plant, the classical ecotypic response (superior performance for each population in its home site) was not observed during that time period. Instead, there was some evidence, particularly with respect to leaf area growth rates, that the low elevation population grew significantly better than the high elevation population at the high elevation site. One intriguing possible explanation is that the high elevation climatic conditions are similar to the conditions under which the low elevation population evolved. In other words, the climate may already have shifted such that both populations are no longer well-adapted to their home sites. Alternative explanations, such as escape of the low elevation population from pathogens or predators cannot be ruled out, however. Continued demographic censusing of these populations over many years may be necessary to conclude that a mismatch of genotype and environment has occurred due to climate change.