Geographic variation and dispersal history in Fennoscandian populations of two forest herbs

Carex digitata and Melica nutans are forest understorey herbs with wide European distributions and their northern range margins in Fennoscandia. The species have closely similar habitat requirements, occur in small populations in old forest stands on base-rich to neutral soils and have restricted dispersal abilities at the present day. This study investigates the structure of allozyme variation (12 and 8 loci, respectively) in material of both species (38 and 37 populations, respectively) from throughout southern Sweden and southern Finland. Both species show a relatively low overall genetic diversity (HT excluding monomorphic loci = 0.17 and 0.18, respectively). The hierarchic structuring of allelic diversity in the species is similar, with a relatively high between-population component of diversity (G(ST) = 0.36 and 0.37. respectively). Neither of the species shows a clear intraspecific pattern of geographic differentiation. The lack of large-scale patterns of geographic differentiation is not consistent with a simple scenario of discrete and independent waves of immigration into Fermoscandia. However, particularly in M. nutans, a group of populations from a lowland belt across southwestern Finland and southern central Sweden is somewhat differentiated from populations to the north and south. A number of rare alleles in both species are widely, but patchily distributed in low frequencies. Hybridization may account for the scattered occurence of some of the rare alleles in Carex digitata, but cannot explain the distribution of rare alleles in Melica nutans.