Postglacial recolonization and cpDNA variation of silver birch, Betula pendula.

Chloroplast PCR-RFLP markers were used to reconstruct the history of the silver birch, Betula pendula Roth, in Europe since the last glacial maximum (LGM). In birch, fossil pollen maps do not reveal a clear chronological sequence of postglacial spread. If anything, the pollen record suggests that most of Europe was recolonized by birches as early as 10000 bp, probably from populations that remained close to the ice sheets during the LGM. The geographical distribution of haplotypes supports a scenario of early colonization. Two of the 13 haplotypes that were observed are common, representing 35% and 49% of the total sample, respectively. Although one of the common haplotypes is predominant in the NW and the other in the SE, both are present throughout most of the investigated geographical area. Rare haplotypes are geographically restricted. The distribution of the haplotypes reveals five genetic boundaries between groups of haplotypes and allows us to infer patterns of postglacial recolonization. Europe was re-occupied by two main waves of recolonization: one eastern and one western, with origins at intermediate latitudes. Populations in the Iberian Peninsula and in Italy did not take part in the postglacial recolonization of Europe.