Biogeographic history of green alder (Alnus alnobetula (Ehrh.) K. Koch s.l.) in Eurasia and North America: evidence from genetic and morphological analyses

The green alder (Alnus alnobetula s.l.) is a cold-resistant boreal-arctic shrub species with a complex intraspecific taxonomy and a wide distribution range covering northern Eurasia and North America. In this study, we assess the level and distribution of diversity in five subspecies (A. alnobetula subsp. fruticosa, kamtschatica, mandschurica, maximowiczii, and sinuata) throughout 34 populations within the native species range by using 7 characters of leaf morphology and 11 nuclear microsatellites (nSSR). The differentiation in both sets of characters that we found has proven to be inconsistent with our previously obtained chloroplast DNA (cpDNA) data. Only three groups were identified using nSSR vs. five cpDNA haplogroups. Both morphological and nSSR analyses support the recognition of A. alnobetula subsp. fruticosa from the western part of the Eurasian distribution range (northwestern Russia, the Urals, and Siberia) and A. alnobetula subsp. maximowiczii from the eastern part (most of Sakhalin Island, the Kuril Islands, and most of Kamchatka). Among other East Asian subspecies such as A. alnobetula subsp. kamtschatica and subsp. mandschurica, as well as in A. subsp. sinuata from North America, considerable genetic and morphological admixture has been recorded. The discordance between the patterns inferred from cpDNA and nSSR data reflects limited gene dispersal via seeds and extensive gene flow via pollen between major glacial refugia.