Distinguishing fossil Betula nana and B. pubescens using their wingless fruits: implications for the late-glacial vegetational history of western Norway

Abstract

Principal components analysis of 7 size and shape variables measured on 154 modern fruit bodies of Betula species and hybrids shows that B. pubescens and B. nana can be distinguished from each other on the basis of their fruit bodies, and that B. pubescens ssp. tortuosa and hybrids either fall within the range of B. pubescens or have an intermediate position on PCA axes 1 and 2. Passive positioning on the modern PCA axes of fossil fruit bodies from late-glacial sediments at Eigebakken, south-west Norway, shows that only B. nana was locally present in the Allerød. This contradicts earlier inferences from pollen analyses alone of birch woodland development in the Allerød in south-west Norway. There is no macrofossil evidence for tree-birches in the Allerød near Eigebakken. The relatively large amounts of Betula pollen, including B. pubescens, at Eigebakken are thus probably derived by long-distance transport from tree-birches in Denmark, south Sweden, and Britain, whose presence is proved there by macrofossils. Consequently, earlier estimates of mean July temperature during the Allerød in southwestern Norway should be reduced to around 7.5–10°C. In contrast, the PCA shows that fruit bodies from Holocene sediments at Eigebakken and at Kråkenes, western Norway, are mostly derived from B. pubescens. Birch trees were able to spread quickly and effectively across Norway in the early Holocene.