Birch hybridization in Thistilfjördur, North-east Iceland during the Holocene

Abstract

Past episodes of birch hybridization in Iceland were studied by pollen analysis. The low stature and polycor-mic form of downy birch Betula pubescens in the subarctic probably results from such hybridization with the dwarf birch Betula nana. Two previous studies in different regions in Iceland revealed events of hybridization connected to early B. pubescens expansion. The present study examined a peat monolith from Thistilfjördur, North-east Iceland, covering the last ten thousand years. Betula pollen was measured, proportions of B. nana and B. pubescens calculated and the presence of hybrids estimated. Betula pubescens expansion started around 7.2 cal ka BP with a peak in non-triporate pollen, indicating hybrids. Low Betula pollen concentrations followed. A second period of considerable hybridization is indicated around 3.3 cal ka BP. Both peaks were associated with B. pubescens expansion.