Early Holocene hybridisation between Betula pubescens and B. nana in relation to birch vegetation in Southwest Iceland

Abstract

The aims of the present research were to gain better insight into the early Holocene vegetation history of Betula in South Iceland and to investigate whether hybridisation between downy tree-birch (Betula pubescens) and dwarf birch (Betula nana) could be discovered in this region, similar to that previously found in North Iceland. A peat monolith spanning the period from about 10.3 to 7.6calkaBP from Eyvík, Southwest Iceland, provided 44 samples for pollen analysis. The samples were dated by known tephra layers, Saksunarvatn and Seydishólar tephras, and two 14C datings. The macrofossil and spore/pollen results showed several progressive phases of vegetation development in the area until birch woodland became established around 9.5calkaBP. The timing of birch colonisation is in good agreement with results from other sites in Iceland. From these Eyvík samples, all morphologically normal Betula pollen grains were size-measured and abnormal pollen grains recorded. Species proportions within samples were estimated statistically on the basis of pollen size. As birch woodland in this area became established rather late, statistical analysis of Betula pollen size was only possible in the uppermost 18 samples, approximately spanning the period between 7.6 and 9.5calkaBP. The frequency distribution of pollen size indicated populations of B. nana predating those of B. pubescens. The proportion of B. pubescens pollen was oscillating but increased with time. In two samples at the top of the peat monolith, more than half of the measured Betula pollen grains belonged to B. pubescens. Evidence of hybridisation based on high frequencies of non-triporate Betula pollen was found in two samples dated at about 8.8 and 7.7calkaBP. Conditions appeared to be comparable to the corresponding hybridisation events previously reported for North Iceland, i.e. in warm periods when downy birch was expanding near dwarf birch habitats.