Microscopical palynology: Birch woodland expansion and species hybridisation coincide with periods of climate warming during the Holocene epoch in Iceland.

Introgressive hybridisation between arctic dwarf birch (Betula nana) and European downy birch (B. pubescens) is relatively common in natural woodlands in Iceland. As dwarf birch is a diploid species and downy tree birch a tetraploid species, their hybrids are triploid. In the introgression process, triploid hybrids, which are partially fertile, can backcross with the parental species, producing progenies comprising introgressed diploid, triploid and tetraploid plants. Triploid plants produce both normal triporate pollen (with three pores) and abnormal, aborted pollen, due to dysfunctional meiosis. The type of pollen abnormality that can be detected and quantified is non‐triporate pollen (with four or more pores in the pollen wall). We therefore used the presence of non‐triporate pollen as a marker to trace birch hybridisation in the past. In the current study we examined fossil pollen in samples from Holocene sediments from three locations: Grímsnes (SW), Eyjafjördur (N) and Thistilsfjördur (NE Iceland). All three peat monoliths had the starting age of 10.3 cal. ka BP. Ages were calibrated based on known tephra layers and by radiocarbon dating. The size of Betula pollen grains was measured, and the species proportions calculated from size. Non‐triporate grains were detected in samples from all three locations and throughout the Holocene, but with different frequencies. The peaks of intense hybridisation followed birch woodland expansion in two major periods of the Holocene, both coinciding with a warming of climate. The first period occurred in the Early Holocene, around 9.5–7 cal. ka BP, when the climate warmed rapidly after deglaciation. The second period occurred around 5–3.5 cal. ka BP, well within the mid‐Holocene Northern Hemisphere warming. A new wave of birch hybridisation appears to have started in the last few decades as the climate has warmed. Birch woodlands are likely to become more widespread. Introgressed shrub birch is likely to be more competitive over dwarf birch. LAY DESCRIPTION: We microscopically examined birch (Betula) pollen in samples from peat sediments extracted from three geographically diverse locations in Iceland and throughout the Holocene epoch, which began 10,000 years ago. By measuring the size of individual pollen grains, we were able to differentiate pollen of the shrub‐like downy birch (B. pubescens) from its closely related dwarf birch species (B. nana). The results revealed an establishment and a rapid expansion of birch woodland predominated by B. pubescens soon after the retreat of the Icelandic Pleistocene icesheet. We also found abnormal looking pollen grains in the samples, essentially those having more than the usual three pores on the pollen wall, so‐called non‐triporate pollen. This type of pollen abnormality is common among pollen produced by triploid birch hybrids, the progeny of hybridisation between diploid dwarf birch and tetraploid downy birch. Non‐triporate grains were detected in samples from all three locations and throughout the Holocene, but with different frequencies. The results showed peaks of intense hybridisation following woodland expansion in the initial period of the Holocene, from about 9000 years ago, and again in the warming period of the mid‐Holocene Thermal Maximum, the period between 5000 and 3000 years ago. Triploid hybrids that were produced during the intense period of hybridisation could backcrossed with the dwarf birch or the downy birch, allowing gene flow by introgression between the two species, presumably making birch more adaptive to environmental changes. Thus, climate warming in the current era is expected to promote this introgressive hybridisation resulting in significant change of landscape of the birch woodland in Iceland. [ABSTRACT FROM AUTHOR]