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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\u2010triporate pollen (with four or more pores in the pollen wall). We therefore used the presence of non\u2010triporate 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: Gr\u00edmsnes (SW), Eyjafj\u00f6rdur (N) and Thistilsfj\u00f6rdur (NE Iceland). All three peat monoliths had the starting age of 10.3\u00a0cal. ka BP. Ages were calibrated based on known tephra layers and by radiocarbon dating. The size of <jats:italic>Betula<\/jats:italic> pollen grains was measured, and the species proportions calculated from size. Non\u2010triporate 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\u20137\u00a0cal. ka BP, when the climate warmed rapidly after deglaciation. The second period occurred around 5\u20133.5\u00a0cal. ka BP, well within the mid\u2010Holocene 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.<\/jats:p>","DOI":"10.1111\/jmi.13175","type":"journal-article","created":{"date-parts":[[2023,2,13]],"date-time":"2023-02-13T13:25:11Z","timestamp":1676294711000},"page":"128-141","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["Microscopical palynology: Birch woodland expansion and species hybridisation coincide with periods of climate warming during the Holocene epoch in Iceland"],"prefix":"10.1111","volume":"291","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3133-2185","authenticated-orcid":false,"given":"Kesara","family":"Anamthawat\u2010J\u00f3nsson","sequence":"first","affiliation":[{"name":"Institute of Life and Environmental Sciences University of Iceland  Reykjav\u00edk Iceland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lilja","family":"Karlsd\u00f3ttir","sequence":"additional","affiliation":[{"name":"Institute of Life and Environmental Sciences University of Iceland  Reykjav\u00edk Iceland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"\u00c6gir Th\u00f3r","family":"Th\u00f3rsson","sequence":"additional","affiliation":[{"name":"Institute of Life and Environmental Sciences University of Iceland  Reykjav\u00edk Iceland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Margr\u00e9t","family":"Hallsd\u00f3ttir","sequence":"additional","affiliation":[{"name":"Institute of Life and Environmental Sciences University of Iceland  Reykjav\u00edk Iceland"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2023,2,28]]},"reference":[{"key":"e_1_2_6_2_1","first-page":"304","volume-title":"Betula Linnaeus. 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