Delayed postglacial colonization of Betula in Iceland and the circum North Atlantic

Abstract As the Arctic continues to warm, woody shrubs are expected to expand northward. This process, known as “shrubification”, has important implications for regional biodiversity, food web structure, and high-latitude temperature amplification. While the future rate of shrubification remains poorly constrained, past records of plant immigration to newly deglaciated landscapes in the Arctic may serve as useful analogues. We provide one new postglacial sedimentary ancient DNA (sedaDNA) record of vascular plants from Iceland and place a second Iceland postglacial sedaDNA record on an improved geochronology; both show Salicaceae present shortly after deglaciation whereas Betulaceae first appears more than 1000 years later. We find a similar pattern of delayed Betulaceae colonization in eight previously published postglacial sedaDNA records from across the glaciated circum North Atlantic. In nearly all cases, we find that Salicaceae colonizes faster than Betulaceae and that Betulaceae colonization is increasingly delayed for locations farther from glacial-age woody plant refugia. These trends in Salicaceae and Betulaceae colonization are consistent with the plant families’ species diversity, environmental tolerances, seed sizes, and soil preferences. As these reconstructions capture the migration of vascular plants during a past period of high latitude warming, a slow response of some woody shrubs to ongoing warming may delay Arctic shrubification and future changes in the structure of tundra ecosystems and temperature amplification.