Constraints on post-glacial boreal tree expansion out of far-northern refugia

Abstract

Aim To use a variety of data sources to infer how northern boreal trees recovered their range upon deglaciation and/or Holocene warming.
Location Scandinavia, Alaska/north-west Canada (eastern Beringia).
Methods Mapped fossil occurrences for Picea (spruce) were assessed against available palaeoenvironmental and phylogeographic information.
Results For Scandinavia, Last Glacial Maximum (LGM) evidence of Picea is confined
to one DNA record, but late-glacial and early-Holocene records include scattered macrofossils. Holocene pollen data show a clear east–west increase to high values. A haplotype unique to the Scandinavian Peninsula is recognized. For eastern Beringia pre- and post-LGM macrofossils occur, but the LGM fossil record comprises only scattered low pollen values. Early Holocene pollen values increase markedly c. 11 cal yr bp (north-west Canada) and c. 10 kcal yr bp (central Alaska). Also at this time three sites on the Bering Land Bridge indicate the presence of Picea where it is now absent. Several unique regional haplotypes were recorded; while most are rare one is common in some modern populations.
Main conclusions Small Picea populations probably occurred in pre-Holocene Scandinavia, but pollen patterns argue against immediate expansion with the onset of warmer conditions. Despite relatively weak fossil evidence, refugial populations are also probable in eastern Beringia, particularly given the extent of unglaciated terrain. Post-glacial pollen patterns are more nuanced, suggesting two spatially and temporally distinct expansions, one possibly consistent with a unique central Alaskan haplotype, and subsequent westerly ‘filling-in’. The presence of macrofossils and/or neutral markers does not require that current northern populations are derived primarily from refugial ones, particularly where pollen patterns show delayed directional expansion of large populations though time. Refugial populations initially responded weakly to major post-glacial environmental change; if subject to genetic isolation and strong selection pressure they may have had little potential to do otherwise, instead being largely replaced by in-migrating populations with greater genetic diversity.

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ORCID for Mary E. Edwards: orcid.org/0000-0002-3490-6682

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