Rethinking long‐term vegetation dynamics: multiple glacial refugia and local expansion of a species complex

Abstract

Evidence is accumulating that some arcto‐boreal plant taxa persisted through the last glacial maximum (LGM) in Alaska and adjacent Canada. However, the spatial patterns of glacial persistence and associated postglacial colonization remain largely unknown. In this study, we investigated the LGM refugia of an alder (Alnus) species complex (n = 3 taxa) and assess the spatiotemporal dynamics of Alnus in this vast region. Specifically, we conducted high‐throughput DNA sequencing (ddRADseq) on Alnus foliar samples collected from a dense population network to investigate patterns of genetic structure and infer the presence of glacial lineages. Species distribution modeling (SDM) was used to investigate the probability and possible locations of glacial persistence. These analyses were integrated and then compared with fossil pollen data to identify the locations of refugial populations and spatial patterns of postglacial colonization. Our genetic analyses revealed two glacial lineages with separate geographic origins for each Alnus taxon, suggesting that the genus persisted in multiple LGM refugia. Non‐overlapping hindcast distributions based on SDMs further support the presence of multiple, spatially distinct refugia. These ddRADseq and SDM results, in conjunction with reassessment of fossil pollen records, suggest that Alnus expanded from several population nuclei that existed during the LGM and coalesced during the Holocene to form its present range. These results challenge the unidirectional model for postglacial vegetation expansion, implying that climate buffering associated with landscape heterogeneity and adaptation to millennial‐scale environmental variability played important roles in driving late‐Quaternary population dynamics.

Ice‐age persistence and genetic isolation of the disjunct distribution of larch in Alaska

“…Our results join a growing body of research demonstrating iceage persistence in areas previously thought to be climatically unsuitable for tree species during the LGM (e.g., Napier et al, 2019;Provan & Bennett, 2008;Roberts & Hamann, 2015;Stewart & Lister, 2001;Zeng, Wang, Liao, Wang, & Zhang, 2015). However, unlike other studies of boreal and temperate taxa that considered northern populations as "cryptic microrefugia" (Tzedakis, Emerson, & Hewitt, 2013), With no "rescue effect" possible from external gene flow, an "Allee effect," or decreased fitness associated with inbreeding depression in smaller population sizes, threatens the continued existence of these remnant populations (Stephens, Sutherland, & Freckleton, 1999).…”

“…These studies have called into question the role of long-distance migration as the dominant species response to shifting climates (Christmas, Breed, & Lowe, 2016;Corlett & Westcott, 2013;de Lafontaine, Napier, Petit, & Hu, 2018). In particular, a growing number of phylogeographic surveys have demonstrated populations persisted in situ, in refugia closer to the ice sheets during the LGM than previously discernible from other lines of evidence (e.g., Anderson, Hu, Nelson, Petit, & Paige, 2006;de Lafontaine, Ducousso, Lefèvre, Magnanou, & Petit, 2013;McLachlan, Clark, & Manos, 2005;Napier, de Lafontaine, Heath, & Hu, 2019).…”

Persistence of arctic-alpine flora during 24,000 years of environmental change in the Polar Urals

“…23–25 . Previous studies have often had to rely on interpolation and/or extrapolation to argue for long-term species persistence, based on sporadic occurrences of a taxon in the fossil record and its presence within the present-day vegetation mosaic 26–28 .…”

Ancient DNA from lakes: Insights into plant response to past climate changes

“…recently, several lines of evidence based on fossil analysis, species distribution modeling, and phylogeographical surveys have been used to propose the existence of these small microrefugia at northern latitudes in the Northern Hemisphere (e.g. Napier et al 2019). in Scandinavia, in central Sweden, several megafossils of mountain birch (Betula pubescens Ehrh.…”