Ancient plant DNA reveals High Arctic greening during the Last Interglacial

Summer warming is driving a greening trend across the Arctic, with the potential for large-scale amplification of climate change due to vegetation-related feedbacks [Pearson et al., Nat. Clim. Chang. (3), 673–677 (2013)]. Because observational records are sparse and temporally limited, past episodes of Arctic warming can help elucidate the magnitude of vegetation response to temperature change. The Last Interglacial ([LIG], 129,000 to 116,000 y ago) was the most recent episode of Arctic warming on par with predicted 21st century temperature change [Otto-Bliesner et al., Philos. Trans. A Math. Phys. Eng. Sci. (371), 20130097 (2013) and Post et al., Sci. Adv. (5), eaaw9883 (2019)]. However, high-latitude terrestrial records from this period are rare, so LIG vegetation distributions are incompletely known. Pollen-based vegetation reconstructions can be biased by long-distance pollen transport, further obscuring the paleoenvironmental record. Here, we present a LIG vegetation record based on ancient DNA in lake sediment and compare it with fossil pollen. Comprehensive plant community reconstructions through the last and current interglacial (the Holocene) on Baffin Island, Arctic Canada, reveal coherent climate-driven community shifts across both interglacials. Peak LIG warmth featured a ∼400-km northward range shift of dwarf birch, a key woody shrub that is again expanding northward. Greening of the High Arctic—documented here by multiple proxies—likely represented a strong positive feedback on high-latitude LIG warming. Authenticated ancient DNA from this lake sediment also extends the useful preservation window for the technique and highlights the utility of combining traditional and molecular approaches for gleaning paleoenvironmental insights to better anticipate a warmer future.

The Arctic is greening as shrub biomass increases and vegetation ranges shift north in response to summer warming (1, 2). This process—one of the clearest terrestrial manifestations of climate change thus far—has major implications both for local ecosystems and for global energy balance and biogeochemical systems (3–5). In particular, taller shrubs darken otherwise snow-covered surfaces, contributing to the albedo feedback (6, 7), and enhanced evapotranspiration is expected to result in a positive greenhouse feedback (8). Shrub cover also impacts soil thermal regime, which may impact permafrost vulnerability (9–11). Because feedbacks related to Arctic greening are complex and potentially large in magnitude, estimating the extent and rate of northward shrub migration is a vital component of predicting future warming.Past warm periods serve as valuable analogs for understanding the extent of Arctic greening under well-constrained climate conditions. The Last Interglacial (LIG; Marine Isotope Stage [MIS] 5e, 129 to 116 ka [thousands of years before present]) was ∼1 °C warmer than the preindustrial period globally, but the Arctic experienced amplified warming due to higher summer insolation anomalies and positive feedbacks at high latitudes (12, 13). The Eastern Canadian Arctic and Greenland, in particular, were likely ∼4 to 8 °C warmer in summer than present (Fig. 1) (14–18). LIG sediment records from this region thus provide an archive of the vegetation response to Arctic warming at levels comparable to predicted 21^st-century climate change (19).Open in a separate windowFig. 1.Map of Baffin Island and Lake CF8 study area. The symbols represent maximum LIG temperature anomalies based on terrestrial proxy records (shape indicates proxy type) from Baffin Island and Greenland (see SI Appendix, Table S1 for metadata). The shaded regions indicate Arctic bioclimate subzones delineations (29), including modern Betula range in subzones D and E. We note that a small outlier population of Betula occurs east of the D/E boundary on Baffin Island (not captured by vegetation map resolution) (38).While most High Arctic lake basins were scraped clean by ice sheet erosion during the last glaciation and thus only contain postglacial sediments, lakes with small, low-relief catchments within regions of cold-based, slow-flowing ice were protected from erosion. Several such sites have been discovered on eastern Baffin Island, Arctic Canada and contain stratified records of multiple interglacials (20–22). Previous work from Lake CF8 on northeastern Baffin Island (Fig. 1 and SI Appendix, Fig. S1) demonstrates that its sediment record spans at least three interglacials (∼200 ka), including a substantially warmer-than-present LIG as indicated by chironomids, diatoms, and geochemical proxies (15, 23).We targeted the multi-interglacial record from Lake CF8 to assess the vegetation response to pronounced warmth during the LIG and moderate warmth during the Holocene. Pollen produced by some key shrubs and trees, including Betula (birch), is efficiently wind-transported and thus present in lake sediments far north of their ranges (24, 25). We therefore analyzed both sedimentary ancient DNA (sedaDNA), which is sourced locally from within the lake catchment and does not include pollen-derived DNA (26), and fossil pollen to generate a robust vegetation record spanning the last ∼130 ka. Taken together, DNA-inferred plant communities and pollen-inferred July air temperatures provide insight into Arctic plant range shifts under strong summer warming.