Arctic shrub colonization lagged peak postglacial warmth: Molecular evidence in lake sediment from Arctic Canada

Abstract

Arctic shrubification is an observable consequence of climate change, already resulting in ecological shifts and global-scale climate feedbacks including changes in land surface albedo and enhanced evapotranspiration. However, the rate at which shrubs can colonize previously glaciated terrain in a warming world is largely unknown. Reconstructions of past vegetation dynamics in conjunction with climate records can provide critical insights into shrubification rates and controls on plant migration, but paleoenvironmental reconstructions based on pollen may be biased by the influx of exotic pollen to tundra settings. Here, we reconstruct past plant communities using sedimentary ancient DNA (sedaDNA), which has a more local source area than pollen. We additionally reconstruct past temperature variability using bacterial cell membrane lipids (branched glycerol dialkyl glycerol tetraethers) and an aquatic productivity indicator (biogenic silica) to evaluate the relative timing of postglacial ecological and climate changes at a lake on southern Baffin Island, Arctic Canada. The sedaDNA record tightly constrains the colonization of dwarf birch (Betula, a thermophilous shrub) to 5.9 ± 0.1 ka, ~3 ka after local deglaciation as determined by cosmogenic 10 Be moraine dating and >2 ka later than Betula pollen is recorded in nearby lake sediment. We then assess the paleovegetation history within the context of summer temperature and find that paleotemperatures were highest prior to 6.3 ka, followed by cooling in the centuries preceding Betula establishment. Together, these molecular proxies reveal that Betula colonization lagged peak summer temperatures, suggesting that inefficient dispersal, rather than climate, may have limited Arctic shrub migration in this region. In addition, these data suggest that pollen-based climate reconstructions from high latitudes, which rely heavily on the presence and abundance of pollen from thermophilous taxa like Betula, can be compromised by both exotic pollen fluxes and vegetation migration lags. K E Y W O R D S ancient DNA, Arctic shrubification, deglaciation, dispersal, paleoclimate, paleothermometry, paleovegetation | 4245 CRUMP et al.

Lake Sedimentary DNA Research on Past Terrestrial and Aquatic Biodiversity: Overview and Recommendations

“…DNA-based studies of past vegetation have mainly focused on arctic, boreal, and alpine regions because of their high sensitivity to climate change, providing new insights on past vegetation dynamics and species distributions e.g., [16,26,27,[43][44][45][46][47][48][49]. At high-latitude regions, sedaDNA has for example contributed to increased knowledge on the occurrence of insect-pollinated plants, which are typically underestimated in pollen analyses [24,27,50].…”

“…Molecular reconstructions of past flora and fauna based on sedaDNA have been compared against a range of biological and geochemical sediment proxies, often showing complementarity between the different approaches [27][28][29]33,48]. In particular, comparisons with well-established proxies such as plant macrofossils, pollen and diatom enumeration, coprophilous fungal spores, or specific biomarker identification, have provided unique opportunities to evaluate the nature and reliability of the DNA signal obtained from sediments.…”

“…Thus, targeting long barcodes (typically >150 bp; [213]), which generally have higher taxonomic resolution [198,214,215] is not a viable option for sedaDNA approaches, even if shorter barcodes generally have lower taxonomic resolution [201,216,217]. An alternative is to use multiple barcodes and compare the results [48]. If shotgun metagenomics is used, then a representative sample of all DNA fragments present in an extract is sequenced, independent of the length.…”

Holocene Vegetation and Plant Diversity Changes in the North-Eastern Siberian Treeline Region From Pollen and Sedimentary Ancient DNA

“…Several pollen studies from the high northern latitudes document an early-to mid-Holocene treeline advance (Jones et al, 2011;Naidina and Bauch, 2011;Salonen et al, 2011) though studies from northern Siberia are still relatively scarce (Clayden et al, 1997). An open question exists with respect to lagged vegetation-climate relationships (Elmendorf et al, 2012;Herzschuh et al, 2016;Crump et al, 2019;Herzschuh, 2020). While it is assumed that the broad vegetation composition can be inferred from arctic pollen records despite the biases from different pollen productivities and dispersal characteristics (Klemm et al, 2013;Niemeyer et al, 2015), plant diversity estimates are challenging because of low taxonomic resolution in some studies (Naidina and Bauch, 2001;Rudenko et al, 2014).…”

Soil and Air Temperature Calibrations Using Branched GDGTs for the Tropical Andes of Colombia: Toward a Pan‐Tropical Calibration

“…We achieved the chromatographic separation, identification, and quantification of brGDGTs by using a slightly modified version of the protocol described by Hopmans et al (2016). Rather than starting at 18% hexane:isopropanol (9:1, v/v) (Hopmans et al, 2016), we began the eluent gradient with 30% hexane:isopropanol (9:1, v/v) to shorten overall run times without compromising the chromotographic separation of brGDGTs (Crump et al, 2019; Harning et al, 2019; Figure S1 in the supporting information). We re‐equilibrated the HPLC column for 20 min between runs.…”