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

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 (brGDGTs) 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 10Be 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.