Steppe-tundra composition and deglacial floristic turnover in interior Alaska revealed by sedimentary ancient DNA (sedaDNA)
A challenge in tracing vegetation dynamics over long timescales (which often cover larger environmental changes than recent observations) is to obtain enough information on past floristic composition to allow a detailed understanding of past communities and their transitions. The late-Pleistocene herbaceous vegetation of ice-free Beringia was part of an intriguing late-Quaternary “no-analogue” biome; both climate and top-down biotic control (e.g., via megafaunal grazing and trampling) are suggested as major environmental influences and implicated in its end-Pleistocene demise. Sedimentary ancient DNA (sedaDNA) can provide floristic records far richer than—yet consistent with—those typically obtained from pollen. The 15,500-year lake-sediment record of sedaDNA from interior Alaska, reported here, provides an exceptionally rich record of this no-analogue vegetation and its replacement during deglaciation. With insight from contemporary vegetation studies and known palaeohydrological changes we reconstruct herbaceous communities and their transition to woody-dominated communities after ca. 14,000 calibrated years before present (cal. yr BP). Initially, vegetation was dominated by forbs found in modern tundra and/or subarctic steppe vegetation (e.g., Hedysarum, Potentilla, Draba, Eritrichium, Plantago, Bupleurum, Anemone patens), and graminoids (e.g. Agrostidinae, Bromus pumpellianus, Festuca, Calamagrostis, Puccinellia), with Salix the only prominent woody taxon. Taxon ecology indicates xeric (occasionally mesic) warm-to-cold habitats. The mixed (steppe-tundra) character of the fossil assemblages is explained as a topo-mosaic of insolation-affected communities, some of which resemble those on dry, open, low-to-mid-elevation hillslopes today. At ca. 14,000 cal. yr BP, just after the first-documented human arrivals and coincident with an increase in effective moisture, Betula expanded. Graminoids became less abundant, but while there was some turnover in forbs, many open-ground taxa persisted; this new mosaic of woody and herbaceous growth forms is compatible with the observed persistence of megafaunal species, which were important resources for early humans. The greatest period of taxonomic turnover, marking the transition to regional forest and a further moisture increase, began ca. 11,000 cal. yr BP. Populus expanded, along with new shrub taxa (e.g., Shepherdia, Eleagnus, Rubus, Viburnum).  Picea then expanded ca. 9500 cal. yr BP, with the entry of further shrub and forb taxa typical of evergreen boreal woodland (e.g., Spiraea, Cornus, Galium, Linnaea). Most taxa in the present catchment flora were established by ca. 5 cal. yr BP, but Larix first appeared ca. 1500 cal. yr BP. Prominent fluctuations in aquatic taxon composition and richness between ca. 14,000 and ca. 9,500 cal. yr BP appear related to lake-level changes prior to the lake reaching its high, near-modern depth ca. 8,000 cal. yr BP.