Steppe-tundra composition and deglacial floristic turnover in interior Alaska revealed by sedimentary ancient DNA (sedaDNA)

Abstract

When tracing vegetation dynamics over long timescales, obtaining enough floristic information to gain a detailed understanding of past communities and their transitions can be challenging. The first high-resolution sedimentary DNA (sedaDNA) metabarcoding record from lake sediments in Alaska—reported here—covers nearly 15,000 years of change. It shows in unprecedented detail the composition of late-Pleistocene “steppe-tundra” vegetation of ice-free Alaska, part of an intriguing late-Quaternary “no-analogue” biome, and it covers the subsequent changes that led to the development of modern spruce-dominated boreal forest. The site (Chisholm Lake) lies close to key archaeological sites, and the record throws new light on the landscape and resources available to early humans. Initially, vegetation was dominated by forbs found in modern tundra and/or subarctic steppe vegetation (e.g., <i>Potentilla, Draba, Eritrichium, Anemone patens</i>), and graminoids (e.g., <i>Bromus pumpellianus, Festuca, Calamagrostis, Puccinellia</i>), with <i>Salix</i> the only prominent woody taxon. Predominantly xeric, warm-to-cold habitats are indicated, and we explain the mixed ecological preferences of the fossil assemblages as a topo-mosaic strongly affected by insolation load. At ca. 14,500 cal yr BP (calendar years before C.E. 1950), about the same time as well documented human arrivals and coincident with an increase in effective moisture, <i>Betula</i> expanded. Graminoids became less abundant, but many open-ground forb taxa persisted. This woody-herbaceous mosaic is compatible with the observed persistence of Pleistocene megafaunal species (animals weighing ≥44 kg)—important resources for early humans. The greatest taxonomic turnover, marking a transition to regional woodland and a further moisture increase, began ca. 11,000 cal yr BP when <i>Populus</i> expanded, along with new shrub taxa (e.g., <i>Shepherdia, Eleagnus, Rubus, Viburnum</i>). <i>Picea</i> then expanded ca. 9500 cal yr BP, along with shrub and forb taxa typical of evergreen boreal woodland (e.g., <i>Spiraea, Cornus, Linnaea</i>). We found no evidence for <i>Picea</i> in the late Pleistocene, however. Most taxa present today were established by ca. 5000 cal yr BP after almost complete taxonomic turnover since the start of the record (though <i>Larix</i> appeared only at ca. 1500 cal yr BP). Prominent fluctuations in aquatic communities ca. 14,000–9,500 cal yr BP are probably related to lake-level fluctuations prior to the lake reaching its high, near-modern depth ca. 8,000 cal yr BP.