A 16000-year record of vegetational change in south-western Alaska as inferred from plant macrofossils and pollen

Pollen and macrofossil analyses of a sediment core from Beaver Pond (60° 37' 14″ N, 154° 19' W, 579m a.s.l.) reveal a record of regional and local postglacial vegetation change in south-western Alaska. The chronology is based on five AMS (accelerator mass spectrometry) ¹⁴C ages obtained from terrestrial plant macrofossils. Pollen and macrofossil records suggest that open herb and shrub tundra with e.g. Poaceae, Cyperaceae, Artemisia, Vaccinium and Salix prevailed on the landscape before ca. 14000 cal a BP. The shift from herb- to shrub-dominated tundra (Salix, subsequent Betula expansion) possibly reflects climatic warming at the beginning of the Bølling period at ca. 14700-14500 and around 13500 cal a BP. Vegetation (Betula shrub tundra) remained relatively stable until the early Holocene. Macrofossil influx estimates provide evidence for greater biomass in Betula shrub tundra during the early postglacial period than today. Charcoal accumulation rates suggest tundra fire activity was probably greater from ca. 12500 to 10500 cal a BP, similar to results from elsewhere in Alaska. The pollen and macrofossil records of Beaver Pond suggest the prevalence of low shrub tundra (shrub Betula, Betula nana, Vaccinium, Ledum palustre, Ericaceae) and tall shrub tundra (Alnus viridis ssp. crispa, Salix) between 10000 and 4000 cal a BP. This Holocene vegetation type is comparable with that of the modern treeless wet and moist tundra in south-western Alaska. The expansion of Picea glauca occurred ∼4000 cal a BP, much later than that of A. viridis (ssp. crispa), whereas in central and eastern Alaska Picea glauca expanded prior to or coincident with Alnus (viridis). At sites located only 200-400km north-east of Beaver Pond (Farewell and Wien lakes), Picea glauca and Betula forests expanded 8000-6000 cal a BP. Unfavourable climatic conditions and soil properties may have inhibited the expansion and establishment of Picea across south-west Alaska during the mid and late Holocene.