A Late-Quaternary Vegetation History from Hanging Lake, Northern Yukon

A 403—cm core was recovered from Hanging Lake in unglaciated northern Yukon. Twenty—one radiocarbon dates indicate that the section is at least 25 000 and possibly 33 000 yr old; they permit the calculation of pollen influxes for the full—glacial in eastern Beringia. Numerical methods were used to divide the pollen stratigraphy into five zones. From prior to 33 000 to 18 450 BP, a herb zone was dominant (zone HL 1) with high percentages of Gramineae, Artemisia, and Cruciferae. However, the low pollen influx, ranging from 5—100 grains°cm^—2°yr^—1, the low organic content of the sediment, and the occurrence of open—ground taxa all indicate that the vegetation cover was sparser than it is today. The arctic—alpine affinities of the herb pollen show that generically the vegetation was akin to modern arctic plant communities. Modern fellfield communities in the northern Yukon and Siberia have a rich and endemic Artemisia flora and they can produce pollen spectra comparable to that of the herb zone. Percent and influx values for spruce, alder, and birch increased slightly during subzone H 1B (21 680—18 450 BP); this subzone probably represents an interstadial. From 18 450 to 14 600 BP, a Salix—Cyperaceae zone (HL 2) occurred, suggesting the development of snowbed and willow scrub communities in sheltered areas. Between 14 600 and 11 100 BP Betula pollen dominated (zone HL 3) indicating the spread of dwarf birches, but the influx data show that this initial increase was modest compared with the subsequent zone and thus dwarf birches were probably restricted at this time to more favorable habitats. The spread of birch together with the increased total pollen influx, the higher organic content of the sediment, and the increased richness of herb pollen indicates that the local flora was more diverse and that the vegetative cover increased. The climate must have warmed. Zone HL 4 (Ericales zone) spanned the period from 11 100 to 8900 BP. Wet heath communities became locally abundant, poplar was more abundant at the beginning of this zone than at any subsequent time, the ranges of Typha latifolia and Myrica gale were greater than today, and pollen influx and sedimentation rate both increased greatly. Spruce became regionally abundant. These varied changes are probably in response to a warmer and wetter climate; they provide evidence for a late Pleistocene to early Holocene warm interval initially recognized by McCulloch and Hopkins (1966). Zone HL 5 (Alnus crispa zone) has been dated from 8900 BP to the present; it reflects the regional expansion of Alnus crispa on organic soils.