Late Quaternary vegetation history of the central Mackenzie Mountains, Northwest Territories, Canada

The sedimentary and pollen records from three lakes in the tundra, forest-tundra and open forest of the central Mackenzie Mountains of Canada, are described. Although two of the lakes lie beyond the mapped limits of late Quaternary glaciation, their records only extend to ∼ 12,000 yr B.P. The third site lies within the limits of the late Wisconsin Laurentide advance and a basal radiocarbon date provides a minimum date of ca. 11,500 yr B.P. for retreat of the northwestern margin of Laurentide ice. Artemisia and Salix-dominated herb-tundra occurred at all elevations from at least 12,000 yr B.P. until ca. 10,200 yr B.P., when shrub tundra dominated by Betula glandulosa expanded throughout the region. Populus balsamifera was also present by this time and probably grew up to and beyond the current treeline. Picea arrived in the central Mackenzie Mountains by 8500 yr B.P., and expanded to the elevation of the current treeline by 8000 yr B.P. Vegetation composition became similar to that of the present by 6000 yr B.P. with the expansion of Alnus at all elevations. Pollen accumulation rates and detrended correspondence analysis suggest Picea population densities in the forest-tundra were greater than present between ca. 8000 yr B.P. and 5000 yr B.P. and have since declined steadily. At the open forest site, Picea population density has not declined in the late Holocene, but the forest composition has shifted towards a greater affinity with higher elevation sites. In general, the changes apparent in the treeline vegetation of the Mackenzie Mountains during the postglacial are consistent with predicted changes in summer insolation based on the Milankovitch theory.