Late-Quaternary Climatic and Vegetational Change in the Lower Mackenzie Basin, Northwest Canada

A 355—cm sediment sequence from Twin Tamarack Lake near Inuvik, Northwest Territories, Canada, provides a 14 500—yr record of pollen percentages and accumulation rates, interpretable in terms of climatic change and plant community dynamics. Four distinct pollen assemblage zones are recognized: a Gramineae—Artemisia—Cyperaceae—Salix zone from 14 500 to 11 800 BP; a Betula glandulosa zone from 11 800 to 8400 BP, subdivided into four subzones (Gramineae—herb, Populus, Salix, and Juniperus); a Picea—Betula zone, from 8400 to 5900 BP; and a Piceae—Betula—Alnus zone from 5900 to 0 BP. The vegetation during the late—glacial at this site (14 500—11 800 BP) was relatively stable, consisting of sparse herb tundra on uplands in a slowly warming climate. Dwarf birch increased during this phase. The early Holocene period of maximum summer radiation produced thermal conditions °10% warmer than present, initiating a change from tundra to woodland at most sites, effected by the efficiently dispersed Populus (chiefly P. balsamifera). Relatively rapid changes in regional vegetation occurred during this period (11 800 to 6500 BP); Juniperus invaded the poplar woodlands, occupying well—drained, favorable sites, and later the slower—dispersed Picea (chiefly P. glauca) arrived and formed stands on alluvial sites, largely replacing poplar. At °7500 BP, Betula papyrifera spread on to the uplands; the accumulation of fuel, accelerated by the spread of conifer woodlands, likely caused an increase in fire frequency, which in turn maintained arboreal birch in the area. Fire opened up the woodlands locally and promoted the spread of Alnus crispa, expressed in the pollen record as a prominent rise in both percentage and PAR values. Picea mariana and other species adapted to mire habitats spread at this time (°7000 BP) as paludification, peat growth, and permafrost aggradation occurred in the extensive lowlands. The termination of summers warmer than at present (°6000 BP) initiated the development of a relatively stable vegetation that has persisted to the present.