The late-Quaternary vegetational history of the Western Interior of Canada

The late-Wisconsinan pollen stratigraphy of the Western Interior of Canada is assembled and 16 representative sites provide a basis for establishing trends of vegetation history. Sites in the southern, prairie region show an early Picea–Artemisia assemblage (12 × 103 to 10 × 103) replaced by a zone dominated by herb pollen. Similarly, sites in the Aspen Parkland and Transitional zones all have the Picea–Artemisia zone from as early as 13.9 × 103 to 10 × 103, followed by a herb zone. It is replaced by an arboreal pollen zone at 4.5 × 103 to 3 × 103 B.P. While there is a broad trend common to all sites in the modern boreal forest, from an early spruce-dominated assemblage to a late postglacial spruce–pine–birch assemblage identical with modern spectra, there are differences in the details of stratigraphy. Two sites in central Alberta have a poplar zone preceding the early spruce zone. Sites near the modern southern forest boundary show a late development (3 × 103 B.P.) of the mixed boreal forest from prairie and hardwood deciduous forest communities. One site, at Flin Flon, Saskatchewan, provides clear evidence for a treeless episode (Artemisia–grass–sedge) preceding the spruce zone.As the late-Wisconsinan glacial ended, an early version of the boreal forest, dominated by spruce and lacking pine, spread from adjacent U.S. on to deglaciated surfaces and till over stagnant ice. It persisted in the southern part of the area until about 10 × 103 and until 6 × 103 in the northernmost portions. In the south the spruce forest was replaced directly by prairie, spreading from the southwest and extending farther north than its modern limit between 7.5 × 103 and 6 × 103. All remnants of ice sheets and glacial lakes had disappeared by about 7 × 103 and the northern part of the area was occupied by a boreal forest undergoing rapid changes in composition from the early spruce-dominated version to the mixed spruce–pine–birch–poplar mosaic prevalent today. Pine probably spread from western réfugia, at least into areas north and west of L. Winnipeg. At about 3 × 103 the southern limit of the forest extended to the south, apparently in response to a climate with cooler and (or) wetter growing seasons. The rapid replacement of the spruce-dominated boreal forest by grasslands in the early postglacial was probably a response to warmer and drier growing seasons.