The Late Quaternary History of Vegetation and Climate at Porcupine Mountain and Clearwater Bog, Manitoba

Abstract

Radiocarbon-dated pollen diagrams from two sites in the southern Boreal forest of Canada have reflected aspects of the local and regional environments since 6,700 and 1,000 BP, respectively. Spruce forest near Porcupine Mountain was replaced by grassland ca. 6,700 BP, with a maximum of prairie taxa occurring just before 5,140 BP and a short-lived reduction of grass and herb pollen shortly after that date. The grassland episode ended at 4,200 BP when spruce forest dominated Porcupine Mountain. The site experienced very rapid Sphagnum peat growth and increased sporogenesis after 2,450 BP.

A tentative climatic interpretation is supplied which suggests that 6,700 to 4,200 BP experienced generally dry, warm summers, with a maximum of this effect just prior to 5,140 BP and a cooler spell following; after 4,200 BP the summer climate was cooler and moister, especially from 2,450 to 2,000 BP. The possibility of a regional increase in soil erosion and sheet flooding prior to 6,700 BP is examined.

Clearwater Bog is underlain by a spruce forest horizon dated 1,200 BP which was established at a time of reduced water level in Clearwater Lake; the Picea timbers were overlain by very humified peat dated 900 BP. Unhumified Sphagnum peat later formed and continued to the modern bog surface. The climatic interpretation is that the summer climate was warm and dry at 1,200 and 900 BP, and that cooler, wetter summers characterized the period since then to the present day.

The suggested climatic sequences are synchronous at many points with the scheme previously developed for southern Keewatin and northern Manitoba, and some of the vegetational changes are provisionally interpreted as the movement of the southern limit of the Boreal forest in phase with the shifts of the Keewatin forest-tundra boundary described earlier. This correlation encourages comparison with other sites in the Northern Hemisphere.