Reconstructing the postglacial history of the vegetation in northwestern Québec
Although the late Holocene vegetation and climate history at treeline in northwestern Québec has been studied in some detail by Serge Payette, his students and associates (Université Laval, Centre d'études nordiques), no pollen diagrams were available from this region. Fourteen new pollen diagrams are providing data about treeline variations in this region. Current work is investigating the spatial and temporal resolution of pollen diagrams in boreal environments.
The transition from forest to tundra is the treeline. In some areas, such as the Mackenzie Delta region of northwestern Canada, this transition can be abrupt - several tens of kilometers. In northwestern Québec, however, treeline is a large transition zone extending across several degrees of latitude, through a broad forest-tundra. The vegetation of this transition was divided into 4 zones by Payette: the boreal forest (lichen woodland), the forest - tundra tree subzone, forest tundra shrub subzone, and tundra. He suggested that this forest-tundra was once forested, and the patchy nature of the transition zone is the result of a late-Holocene deforestation by fire in the presence of a gradual neo-glacial cooling. Palynological studies provided a means to test this hypothesis.
A study of the modern pollen deposition in the region established that there was a relationship between the pollen and the vegetation (Gajewski, K. 1991. Représentation pollinique actuelle à la limite des arbres au Nouveau Québec. Journal canadien des sciences de la Terre 28:643-648). Four pollen diagrams from each of the 4 zones established that the vegetation was more dense in the forest tundra before around 2000 years ago (Gajewski, K., S. Payette et J.C. Ritchie. 1993. Holocene vegetation history at the boreal forest-shrub tundra transition in northern Québec. Journal of Ecology 81:433-443), and a further study of three other pollen diagrams from the tundra (Gajewski, K. et S. Garralla. 1992. Holocene vegetation histories from three sites in the tundra of northwestern Québec, Canada. Arctic and Alpine Research 24:329-336) confirmed that the forest had never extended further north than the present-day limit during the past 6000 years. The late Holocene deforestation of the forest subzone of the forest tundra was confirmed by three other pollen diagrams (Gajewski, K., S. Garralla and V. Milot-Roy. 1996. Postglacial vegetation at the northern limit of lichen woodland in northwestern Québec. Géographie physique et Quaternaire 50:341-350). All of these data are available from the North American Pollen Database or from us.
Current research is concentrating on the temporal and spatial resolution of pollen diagrams as indicators of vegetation change. Three pollen diagrams from the forest-tundra have been sampled at 50 to 100 year intervals to investigate the temporal resolution in tree-line pollen diagrams. In another study 5 pollen diagrams and 50 surface samples from a 5 by 20 km area are being analyzed to determine the spatial resolution of pollen studies.
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