Space-time Millenial-Scale Evolution of Vegetation and Climate Gradients in Boreal and Mixed Forests of Western Québec since 9000 years

English Abstract:

Millenial-scale changes in vegetation gradients and climate conditions were reconstructed in more than 25 lacustrine cores of the boreal and mixed forests, in western Québec, based on the modern analogue technique applied to pollen assemblages. The modern database used includes 1010 sites from eastern Canada, east of 100°W. Pollen assemblages of surface samples and modern climate conditions are available in the database and a cluster (K-means) analysis applied to surface pollen assemblages helped in assessing the modern vegetation of the sites. The reconstructed vegetation types are illustrated on maps and postglacial changes in vegetation gradients are evidenced. The reconstructed climate parameters include July temperature, annual precipitation and summer sunshine. Here, the reconstruction of summer sunshine is used to document large-scale atmospheric circulation patterns. Although climate reconstructions from individual sites indicate some local differences, there are generally consistent features on a regional scale including a progressive 1°C decrease in July temperature since 9000 cal BP, a progressive 150mm/yr increase in annual precipitation and a 3% decrease in summer sunshine. Data also indicate notable climate and vegetation changes at 5000 cal BP. In western Québec, climate was warmer and drier than today before 5000 cal BP and the dominant westward winds were stronger. This climate transition is tentatively attributed to a weakening of atmospheric circulation. Maps of paleo-vegetation point to west to east gradients before 5000 cal BP and to more diversified vegetation. Thermophilous trees were then more abundant than today in forests of western Québec. Reconstructed vegetation also illustrates that modern latitudinal gradients were attained lately at ca. 3500 cal BP. Coniferous trees abundance then increased in vegetation landscapes and modern boreal forest established in western Québec.