Résumé
At the beginning of the Holocene, the Laurentide Ice Sheet was progressively melting and liberating eastern Canadian landscapes. Meltwater accumulated at the glacier front, forming the proglacial Lake Ojibway, which covered western Québec and eastern Ontario for approximately 2,000 years before flushing into James Bay around 8,200 years ago. Paleoecological vegetation dynamics reconstructions of sites covered by Lake Ojibway showed that spruce forests were established from the onset, whereas sites south of the southern extent of Lake Ojibway first went through a steppe period before being afforested. One possible explanation for this rapid postglacial forest establishment in western Québec is that islands present on Lake Ojibway (paleo-islands) were colonized by forests before the drainage of the proglacial lake, hence acting as migration outposts. However, this hypothesis had not yet been tested with empirical data. We studied sediments from two small lakes located on paleo-islands (current hilltops) of Lake Ojibway to reconstruct vegetation development and diversity at the local (macro-remains) and regional (pollen) scales. Afforestation occurred between 9,900 and 9,600 years before present on the paleo-islands and was followed by a transition from open woodlands to denser forests around 9,200 years ago. Pinus banksiana, Picea mariana and Betula papyrifera were thus established on the islands almost 1,000 years before the drainage of Lake Ojibway. We also used diversity indicators and charcoal particles counting to better understand and characterize the vegetation and fire dynamics. Diversity indicators showed opposite trends for macro-remains and pollen, indicating local diversification and regional homogenization of the vegetation, supporting the boreal-mixed wood establishment around 9,200 years ago. Charcoal particles combined with the Charcoal Size Distribution (CSD) method reveal that local fires occurred on the paleo-islands and seemed to trigger the vegetation diversification and densification observed. Our results suggested a high magnitude local fire could trigger the vegetation diversification during the colonization process. We also provided strong empirical evidence for boreal mixed wood forest early establishment on paleo-islands outposts, which likely fostered lowlands plant colonization following Lake Ojibway drainage.