ABSTRACT Following deglaciation, lowland sites in eastern Canada that were covered by proglacial Lake Ojibway recorded direct afforestation with boreal mixedwood taxa, without an initial tundra phase, contrary to sites that were never covered by this lake. Because former islands of proglacial Lake Ojibway were colonized by vegetation before lake drainage around 8200?cal a bp, we hypothesized that these paleo-islands went through a non-arboreal phase which eventually transitioned to boreal mixedwoods, hence providing a source of propagules for rapid and later colonization of the lowlands. We carried out a multi-proxy analysis combining pollen, macro-remains, sedimentary ancient DNA and charcoal to document vegetation composition and dynamics on two paleo-islands. Both study sites recorded progressive vegetation establishment starting with a tundra-like phase with only herbs and shrubs, followed by open forests dominated by either larch or pine, culminating with the establishment of the boreal mixedwoods about 300 years before the final drainage of proglacial Lake Ojibway. Fire regimes seem to have partially driven vegetation dynamics and diversity on the paleo-islands. Paleo-islands were sources of propagules, which helps explain how the former Lake Ojibway lowlands were directly colonized by boreal mixedwoods, without an initial tundra phase.

Postglacial vegetation colonization that followed ice retreat and proglacial lakes drainage in north-eastern America occurred rapidly, more rapidly than expected based on the modern dispersal capacities of boreal mixedwood trees. Paleo-islands from proglacial Lake Ojibway in Québec (Canada) were afforested early, before the final drainage of the lake. We hypothesized that these paleo-islands could have acted as outposts of migration and thus, could explain the rapid afforestation of lowlands observed. To determine if postglacial colonization occurred as a south-north front from the southern margin of Lake Ojibway or if islands acted as migration outposts, we estimated the date of first arrival of the main taxa of the current boreal mixedwood forest. We studied southern sites never covered by proglacial Lake Ojibway, sites that were islands within Lake Ojibway, and northern lowland sites that were liberated after the final drainage of proglacial Lake Ojibway. Taxa arrival was estimated as a sharp rise of the pollen percentage or as the occurrence of macro-remains within the sediments of small lakes dated with radiocarbon. Then we compared migration scenarios where colonization occurred gradually from south to north from the southern margin of proglacial Lake Ojibway and where paleo-islands of Lake Ojibway were first colonized through long-distance dispersal, thus becoming sources of seeds readily available to colonize lowland sites after the final drainage of Lake Ojibway. Finally, we compared the migration rates from the scenarios with the current mean dispersal capacities of the studied taxa. The migration rates estimated without taking the paleo-islands into account are too slow to explain the rapid afforestation observed following the final drainage of proglacial Lake Ojibway. Only the migration rates estimated from the scenarios with paleo-islands were comparable to the current mean dispersal capacity of the boreal mixedwood taxa. Thus, paleo-islands acted as stepping stones during postglacial migration, which explains why the lowlands were rapidly colonized. Larger paleo-islands and those located closer to the southern margin of the proglacial Lake were colonized first, in line with the theory of island biogeography.

Après le retrait du glacier dans l’est de l’Amérique du Nord il y a environ 11 700 ans, la colonisation végétale s’est effectuée plus rapidement que prévu comparativement aux taux de migration estimés à partir des capacités de dispersion actuelles moyennes des arbres boréaux. Ce décalage entre taux de migration réel et estimé se nomme paradoxe de Reid. À l’ouest du Québec, les hypothèses classiques pour le résoudre ne s’appliquent pas ou n’ont pas été démontrées. Aucun refuge glaciaire n’a été mis en évidence (hypothèse 1) et la possibilité d’événements de dispersion à longue distance (hypothèse 2) n’a pas été testée empiriquement. L’afforestation des basses-terres en Abitibi-Témiscamingue a non seulement été rapide, mais aussi sans phase toundrique. Dans ce doctorat, les anciennes îles (paléo-îles) du lac proglaciaire Ojibway (présent entre 10 570 et 8200 ans avant aujourd’hui (cal. BP)), devenues des sommets de collines aujourd’hui, ont été étudiées afin de déterminer si elles ont pu jouer le rôle d’avant-postes de colonisation et ainsi accélérer la colonisation végétale. L’analyse des sédiments lacustres de deux paléo-îles confirme l’établissement des premiers arbres sur les paléo-îles vers 9850-9500 ans cal. BP, soit avant le début du drainage du lac Ojibway. Une analyse multiparamétrique (grains de pollen, macro-restes végétaux et ADN ancien) des sédiments montre l’établissement progressif de la végétation sur les paléo-îles avec d’abord une phase toundrique, suivie par l’afforestation, puis l’établissement de la forêt boréale mixte vers 8200 cal. BP. La végétation initiale diffère entre les sites, sûrement en réponse à différentes conditions édaphiques, topographiques et de dynamiques de feux, puis converge après la vidange du lac proglaciaire Ojibway. Pour finir, la comparaison de différents scénarios de colonisation confirme que les paléo-îles ont joué un rôle d’avant-postes de colonisation. Elles ont facilité et accéléré la colonisation postglaciaire et permettent donc de résoudre le paradoxe de Reid dans l’ouest du Québec. Selon l’étude des distances et surfaces des paléo-îles, il semble que leur colonisation suive la théorie de la biogéographie insulaire.

At the end of the last glacial period in the northern hemisphere, meltwater from receding ice sheets accumulated into large proglacial lakes, potentially limiting postglacial afforestation. We explored whether former islands of proglacial Lake Ojibway (Canada) (hilltops in the current landscape) could have acted as migration outposts and thus accelerated the postglacial migration. We extracted sediments from two small lakes located on “paleo-islands” and used XRF to detect changes in soil erosion and vegetation biomass. We also used plant macro-remains and wood charcoal to determine if (and which) tree species colonized the sites and to detect local fire events. Organic sediment accumulation started around 9657 and 9947 cal. yr BP at Lakes Perché and Despériers, respectively, before the level of Lake Ojibway started to decrease and liberate parts of the studied landscape ca 9400 cal. yr BP. Lithogenic elements (Ti, K, Sr, Fe, Zr, and Rb) decreased between the beginning of organic sediment accumulation and 8800–8700 cal. yr BP, indicating reduced soil erosion, possibly due to soil stabilization by vegetation. Then, the S/Ti ratio, a proxy of organic matter increased around 8800 and 8400 cal. yr BP. The earliest tree macro-remains (Larix laricina and Pinus spp.) were found between 9850 and 9500 cal. yr BP. Local fires were detected around 9820 and 8362 cal. yr BP. Early afforestation occurred on the islands of Lake Ojibway, 200 and 450 years before its level started to decrease, confirming that some islands acted as migratory outposts accelerating postglacial migration.