Histoire postglaciaire de la végétation au sud du lac Abitibi, Ontario et Québec

Abstract

Les lacs Yelle et Clo, situés à environ 10 km au sud du lac Abitibi, ont respectivement livré des séquences continues de sédiments depuis la déglaciation de la région, vers 9 000 BP, et le retrait du lac proglaciaire Ojibway, vers 7 900 BP jusqu’à l’actuel. L’interprétation des diagrammes polliniques révèle que la rive sud du lac Ojibway était occupée, vers 8 900 BP, par une forêt ouverte dominée par l’épinette noire (Picea mariana), dans laquelle le tremble (Populus tremuloides) et le pin gris (Pinus divaricata) jouaient un rôle important. Ce type de paysage s’est maintenu durant environ 1 000 ans dans les collines alors insulaires entourant le lac Yelle, pendant que le niveau du lac Ojibway passait de 355 à 280 m. Lors de la vidange du lac Ojibway, la colonisation végétale des basses terres s’est effectuée rapidement. De 7 900 à 7 200 ans BP, la forêt appartenait déjà au domaine de la sapinière à bouleau blanc, occupant les stations mésiques, mais l’épinette noire et le tremble étaient abondants, de même que le pin gris. De 7 200 à 6 000 BP, la végétation a atteint un maximum de diversité et de thermophilie. Le pin blanc (Pinus strobus) s’est installé, surtout dans les collines et sur les stations xériques. Entre 6 000 et 3 250 BP, le genévrier (Juniperus) est devenu beaucoup plus abondant. Depuis 3 250 ans BP, la forêt s’est refermée, et l’épinette noire et le pin gris ont progressé, aux dépens du pin blanc notamment. La sapinière à bouleau blanc, sur stations moyennes, s’est toujours maintenue au sein du paysage et, malgré des différences, révolution de la végétation a été synchrone entre les deux sites depuis 7 900 ans BP.Complete sequences of sediments representing the time since ice retreat (ca. 9,000 BP) and drainage of proglacial Lake Ojibway (ca. 7,900 BP) have been recovered from Lake Yelle and Lake Clo, two lakes, in a region about 10 km south of lake Abitibi. The interpretation of the pollen diagrams lead to the reconstruction of an open forest dominated by black spruce (Picea mariana), with abundant aspen (Populus tremuloides) and jack pine (Pinus divaricata), at the southern shore of lake Ojibway around 8,900 BP. This type of vegetation lasted about 1 000 years on the islands formed at that time by the hills around Lake Yelle, while the level of Lake Ojibway's level fell from 355 to 280 m. The forest vegetation migrated rapidly into the lowlands after the drainage of the proglacial lake. From 7,900 to 7,200 years BP, the forest was a balsam fir — white birch community, on the mesic sites, but black spruce and jack pine were abundant. From 7,200 to 6,000 BP, the vegetation was at its maximum diversity and thermophily. White pine (Pinus strobus) migrated into the area, especially on hills and on xeric sites. Between 6,000 and 3,250 BP, juniper (Juniperus) became more abundant, the forest canopy being still open. Since 3,250 BP, the forest cover has closed and black spruce and jack pine have increased at the expense of white pine, in particular. The balsam fir-white birch community, on the mesic sites...

Resilience of the boreal forest in response to Holocene fire-frequency changes assessed by pollen diversity and population dynamics

“…7), corresponds to the disappearance of Betula alleghaniensis and the relative increase of B. papyrifera or dwarf species such as B. pumila or B. glandulosa. Similar trends in birch-pollen diameter have been observed in Quebec (Richard 1980;Richard et al 1982) during the late Holocene, and have been interpreted also in terms of the disappearance of B. alleghaniensis (Marcoux and Richard 1995). These Betula records support the hypothesis that broadscale processes were more responsible for the detected changes than local events.…”

“…All regional pollen studies indicate a similar vegetation history since the retreat of the proglacial waters from the areas of the Mixedwood BF (Richard 1980;Liu 1990; present study) and the Coniferous BF (Garralla and Gajewski 1992;Gajewski et al 1993;present study). Pinus strobus, Thuja occidentalis and Taxus (all studies), and Betula cf.…”

Sugar maple (Acer saccharum) forests at their northern distribution limit are recurrently impacted by fire

“…Richard, personal communication, 2011) and the Témiscamingue (8000-6800 years BP; Bennett 1987;Richard 1980) regions when the studied sites were under the influence of a low fire activity. The hypothesis in which a wet climate is less favorable for the propagation of fire and therefore facilitates the expansion of sugar maple seems plausible at the regional scale but remains to be verified at the local scale.…”

“…as the main companion tree species. The maple forest was first established in southern Quebec 9900 years ago (hereafter in calibrated years) (Lavoie and Richard 2000) and migrated northward over the next four millennia reaching the Abitibi and the Haute-Gaspésie areas about 6300 and 5400 years ago, respectively (Richard 1980;Jetté and Richard 1992). Since their first postglacial establishment, southern maple forests (Muller and Richard 2001), as well as the maple forests in marginal regions (Labelle and Richard 1984;Richard et al 1992), appear to have maintained themselves until present time, as Holocene pollen data do not indicate a return to mixed or boreal forests.…”

Local versus regional processes: can soil characteristics overcome climate and fire regimes by modifying vegetation trajectories?

“…3). The rapid afforestation at both sites agrees well with studies in North America (Florin and Wright, 1969; Richard, 1980; Ritchie and MacDonald, 1986) and in northern Europe (Heikkilä et al , 2009) that show that trees were present near the front edge of the retreating ice sheets.…”