Paléophytogéographie et paléoclimats postglaciaires dans l’ouest du Bas-Saint-Laurent, Québec

Abstract

L'analyse pollinique des sédiments de quatre lacs du Bas-Saint-Laurent et de la Côte-du-Sud permet de retracer l'existence d'une phase initiale de toundra s'étendant sur une durée estimée à 500-1000 ans, après à la déglaciation. Cette toundra montre un développement progressif à partir d'un quasi-désert jusqu'à un stade arbustif à bouleau glanduleux, en passant par un stade herbacé riche en plantes arctiques-alpines dont la présence est confirmée par l'analyse macrofossile. L'accumulation organique dans les lacs ne survient que vers 9500-9200 BP, soit plus de 2000 ans après la déglaciation, et correspond à l'arrivée des premiers arbres, d'après les macrorestes. Le tardiglaciaire régional paraît donc avoir été climatiquement très rigoureux par rapport aux régions sises au sud-ouest et au sud, dans les Appalaches. Les forêts sont restées ouvertes jusque vers 8000 BP, constituées surtout de l'épinette noire, des peupliers, du mélèze et du bouleau blanc. La zone pollinique correspondante est dominée par l'aulne crispé, qui a dû jouer un rôle important dans le couvert végétal durant une période caractérisée par une forte incidence des feux. Par la suite, la végétation moderne s'est établie, soit une mosaïque de sapinières et d'érablières avec sensiblement les mêmes séquences physiographiques et les mêmes gradients phytogéographiques régionaux que ceux actuellement constatés dans ce secteur des Appalaches. Les cédrières tourbeuses et les sapinières humides à thuya répandues actuellement dans les basses terres estuariennes et à la marge nord du plateau appalachien ne se sont toutefois développées que vers 3600 ans avant l'actuel. L'Optimum climatique holocene n'est clairement marqué que par une plus grande abondance du pin blanc de 7000 à 3600 ans BP, notamment à la marge nord du plateau appalachien.Postglacial paleophytogeography and paleoclimates in the western part of the Lower Saint Lawrence River region, Québec. Pollen analysis of the sediments of four lakes in the Lower Saint Lawrence River region, north of Maine, shows that tundra vegetation existed during an estimated period of 500 to 1000 years following ice retreat. Progressive development of the tundra from a quasi-desert to a shrub stage with dwarf birch, through an herb stage rich in arctic-alpine taxa is confirmed by macrofossil analysis. Organic sedimentation in the lakes started only around 9500-9200 BP, that is to say more than 2000 years after ice retreat, and is coincident with the arrival of the first trees (macrofossils) in the area. The regional late-glacial climate was thus apparently very harsh, compared to the Appalachian regions located to the south and southwest. Black spruce, the poplars, larch and white birch comprised the forests which remained open until around 8000 BP. The corresponding pollen zone is dominated by green alder which played a major role in the plant cover during a period characterized by a great incidence of fire. Afterwards, the modern vegetation developed, corresponding to a mosaic of fir and maple communities with physio...

Sugar maple (Acer saccharum) at its northeastern range limit: a fire-resilient tree species

“…This direct proof of fire incidence during the lower and the upper Holocene also confirms the abundance of microscopic charcoals from around 8500 years ago and from 2500 years ago until today, collected by Asnong and Richard (2003) from a small lake near the municipality of Gaspé. The same paleofire tendencies characterize the Bas-StLaurent region, situated west of the Gaspé Peninsula, and is highlighted by peaks in microcharcoal abundance in several lakes at the beginning of the Holocene (Richard et al 1992). The same fire frequency is again observable in macrocharcoal datings from the beginning of the Holocene in the region situated in the countryside around Rimouski, in the Saguenay, as well as in the greater Quebec City region (Talon et al 2005;Pilon and Payette 2015;Payette et al 2016;2017).…”

“…Farther west, in the Bas-St-Laurent region, which was marked by close contact between temperate and boreal forest flora, sugar maple was more abundant during the mid-Holocene than it is today, although even then it was not D r a f t predominant in these areas (Richard et al 1992). It is probable that some thermophilous species from the Gaspé region, such as sugar maple and red oak, also took hold in the balsam fir-yellow birch bioclimatic domain from the beginning of the mid-Holocene, around 5000 years ago, without reaching regional dominance.…”

Fire History of Appalachian Forests of the Lower St-Lawrence Region (Southern Quebec)

“…Because of the lack of macrofossil data, it is not possible to assess the regional distribution of sugar maple forests despite favorable local and climatic conditions which most likely prevailed during this period. However, Richard et al [45] suggest that sugar maple was more abundant during the mid-Holocene. Their conclusions on the development of the Appalachian vegetation, near our forest sites, are that forest communities were relatively stable during the Holocene, except during the initial phase of postglacial colonization.…”

“…Pollen data [45] also concluded on the persistence of the same tree species in the nearby Témiscouata (and Kamouraska) region where only minor vegetational changes occurred during the Holocene. The pollen curve of sugar maple west of the Rimouski area, in the Témiscouata uplands, shows the species occurrence early in the Holocene, at ca.…”

“…BP, if not earlier at 8500-9000 cal. BP [45]. It is probable that the species also arrived in the area south of Rimouski during the same period [46].…”