Le couvert végétal du Québec-Labrador il y a 6000 ans BP : essai

Abstract

Une carte du couvert végétal du Québec-Labrador est proposée pour l'époque des 6000 ans BP, à partir des données paléopalynologiques et paléobotaniques. Une Toundra arbustive dense s'interposait entre les mers de Tyrrell ou d'Iberville et les glaces résiduelles qui occupaient alors plus de 100 000 km2 en Ungava (Nunavik) et jusque dans la région de Schefferville. Étroite au sud des glaciers, cette Toundra s'étalait à l'est des glaces sur plus de 400 km jusqu'à la mer du Labrador, au nord du 54° de latitude. La limite des arbres s'établissait donc alors à peu près au centre de l'actuelle zone de Taïga (Forêt boréale ouverte), mais la limite nord de la Forêt fermée correspondait déjà à peu près à la position moderne. Comme la Toundra, la Taïga de l'époque était arbustive, plus dense et plus diversifiée qu'actuellement, sans doute en raison d'un climat favorable, mais aussi d'une plus grande richesse des sols en éléments nutritifs. Les domaines forestiers méridionaux des Pessières, des Sapinières et des Érablières atteignaient dans l'ensemble les limites modernes de leur aire de peuplement, mais l'abondance de certaines essences compagnes différait le plus souvent de l'actuelle. Ces différences sont attribuables 1) à une migration tardive de certaines essences, telles le pin gris (Pinus banksiana) dans les Pessières occidentales ou le hêtre à grande feuilles (Fagus grandifolia) dans les Érablières, 2) à une incidence généralement plus faible des feux naturels, 3) à un entourbement moindre des stations hydriques, et 4) à un climat dans l'ensemble plus clément. L'analyse régionale permet d'identifier des gradients et des champs phytogéographiques plus nuancés éventuellement influencés par le climat, notamment en zones montagnardes et subalpines.A cartographie reconstruction of the vegetational cover of Québec-Labrador for 6 ka BP is based on palynological and plant macrofossil records. A dense Shrub Tundra occupied the stretches of land between marine waters (Tyrrell Sea or d'lberville Sea) and the residual ice masses that still occupied over 100 000 km2, from Ungava (Nunavik) to the Schefferville area. This Tundra belt was narrow along the southern limit of glaciers but expanded eastward to a width of more than 400 km, extending to the Labrador Sea north of latitude 54°. The tree-line thus ran approximately through the middle of the present-day Taiga zone (open boreal forest), but the northern limit of the Closed Forest already corresponded to its modern position. Like the Tundra, the 6 ka BP Taiga was shrubby, more dense, and more diversified than todays, probably due to favorable climatic conditions but also because soils contained more nutrients than today. The southern forest vegetation zones, dominated respectively by spruce, fir, and maple, generally occupied their modern positions, but the abundance of some companion tree-species differed markedly from the modern. The differences are attributed 1) to delayed migration, as in the case of jack pine (Pinus banksiana) in the Western Spruce Forest domain or America...

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

“…6a). The high pollen values of Thuja/ Juniperus observed here are likely due to T. occidentalis (Liu 1990;Richard 1995). The two sites show the same pattern, with an immediate expansion of Populus, P. strobus and Taxus during the afforestation.…”

The Neotoma Paleoecology Database, a multiproxy, international, community-curated data resource

“…These prior efforts, beginning in North America and Europe in the 1970s (Davis, 1976;Bernabo and Webb, 1977;Huntley and Birks, 1983;Sadler et al, 1992;Grimm et al, 2013), resulted in multiple paleoecological databases, each usually restricted to a particular data set type or region-for example, the North American, European, African, and Latin American Pollen Databases (Vincens et al, 2007;Fyfe et al, 2009;Grimm et al, 2013;Flantua et al, 2015); the FAUNMAP and MIOMAP terrestrial vertebrate databases (Graham et al, 1996;Carrasco et al, 2007); the Mexican Quaternary Mammal Database (Arroyo-Cabrales et al, 2007, 2009; NANODe (Forester et al, 2005); the Diatom Paleolimnology Data Cooperative (https://diatom.ansp.org/dpdc/) (Sullivan and Charles, 1994); North American and Latin American packrat midden databases Latorre et al, 2014) and plant macrofossil databases (Jackson et al, 1997(Jackson et al, , 2000; the Base de Données Polliniques et Macrofossiles du Quebec (Richard, 1995); the BUGS insect database (Sadler et al, 1992); and others. Other, newer proxies, such as testate amoebae and organic biomarkers, are just beginning to be gathered for use in broadscale studies and need a common platform for data archiving, sharing, and reuse.…”

Tundra in a Changing Climate

“…In Beringia, the location of the northern forest boundary was close to the present; in the Keewatin and Labrador areas, the ecotone's shift was ~200 km. About 6 ka B.P., on the Labrador Peninsula, there still remained a small residual glacier, which probably explains the anomalous position of the northern forest boundary in the Holocene Optimum in this region [Richard, 1995]. This border reached its most northern position later, after the complete collapse of the glacier [Richard, 1995].…”

“…About 6 ka B.P., on the Labrador Peninsula, there still remained a small residual glacier, which probably explains the anomalous position of the northern forest boundary in the Holocene Optimum in this region [Richard, 1995]. This border reached its most northern position later, after the complete collapse of the glacier [Richard, 1995]. The shrub tundra in the Holocene Optimum extended farther north than now, while the border between the northern and the southern belts of the dwarf shrub tundra was similar to the modern one ].…”