Department of biology, chemistry and geography
Guillaume de Lafontaine (Regular member)
Introduction: The Eastern North American boreal landscape is mainly dominated by black spruce forest. However, some rare and isolated white birch stands located mainly on hillslopes facing south, create a distinctive vegetation mosaic in this conifer-dominated forest. The origin and dynamics of these marginal stands have seldom been investigated and are thus still poorly understood. As a response to contemporary climate change, we expect to see a northwards migration of temperate vegetation leading to a shift in the coniferous forest canopy of the boreal biome to a more deciduous one. In this context, this study will assess the origin and dynamics of marginal white birch stands to determine their evolution and future distribution in the boreal landscape.Objectives: In order to study the origin and dynamic of marginal white birch stands in the boreal biome, a spatial analysis will first quantify their distribution along a latitudinal gradient. Then, white birch stands along the latitudinal gradient will be compared in terms of species composition, stand structure and regeneration potential. Finally, fire disturbance history and tree composition will be reconstructed on recent (last fire) and multimillennial (Holocene) scales.Study sites: Study sites: are located in Eastern North America, along a gradient extending from Baie-Comeau up to north of the Caniapiscau Reservoir. A total of 20 sites were chosen in the closed-crown boreal forest (spruce moss forest and fir-birch forest) and the open boreal forest (lichen woodland and forest tundra) to represent a south-north gradient used to study the range margins of white birch stands in the boreal landscape. To select study sites, we relied on a spatial analysis to detect white birch dominated stands (>80% of canopy cover) surrounded by conifer forest (spruce, balsam fir).Material and methods: Using a combination of digital mapping, satellite images and field observations, sites with a high presence of white birch were selected within the boreal landscape. Sample sites of 400 m2 were used to quantify the vegetation composition, stand structure, understory regeneration and edaphic conditions of white birch stands. Fire history at stand-scale will be reconstructed from an analysis of wood charcoal particles recovered from mineral soil and surface samples. A total of 15 soil samples were collected, filtered, and sorted out to extract charcoals. Charcoal particles will then be identified to the genus or species level based on microscopic wood anatomical features and a sub-sample will be radiocarbon-dated. This method will be used to reconstruct fire and forest vegetation history of the stand and assess the forest composition prior to the last fire event. Expected results: Virtually no study has previously investigated these stands, but we posit three alternatives hypotheses that might explain their presence. The marginal white birch stands could be i) relics of a previous more northern distribution of the fir-birch forest, in which white birch is a companion species; ii) a transitionary successional state following a recent disturbance; or iii) an alternative stable state indicating the onset of a shift towards the infilling of deciduous forests within the boreal biome.
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