E-mail : hugo.asselin@uqat.ca

Vegetation dynamics of a subarctic toposequence: interaction between climate and forest fires
Supervisor : Serge Payette  

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The combined action of climate and forest fires over the last 3000 years is responsible for the structure of the forest tundra landscape in northern Québec. This research uses a vegetation toposequence that includes deforested hill summits dominated by lichens and shrubs, mid-hill spruce lichen woodlands, and spruce-moss forest and peatland communities located at hill bottoms. The dynamics of this toposequence, from its post-fire origin to the present day, are still relatively unknown. The goal of this doctoral thesis was to describe these dynamics at several different spatial scales. At the regional scale, we confirmed the role of fire in opening the forest tundra landscape and determined when this process was the most active. At the local scale, a complex site was studied that included a degrading palsa plateau, a poor fen, mesic black spruce sites, and deforested hill summits, in order to determine the mechanisms influencing the dynamics of a typical subarctic toposequence. A new method of quantifying wood micro-charcoal on pollen slides allowed the differentiation of local fires (originating within the lake watershed) from the background noise attributed to the long-distance transport of charcoal. The opening of the tundra forest landscape was the most pronounced between 2000 and 900 years ago, probably in association with an increase in fire frequency. The anatomical identification of charcoal obtained from hill summits within the study area toposequence, which burned around the year 1000 AD, allowed the pre-fire vegetation composition to be reconstructed. The dominance of black spruce (~32% cover) at that time contrasts strongly with the current dominance of dwarf birch at the site. The analysis of black spruce annual growth rings for living, dead, and subfossil trees along the toposequence indicates that, in addition to fire, permafrost and a regional flooding event also played roles in developing the structure of the landscape .

Publications

Asselin, H., Payette, S. 2006. Origin and long-term dynamics of a subarctic tree line. Écoscience, 13 (2): 135-142. 

Asselin, H., Payette S., 2005. Detecting local-scale fire episodes on pollen slides. Review of Palaeobotany and Palynology, 137: 31-40.


Asselin, H., Payette, S., 2005. Late Holocene deforestation of a tree line site: estimation of pre-fire vegetation composition and black spruce cover using soil charcoal. Ecography, 28: 801-805.


Asselin, H., Payette, S., 2005. Late Holocene opening of the forest tundra landscape in northern Québec, Canada. Global Ecology and Biogeography, 14: 307-313.


Jasinski, J.P.P., Asselin, H. 2004. Alternative view on alternative stable states. Frontiers in Ecology and the Environment, 2: 10-11.


Asselin, H., Payette, S., Fortin, M.-J., Vallée, S., 2003. The northern limit of Pinus banksiana Lamb. in Canada: explaining the difference between the eastern and western distributions. Journal of Biogeography, 30: 1709-1718.