Cette thèse est composée de trois articles qui forment chacun un chapitre et qui ont été soumis ou acceptés dans des revues scientifiques avec comité de lecture. Pour chaque article, je suis le principal responsable de la récolte des données incluant l'échantillonnage au terrain, les analyses de laboratoire, l'interprétation des résultats et la rédaction. Les co-auteurs des articles ont contribué à la recherche par des échanges sur les résultats et interprétation de la recherche, l'aide à l'utilisation des logiciels ou encore en fournissant des données.

Le premier article s'intitule «Quantifying spatial and temporal Holocene carbon accumulation in ombrotrophic peatlands of the Eastmain region, Quebec, Canada» et a été publié dans la revue Global Biogeochemical Cycles en mai 2011. Les co-auteurs de cet article sont Pierre-Luc Dallaire (ex-candidat à la maîtrise), Michelle Garneau et Yves Bergeron (respectivement directrice et codirecteur de recherche). Dans ce chapitre, les données issues de l'étude du terrain à l'aide du géoradar ont été fournies par Pierre-Luc Dallaire.

Le deuxième article est sous presse dans la revue The Holocene et s'intitule «Holocene carbon accumulation rates from three ombrotrophic peatlands in boreal Quebec, Canada: impact of climate-driven ecohydrological change». Il est co-écrit par Michelle Garneau et Robert K. Booth. La contribution de Robert K. Booth (Lehigh University, États-Unis) a consisté à l'application d'une fonction transfert appliquée aux assemblages de thécamoebiens et permettant d'obtenir une reconstruction de la nappe phréatique dans le temps.

Le troisième article, «Did fires drive Holocene carbon sequestration in boreal ombrotrophic peatlands of eastern Canada?» a été soumis pour publication à la revue Quaternary Research en décembre 2010. Les co-auteurs sont Michelle Garneau, Yves Bergeron et Adam A. Ali (Université Montpellier 2). Adam A. Ali a contribué à l'application du logiciel utilisé à l'interprétation des séquences de macrocharbons.

The global boreal forests comprise large stocks of organic carbon that vary with climate and fire regimes. Global warming is likely to influence several aspects of fire and cause shifts in carbon sequestration patterns. Fire severity or forest floor depth of burn is one important aspect that influences both carbon emission during combustion as well as post­fire ecosystem regeneration. Numerous publications on projections of future area burned exist, whereas scenarios on twenty-first century fire severity are more scarce, and the stand-typical response to severe fire weather is rarely taken into account. This paper aims to syn­thesize knowledge on boreal forest carbon stocks in relation to changes in fire severity for Quebec, Canada. Besides warming, this region may be subjected to an important increase in future precipitation. Future fire severity and area burned may well increase as fire weather will be drier, especially near the end of the twenty-first century. Moreover, the fire season peak may shift towards the late summer. Intense burning will favour tree cover de­velopment while the forest floor carbon stock may become less important. As a result, to­tal Quebec boreal carbon sequestration may diminish. The development of dynamic vege­tation models may improve scenarios on twenty-first century changes in carbon sequestra­tion driven by climate change and fire severity and frequency effects.

The boreal forest comprises a number of biomes that possess large quantities of terrestrial carbon (C). Climate scenarios show that changes in climate will be largest in the higher northern latitudes and can be an important factor in determining fire occurrence in these regions, being the most important disturbance factor in the boreal biomes. Of all fire traits, fire severity is the one that directly affects both direct C release to the atmosphere as well as the potential for postfire vegetation establishment.

Using climate scenarios and scenarios for future forest floor C contents obtained from the scientific literature, I investigated the factors that influence the fire severity regime and estimated the effect of climate change on the occurrence of severe burning and the effects on the C stock for each of the Québec boreal biomes.

Climate scenarios show an increase in both mean summer temperature and mean summer precipitation for most of the Québec boreal forest. In addition, the increase in winter precipitation may influence the deep forest floor moisture content and cause the potential for deep burning to shift during the fire season. A lower-fire severity regime was confirmed by Fire Weather Index scenarios that indicate a general lower intensity of spreading fires. The occurrence of less frequent severe fires may well induce thicker forest floors and lower tree biomass, resulting in increasing boreal C stocks. However, the direct influence of increasing summer temperatures may cause an increase in tree production.

The moisture index used was not directly applicable in estimating forest floor consumption by fire. In addition, the link between fire severity and fire frequency needs to be taken into account in determining the long-term boreal C stocks. Therefore, further research is required to define more precise scenarios on the potential for future high-severity fire presence as well as a better picture of long-term stand development concerning C dynamics in the absence of fire.