Catastrophic fire years that have taken place during the last decade in Siberia, and more generally within the boreal forest, have been directly linked to global warming and had strong repercussions on boreal ecosystems and human populations. In this context the study of the past dynamics of these fires is essential for understanding their links with climate, vegetation and human activity changes on longer time scales than the last few decades. However, few studies on fire dynamics are available for Siberia, and none have been conducted for the entire Holocene period. This study presents the first fire history reconstruction of this area during the Holocene based on charcoals sequestered in sediments of two lakes located on the southern shore of Lake Baikal, in Siberia. The results show a similar trend in the two lakes, with high frequency and high peak magnitude during the Early Holocene and low magnitudes after 6500 cal. yr BP. This difference is interpreted as crown fires versus surface fires. According to pollen records (Dulikha, Vydrino, Ochkovoe) available near the studied lakes, a vegetation transition occurred at the same time. Picea obovata, which has a tree structure prone to crown fires, was dominant during the Early humid Holocene. After 6500 cal. yr BP, conditions were drier and Pinus sylvestris and Pinus sibirica became the dominant species; their tree structure favors surface fires. In addition to vegetation dynamics, the nearby pollen sequence from Dulikha has been used to provide quantitative estimates of past climate, indicating an Early to Middle Holocene climatic optimum between 8000 and 5000 cal. yr BP and an increase in temperatures at the end of the Holocene. These results have been compared to outputs from regional climate models for the Lake Baikal latitudes. Fire dynamics appear to have been more linked to the vegetation than climatic conditions. Over the past 1500 years, the greater presence of human populations has firstly resulted in an increase in the fire frequency, then in its maintenance and finally in its suppression, which may possibly have been due to very recent fire management, i.e., after ca 500 cal. BP.

At the end of the last glacial period in the northern hemisphere, meltwater from receding ice sheets accumulated into large proglacial lakes, potentially limiting postglacial afforestation. We explored whether former islands of proglacial Lake Ojibway (Canada) (hilltops in the current landscape) could have acted as migration outposts and thus accelerated the postglacial migration. We extracted sediments from two small lakes located on “paleo-islands” and used XRF to detect changes in soil erosion and vegetation biomass. We also used plant macro-remains and wood charcoal to determine if (and which) tree species colonized the sites and to detect local fire events. Organic sediment accumulation started around 9657 and 9947 cal. yr BP at Lakes Perché and Despériers, respectively, before the level of Lake Ojibway started to decrease and liberate parts of the studied landscape ca 9400 cal. yr BP. Lithogenic elements (Ti, K, Sr, Fe, Zr, and Rb) decreased between the beginning of organic sediment accumulation and 8800–8700 cal. yr BP, indicating reduced soil erosion, possibly due to soil stabilization by vegetation. Then, the S/Ti ratio, a proxy of organic matter increased around 8800 and 8400 cal. yr BP. The earliest tree macro-remains ( Larix laricina and Pinus spp.) were found between 9850 and 9500 cal. yr BP. Local fires were detected around 9820 and 8362 cal. yr BP. Early afforestation occurred on the islands of Lake Ojibway, 200 and 450 years before its level started to decrease, confirming that some islands acted as migratory outposts accelerating postglacial migration.

Following deglaciation, lowland sites in eastern Canada that were covered by proglacial Lake Ojibway recorded direct afforestation with boreal mixedwood taxa, without an initial tundra phase, contrary to sites that were never covered by this lake. Because former islands of proglacial Lake Ojibway were colonized by vegetation before lake drainage around 8200 cal a bp, we hypothesized that these paleo‐islands went through a non‐arboreal phase which eventually transitioned to boreal mixedwoods, hence providing a source of propagules for rapid and later colonization of the lowlands. We carried out a multi‐proxy analysis combining pollen, macro‐remains, sedimentary ancient DNA and charcoal to document vegetation composition and dynamics on two paleo‐islands. Both study sites recorded progressive vegetation establishment starting with a tundra‐like phase with only herbs and shrubs, followed by open forests dominated by either larch or pine, culminating with the establishment of the boreal mixedwoods about 300 years before the final drainage of proglacial Lake Ojibway. Fire regimes seem to have partially driven vegetation dynamics and diversity on the paleo‐islands. Paleo‐islands were sources of propagules, which helps explain how the former Lake Ojibway lowlands were directly colonized by boreal mixedwoods, without an initial tundra phase.

Histoire des paléo-îles du lac proglaciaire Ojibway: Après le retrait du glacier de l’Amérique du Nord au début de l’Holocène (il y a environ 11 700 ans avant aujourd’hui), la colonisation végétale s’est effectuée plus rapidement que prévu comparé aux taux de migration estimés à partir des capacités de dispersion actuelles moyennes de plusieurs arbres. Ce décalage entre migration observée et migration estimée (paradoxe de Reid) a été observé à l’ouest du Québec où les hypothèses classiques pour le résoudre ne peuvent être appliquées. En effet, aucun type de refuge proglaciaire n’a été mis en évidence et la présence du grand lac proglaciaire Ojibway de 10 570 et 8200 ans avant aujourd’hui (cal. BP) a empêché toute colonisation végétale et événement de dispersion de longue distance. Pourtant, l’afforestation des plaines inondées a été quasiment directe après le drainage du lac proglaciaire Ojibway. Dans cette thèse de doctorat, les anciennes îles du lac proglaciaire Ojibway (les paléo-îles) ont été étudiées afin de déterminer si elles ont pu jouer le rôle d’avant-postes de colonisation et ainsi accélérer le processus de colonisation végétale. L’analyse des sédiments lacustres de deux paléo-îles de la région de l’Abitibi confirme l’établissement des premiers arbres sur les paléo-îles vers 9500 et 9850 ans cal. BP avant le début du drainage du lac proglaciaire Ojibway. Une analyse multiparamétrique (pollen, macro-restes et ADNseda) des sédiments montre l’établissement progressif de la végétation sur les paléo-îles avec une succession de phase toundrique non-arborée, afforestation et diversification avant l’établissement de la forêt boréale vers 8200 cal. BP. Malgré des phases de succession commune, la végétation diffère les deux sites, sûrement en réponse aux conditions édaphiques et dynamiques de feux différentes. La dynamique de la végétation est liée aux variations de climat. Pour finir, la comparaison de différents scénarios de colonisation et leurs taux de migration associés, confirme que les paléo-îles ont bien joué un rôle d’avant-postes de colonisation. Elles ont pu faciliter et accélérer la colonisation postglaciaire et permettent donc de résoudre le paradoxe de Reid dans l’ouest du Québec. Selon l’étude des distances et surfaces des paléo-îles, il semblerait que leur colonisation suive la théorie de la biogéographie insulaire. Cette thèse contribue à l’avancement des connaissances des mécanismes de colonisation postglaciaire et discute également du rôle des paléo-îles dans les paysages actuels et futurs et des perspectives d’étude possibles.

Holocene fire history of the southern Lake Baikal regionThe catastrophic fire years that have taken place during the last decade in Siberia and in the boreal forests in general, directly linked to global warming, have had dramatic repercussions on the human populations of these regions. Past fire reconstruction studies are currently the only way to study the past dynamics of these fires and to understand their link with climate, vegetation and human activities. However, few studies of the dynamics of these fires are available in Siberia, and none have been carried out on the scale of the Holocene. This study aims to present the first reconstruction of the fire history during the Holocene based on sedimentary charcoals from two lakes localised on the southern shore of Lake Baikal, in Siberia. Two lakes have been sampled, Lake Ébène and Lake Jarod. The results showed a similar trend between the two lakes,with severe and intense crown fires during the early Holocene and less severe surface fires after 6 500 cal. yr BP. According to pollen reconstructions carried out near the studied lakes, a vegetation transition occurred at the same time. Picea obovata was dominant during the early humid Holocene. After 6 500 cal. yr BP, conditions were drier and Pinus sylvestris and Pinus sibirica became the dominant species. Over the past 1 500 years, the greater presence of human populations has firstly resulted in an increase of the fire frequency, then probably in its maintenance after 600 cal. yr BP in lake Ébène and to finish, in its suppression after 900 cal. yr BP in Lake Jarod. The decrease of fire frequency at the end of the 20th century could be explained by new fire management policies.