Risques passés et futurs de feux de forêts et leurs incidences sur la résilience de la forêt boréale de l’Est Canadien

Abstract : Changes in forest composition and structure are projected in response to the future climate likely more conducive to fire and water stress. A decrease in carbon and biomass stocks could significantly affect the forest industry and global warming by high carbon emissions during fires. However, despite its ecological and socio-economic importance, the future of the forest is uncertain because the impacts of climate change on ecosystem processes and standing biomass are still poorly understood. The primary objective is therefore to assess the potential effects of climate change on vegetation dynamics and fires, and to characterize their joint effects on the resilience of eastern Canada's boreal forest on both sides of the northern limit of managed forests (NLMF). Simulations were carried out with the LPJ-LMfire dynamic global vegetation model and focused on three specific objectives: (1) to reconstruct fire activity during the 20th century and analyze changes in spatial and temporal fire trends related to vegetation and climate, (2) to analyze the forest response projection to climate change and to fire increase to assess if abrupt changes in biomass of dominant species could occur, (3) to simulate trajectories of past fires and vegetation in response to Holocene climatic variations to understand the relationship between climate, fire and vegetation. For the first time, simulations are performed on the eastern boreal forest with LPJ-LMfire over 6000 years and at high spatial resolution (100 km 2) over a study area stretching west to east, from Manitoba to Newfoundland. The plant functional types for the four dominant tree genera (Picea, Abies, Pinus, Populus) have been parameterized. The predictive capabilities of the model were tested over the 20th century by comparing simulated annual rates of combustion and biomass with independent observations. The same variables, simulated over the past 6,000 years, have been compared to paleoecological reconstructions from lacustrine records of microcharcoals and pollen. Finally, the present version of LPJ-LMfire has been used with IPCC climate scenarios to analyze trajectories along the 21st century. The results show that LPJ-LMfire correctly reproduces the spatio-temporal trends in fire frequency observed in the 20th century, particularly in Manitoba and Ontario. The simulated spatial distribution of plant biomass is also consistent with observations, except at the northern limit of trees where it is overestimated, especially for Picea. The trajectories of simulated fires and vegetation over the last 6,000 years were spatially shifted compared to paleoecological reconstructions: too far south in the west and too far north in the east. The observed difference would be due to the IPSL-CM5A-LR climate data provided as input of LPJ-LMfire. Climate variability and lightning impacts are the determining factors in the distribution of fire frequency during the 20th century, while vegetation feedbacks on fires control the distribution of their frequency over long time scales. Our results contradict the predicted increase in future fire risk, suggesting a decrease in fire frequency by 2100, especially in the south, associated with an increase in the proportion of deciduous taxa and an opening of landscapes that should limit ignition and spread of fires. The frequency and intensity of droughts induced by climate change are expected to increase and favor tree mortality south of the NLMF. Rising temperatures and atmospheric CO2 will only temporarily increase forest productivity. By 2100, declining biomass stocks and increasing broadleaf proportion south of the NLMF could threaten the economy of the forest sector. Silvicultural practices that preserve productivity and boreal forest resilience are therefore recommended to maintain sustainable forest management.
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Émeline Chaste. Risques passés et futurs de feux de forêts et leurs incidences sur la résilience de la forêt boréale de l’Est Canadien. Milieux et Changements globaux. PSL Research University; Université du Québec à Montréal, 2018. Français. ⟨NNT : 2018PSLEP047⟩. ⟨tel-02174929⟩

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