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Détermination de l'impact potentiel du changement climatique sur la mortalité des principales essences forestières européennes

Abstract : Forest ecosystems are one of the main providers of terrestrial ecosystem services, whose functioning has already been altered by recent climate change. Increases in tree mortality rates have been highlighted in different biomes worldwide, as well as increases in the frequency of massive mortality events following droughts. However, tree mortality is a multi-causal process. It is difficult to quantify the importance of the different factors that can possibly lead to tree death, and particularly the importance of climate change in comparison with forest dynamics and competition-related effects, environmental or biotic factors. This thesis aims at assessing the drivers of background tree mortality, which is the mortality observed in a stand in the absence of extreme perturbation, for the main European tree species. We used data from the French forest inventory of the National Geographic Institute (IGN) and historic climate data from Météo-France spanning the years 1961 to 2015. First, we modelled background tree mortality for 43 tree species in order to identify the drivers of background tree mortality. We used 372.974 trees, including 7.312 dead trees surveyed between the years 2009 and 2015. We found that factors related with competition, tree development stage, stand structure and species composition and logging intensity explained 85% of the recent tree mortality. Environmental factors (soil and climate conditions) accounted for 9% of the total modelled mortality. Temperature increases and rainfall decreases since the period 1961 – 1987 had a significant effect on the mortality of 45% of the 43 species and explained in average 6% of the total modelled mortality. Secondly, we focused on the link between trees locations along temperature and rainfall gradients and their sensitivity to changes of temperature and rainfall. We found that, for 9 species out of 12, temperature increases and rainfall decreases effects were more important in areas with high mean temperature and low mean rainfall. These results show that climate change-related tree mortality could be exacerbated towards the species’ warm and dry edges. Finally, we sought to evaluate how climate change-related tree mortality varied along trees social statuses and sizes gradients. We found that suppressed trees were more sensitive to temperature increases than dominant trees. On the contrary, dominant trees, and particularly large dominant trees, appear to be more sensitive to rainfall decrease than suppressed trees. Overall, our results show that climate change-related tree mortality is globally more important for suppressed than dominant trees. We highlighted the existence of a link between recent temperature increases and rainfall decreased and observed tree mortality rates on around half of the species of the French forest. We also showed that these effects were exacerbated towards the warm and dry edges of the species ranges. Finally, we showed that these effects differed according to trees social statuses and development stages. These results allow us to better understand the impacts of climate change on French and European forest and to better anticipate their effects through the adaptation of silvicultural practices.
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Adrien Taccoen. Détermination de l'impact potentiel du changement climatique sur la mortalité des principales essences forestières européennes. Biodiversité et Ecologie. AgroParisTech, 2019. Français. ⟨tel-02433527v1⟩

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