Reponse du Douglas a des evenements climatiques extremes

Abstract

La température de la terre a augmenté de 0,7 °C au cours du siècle dernier, ce qui correspond à une vitesse de réchauffement jamais observée auparavant. Les prévisions et la particularité du changement climatique global, ses événements extrêmes plus intenses et fréquents, posent beaucoup de questions pour l’avenir des écosystèmes forestiers et leur capacité d'adaptation. Ce seront principalement les contraintes hydriques, comme celles qui ont accompagné la canicule qui s’est produite en Europe pendant l’été 2003, qui limiteront la croissance et la survie des arbres dans le futur. On a observé des symptômes qui allaient du rougissement des feuilles jusqu'au la mort totale des individus en passant par la mort de la pousse apicale et de parties de houppiers. On s’attend à ce que des événements climatiques de cette ampleur se produisent plus fréquemment dans les années à venir.

Strong hydrological constraints, as those which accompanied the heat and drought wave which occurred in Europe during the summer 2003, affected the trees to strong functional and physiological adjustments, by limiting not only growth but also by affecting their survival. It is expected that limitations in water availability during trees' growing season will become more intense and frequent, arising many questions for the future forest ecosystems and their adaptation. Since the process of climate change is directional and fast, and the time frame of the trees is long, phenotypic plasticity should play a central role in the capacity of trees to adjust with the new climatic context. Our species of study, Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) is a highly productive species. Its ecological requirements and its symptoms presented after the heat wave of 2003, make of Douglas-fir an appropriate species well adapted for the study of the trees response to water deficit to which will be exposed the majority of them in a near. Wood is a tissue integrating multiple processes, which we used to study how trees can acclimate and adapt to environmental constraints. Certain basic properties as density are related to adaptive functional processes related to xylem structure and thus are important for survival. Density is the consequence of cambial activity which itself responds to the variations of environment. Wood density was implied in the capacity of Douglas-fir trees to survive to the 2003 heat wave. The trees which survived have a higher trunk density than those which died. Surviving trees formed higher density rings than dead trees during several years before the 2003 heat wave. The ring 2003 shows that Douglas-fir acclimated itself by stopping its growth exceptionally early that year. Through an innovating approach, it was possible to connect the intra-ring density variations with the climate variations. These density variations were interpreted as tree response to water deficit during the growing season, in other words, as a measurement of its phenotypic plasticity (dendroplasticity). Clonal tests enabled us to study genetic variation of tree response to the heat wave and thus of dendroplasticity and also to compare it with genetic variation of microdensity variables. Microdensity profiles were largely used in genetic improvement programs for the selection of trees with desirable density characteristics. We show how microdensity profiles can be used to predict the survival capacity of Douglas-fir trees to climatic constraints allowing thus to integrate morphometric, physiological and adaptation to abiotic constraints characters.