Projet de recherche :

Anatomical and productivity responses of aspen to recurrent forest tent caterpillar outbreaks in boreal mixed forests

Forest tent caterpillars (Malacosoma disstria) exhibit cyclic population outbreaks lasting 4-6 years, every 10-12 years. Outbreak events represent a major disturbance in boreal forests, causing severe defoliation, impacting productivity, and affecting stand composition. Severe defoliation can cause tree mortality, creating gap dynamics within stands, which alters productivity. In the northern regions, trembling aspen (Populus tremuloides) is a primary host species for FTC and is subject to consequences of recurrent defoliation. Defoliation by FTC occurs early in the growing season, and surviving trees must reallocate new carbon and reserves towards canopy regeneration, at the expense of structural wood. Defoliation induced carbon reallocation is known to reduce ring width and induce formation of pale rings. Effects of recurrent defoliation on chemical composition of the secondary cell wall, an important determinant of wood structural

properties, remain poorly understood, particularly in the context of interacting disturbances. To assess how FTC impacts trembling aspen productivity, wood cores will be retrieved from trembling aspen located in permanent plots impacted by FTC outbreaks. Outbreak histories will be identified using field observations and vegetation indices (NVDI and EVI), satellite-based measures used to assess vegetation greenness and health. Cores will be prepared into thin sections, and key anatomical wood qualities will be assessed, which may include vessel size and density and proportion of fibers and parenchyma. Spectrophotometer techniques may be used to evaluate relative proportions of structural and nonstructural sugars. Anatomical chronologies will then be created and cross-referenced with outbreak histories to relate variation with intensity. This will provide a better understanding of the cumulative impact of outbreak cycles, physiological trade-offs induced by disturbances, and early characteristics of declining resilience. This is increasingly important as climate change influences outbreak dynamics, including frequency and severity and is expected to decrease the availability, quality, and long-term reliability of conifer wood supply.