COS 15-5 - Modeling the impact of climate change on the defoliation of black spruce by spruce budworm

Background/Question/Methods

Current models predict profound effects of global change on forest ecosystems with changes in natural disturbances regime. Recognized as the most important defoliator in North America, spruce budworm (Choristoneura fumiferana) (SBW) outbreaks have a major impact on the dynamism of the Canadian boreal forest. SBW defoliation has caused a decrease in the growth of dominant tree species, mortality, and loss in forest productivity, and these impacts are increasing with the changing global scenario. But how is the growth of boreal tree species like black spruce (Picea mariana) influenced by the climate during an outbreak period? Our goal was to evaluate how the interaction effect between climate and defoliation severity modulated the growth of black spruce during the late 20^th century outbreak in Quebec. For this, we compiled dendrochronological (2277 trees), defoliation, and climate data for 164 sites distributed over approximately 900,000 km². After computing site-specific growth indices, cumulative defoliation over 5 years, and climatic parameters, we used linear mixed-effects models to determine the interaction effects on the growth.

Results/Conclusions

Our analysis revealed that at the maximum outbreak severity, the growth of black spruce was reduced by 2.7% per year. This was amplified by high values of the preceding summer minimum temperatures and the preceding summer CMI (Climate Moisture Index) that imposed an additional growth reduction of 2.1% and 0.6% (standardized effect) respectively. In contrast, the defoliation effect was attenuated by high values of the preceding spring minimum temperature (1.8%%) and the preceding summer maximum temperature (1.2%%). The increase in summer minimum temperature might have shortened the hibernation window of budworm, which triggered the earlier emergence of SBW larva; whereas increased moisture might have enhanced activities of other pests on trees imposing stresses at the meantime when budworm was feeding. This implies that the effect of defoliation on black spruce growth is different depending on the climatic condition that is proximal to the larval emergence period. Our research will have major implications in predicting the impact of shifting climate scenarios in the range and severity of future outbreak periods, which is crucial for managing boreal forests under climate change.