Long-term impact of a major ice storm on tree mortality in an old-growth forest

In January 1998, eastern North America experienced the worst ice storm ever recorded, which resulted in major damage to forests throughout the region. However, we know little about the long-term effects of such a severe weather event, or how its potential interaction with other biotic factors impacted tree mortality. In this study, we investigated the long-term influence of the 1998 ice storm on tree mortality in Boise-des-Muir ecological reserve, an old-growth forest in southern Quebec (Canada). Mortality rates for four out of the five tree species studied showed a striking increase a few years after the ice storm, between 2000 and 2004. After this peak, tree mortality rates remained higher than before 1998. These results indicate a lagged and persistent effect of the ice storm on tree mortality and suggest that the tree community of Boise-des-Muir was more severely impacted than previously reported. Tree species were differentially damaged by the ice storm, and more than 25% of all tree species lost more than half their crown. Using a Cox proportional hazard model, we evaluated the impact of different levels of initial canopy damage from the ice storm on long-term tree mortality while taking into account other important biotic factors, such as tree size, tree density, indices of species-specific competition, and the presence of the beech scale insect. Species differed in their response to the storm, suggesting fundamental differences in sensitivity to extreme weather events. In particular, eastern hop-hornbeam (Ostrya virginiana) and American beech (Fagus grandifolia) were the species most vulnerable to the ice storm, and severe canopy loss led to a large increase in mortality risk. These findings suggest that the difference in tree survival following a natural disturbance likely depends not only on the severity of the damage itself, but also on species-specific ability to repair injuries and maintain physiological processes. We also found that tree size and species-specific competition influenced tree mortality significantly but did not interact with level of canopy loss. Consequently, multiple biotic factors remain important to explain mortality even after a major disturbance and should be considered when evaluating tree mortality. Our results show that the impact of the ice storm on tree mortality was species-specific, lagged and persisted for several years after the disturbance, potentially resulting in long-term compositional changes in the forest. This underscores the necessity of conducting long-term monitoring studies to understand the complete impacts of ice storms and other extreme climatic events on forests.