The Interaction Between Insect Outbreaks and Debris Slides in a Glacial Valley of the Eastern Canadian Shield

Abstract

Disturbance interactions, including compound and linked disturbances, are increasingly recognized to have important effects on landscape structure and function. Most research on linked disturbances has focused on the interaction between insect outbreaks and fire, and between fire or forest harvesting and other physical processes such as soil erosion and landslides, but to our knowledge, none have explored the possible interaction between insect outbreaks and landslides. We tested the hypothesis that severe outbreaks of the spruce budworm, an insect defoliator, could increase the probability of debris slide occurrence in the southern boreal forest of eastern Canada. We mapped 28 debris slides that occurred between 1996 and 2002 along a deep U-shaped glacial valley (Jacques-Cartier River valley, Quebec) and compared their characteristics with those of 50 random sites unaffected by debris slides using maps of the abiotic (topography, surficial deposits, and so on) and biotic (forest cover and cumulative spruce budworm defoliation from aerial surveys) factors that could influence the probability of debris slide occurrence. Logistic regression showed that probability of debris slide occurrence increased with mean slope, outbreak severity, and abundance of balsam fir, and that it was associated with thin surficial deposits and northeast aspects. Spruce budworm outbreaks probably increase susceptibility to debris slides primarily by reducing the biomass of soil-stabilizing root systems, and possibly also by reducing forest transpiration, which in turn increases soil water content. Future research should focus on the mechanisms underlying insect outbreak-landslide interactions.