Revisiting the relationship between spruce budworm outbreaks and forest dynamics over the Holocene in Eastern North America based on novel proxies

Abstract This chapter revisits the hypothesis put forward in 2007 that there is a close relationship between the structure (species composition and their distribution in the overstory) of mature boreal forest stands and the occurrence—as well as the periodicity, synchronicity, and amplitude—of insect outbreaks within the forest. New data from original dendroecological and paleoecological studies of spruce budworm outbreaks over the last 10 000 years strengthen this hypothesis. The variation in spatial and temporal dynamics of outbreaks is the reflection of the changes in forest structure driven by fire and insect outbreaks. There is a close inverse relationship between fire frequency and outbreak frequency over time. High fire frequency favors the presence of non-host species of the insect while low fire frequency favors the development of fire intolerant species such as balsam fir, the preferred host of the budworm, which influences outbreak frequency and impact. This would explain why the outbreaks of the 20th century were so widespread, even spreading further north due to climate warming.