The eastern spruce budworm (SBW, Choristoneura fumiferana) is a major disturbance agent of North American boreal forests. Massive outbreaks of this native defoliator insect affect vast areas every 30-40 years and can last up to 15 years. Despite the major ecological and socio-economic consequences of SBW outbreaks, the drivers of their regional dynamics are little understood.
We used aerial survey maps to compare three SBW outbreaks that occurred in different regions and time periods in Quebec, Canada: Western Quebec in 1968-1977 (W-1970); Eastern Quebec in 1972-1981 (E-1970), which is the follow-up of the W-1970 regional outbreak, and Eastern Quebec in 2007-2016 (E-2000). We asked two questions: (1) what factors explain the probability of infestation in new areas and do these factors vary between the three outbreaks?; (2) what factors explain the temporal patterns of defoliation and do these factors vary between the three outbreaks?
For each outbreak, (1) we selected, for each year, newly defoliated and unaffected sites, and performed logistic regressions using forest composition maps, digital elevation model (DEM), and climate data as explanatory variables; (2) we performed multivariate regressions between the annual increase of defoliation severity and the same environmental variables. We then compared regression results between outbreaks.
Results/Conclusions
Elevation was the most important factor explaining the probability of infestation of new areas, especially in the Eastern region (E-1970 and E-2000), where the rugged topography greatly influences microclimate. Precipitation influenced the probability of infestation of new areas at the beginning of both regional outbreaks (W-1970 and E-2000) whereas temperature was the most significant climatic variable at the end of the regional outbreak (E-1970; the 2000s outbreak is still ongoing). During a given outbreak, forest composition was important in the first years but was later outweighed by temperature. Temporal patterns of defoliation differed significantly between outbreaks: annual increase of defoliation severity was faster in the 1970s outbreaks than in the E-2000 outbreak. The annual increase of defoliation severity was the fastest for sites affected 2-3 years after the beginning of the regional outbreaks, possibly because insect density had greatly increased. It was also faster at the beginning of the W-1970 and E-1970 outbreaks and slower at the end, compared to the ones of E-2000.
Understanding the regional spatio-temporal dynamics of SBW outbreaks could help anticipate the location of new infestations and help insect pest management and forest management during and after the outbreaks.