The dynamics of forest tent caterpillar outbreaks in Québec, Canada

Historical patterns of forest tent caterpillar defoliation over the period 1938–2002 in the province of Québec, eastern Canada, were analyzed in relation to forest inventory data. The extent of defoliation over time was largely nonstationary and only somewhat periodic; however six major defoliation episodes could be identified. Individual outbreaks tended to span only 36.6% (±13.1% S.E.) of the total area defoliated, suggesting they are frequently terminated before attaining their maximum potential extent. Although outbreaks tended to recur periodically, they were not perfectly synchronized across the province. Two core regions, 14 000 and 20 000 km2 in size, located in the northwestern, aspen-dominated boreal forest region and the southeastern, maple-dominated mixedwood forest region, were found to exhibit cyclic patterns of defoliation, with periodicities of 9 and 13 years, respectively. These oscillations were characterized by strong second-order negative feedback, suggesting regulation by lagged density-dependent processes. Outbreak cycles in the two core regions were in phase with one another (r = 0.39) until 1963, when a sudden, large-scale outbreak collapse occurred in the North during the initial phase of the third cycle. Since that time outbreak oscillations have been completely out of phase (r = −0.16), leading to a persistent wave-like pattern of outbreak spread back and forth between regions along a northwest–southeast axis. Within core regions, cycle amplitude varied in a slow and smooth manner, with the phasing pattern of amplitude modulation differing substantially between regions. Although the timing of population cycle peaks appears to be highly predictable, at least within the core regions, the levels of defoliation experienced during these peaks appears to be unpredictable and may be modulated by factors yet to be identified.