Long-term impact of insect defoliation on growth and mortality of eastern larch in boreal Québec

Tree-ring analysis was used to relate the pattern of eastern larch (Larix laricina) growth to insect defoliation over the last three centuries in boreal Québec. Four sampling sites were selected along a moisture gradient, from a well-drained site to a forested peatland near Lake Bienville, in an area where larch is currently recovering from a recent outbreak of the larch sawfly (Pristiphora erichsonii), a notoriously recurrent defoliator of eastern larch. In the two dry sites, the eastern larch tree-ring series clearly showed eight periods of defoliation-induced growth suppression since 1782. Growth depressions were not as consistent in the more humid sites, especially at the turn of the century (i.e. between 1883 and 1911), when only one infestation was clearly expressed among three successive ones at the dry sites. Time series filtering indicated sharp synchrony of defoliation events among sites. Radial and apical growth during infestations correlated in a complex way, with radial growth being most severely suppressed at the apex and at the root collar. Defoliation of larch caused significant tree mortality in the dry sites. Mortality was directly related to defoliation for a significant fraction of the dead trees sampled. There was a variation in the number of years of suppressed growth without recovery prior to death, as well as in the number of severe infestations that were successfully overcome, before the final assault. The age structure of the extant larch population at one of the dry sites indicated a lack of recruitment over the last century. Our study suggests that larch sawfly outbreaks had a considerable long-term impact on the forest structure. In the absence of fires, the long-term effect may be a gradual substitution of the mixed black spruce—eastern larch forest by a monospecific black spruce forest.