Boreal peatland margins as repository sites of long-term natural disturbances of balsam fir/spruce forests

Background/Question/Methods: In maritime boreal areas of eastern Canada where balsam fir (Abies balsamea) and white spruce (Picea glauca) often form mixed stands, large forest fires are rare. Episodic outbreaks of insect defoliators, namely the eastern spruce budworm (Choristoneura fumiferana) and hemlock looper (Lambdina fiscellaria), local tree falls and windthrow are the main disturbances of forest stands. In several regions from Ontario to Newfoundland and throughout Québec, insect outbreaks are well documented for the last 200–300 years, based on major reductions in radial growth of host trees. Because tree-ring records going back in time are limited by the age of trees, tracing past insect infestations over the last millennia by using affected trees is a difficult task. For these reasons, dendroecological and paleoecological methods must alternate in an attempt to document insect activity during the Holocene. A multidisciplinary, high-resolution paleoecological study (Lepidoptera and plant remains, macroscopic charcoal, pollen) was conducted on a 4000-yr peat monolith extracted from the margin of an ombrotrophic peatland on Anticosti Island (Gulf of St. Lawrence, eastern Canada) to reconstruct the long-term natural disturbances of a balsam fir/spruce forest. Because insect remains (head capsules, feces) and macroscopic charcoal are not dispersed over long distances, the study of a peat monolith collected at the forest interface increased the chances to identify past insect activity and forest fires faithfully. We hypothesized that an activity of insect defoliators was the main disturbance factor of conifer forests during the Late Holocene.

Results/Conclusions: The earliest remains of spruce budworm and hemlock looper were found ca. 3220 and 2350 cal. yr BP, respectively. Peaks of insect head capsules occurred from ca. 1640 to ca. 625 cal. yr BP. Low balsam fir pollen concentrations during this period suggest a lengthy episode (~1000 yr) of high insect activity, resulting in extensive fir dieback and mortality. This interval partly corresponds to the Medieval Warm Period, which suggests a possible link between climate and sustained insect activity through increased food availability. The long-term dynamics of the pristine balsam fir/spruce forests were mainly governed by the activity of insect defoliators. The low occurrence of forest fires in the maritime environment of Anticosti Island allowed the development of mature coniferous stands propitious for insect infestations. Insect head capsules appeared to be a useful and effective tool for establishing insect presence and activity during the Holocene.