Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

A new paleoecological indicator was used for retracing the long-term history of spruce budworm (Choristoneura fumiferana Clem.) outbreaks from forest humus. Macrofossil analyses were performed on 28 forest soil profiles (10-20 cm) collected in four balsam fir (Abies balsamea (L.) Mill.) stands located north of Lake Saint-Jean, Quebec. Direct evidence of the presence of spruce budworm was found. Few pupa fragments were present in the profiles, but the most abundant and well-preserved macrofossils collected were spruce budworm feces. The feces found were associated in part with the three outbreaks that occurred during the last century around 1914, 1952, and 1979. Good correspondence was obtained between the macrofossil results and the known dynamics of two stands, Lib20 and Lib23. However, the method failed to detect specific outbreaks in the other two stands, Lib21 and Lib24. A complementary macrofossil study was conducted on deeper humus profiles (47-70 cm) collected on two islands of Mingan Archipelago. The outbreak history of the last century was retraced, and it was established that spruce budworm was present in the Mingan Archipelago since at least 1520 years BP. The greatest numbers of spruce budworm feces were found in the first 20 cm of the humus profiles corresponding approximately to the 20th century period. Afterward, an important reduction in numbers of feces was observed. This may be attributable to lower levels of spruce budworm populations before the 20th century or the intensification of decomposition with increasing humus depth.