Defoliation of tamarack by outbreak populations of larch sawfly in subarctic Quebec: measuring the impact on tree growth

Abstract

In conjunction with other major perturbations, outbreaks of specialist insect defoliators could play a key role in the long-term evolution of northern forest communities. Sawfly oviposition marks up to 20 years old in shoots of tamarack, Larixlaricina (Du Roi) K. Koch, provide a direct record of a large-scale epidemic cycle of the larch sawfly, Pristiphoraerichsonii (Htg.), during the 1980s in the Grande Rivière de la Baleine region, northern Quebec. Because no significant tree mortality was noted in the years following peak defoliator populations in 1985–1986, a study was undertaken to determine whether insect population changes during the outbreak period were correlated with growth changes under various climatic and soil conditions in three coastal sites and one continental sampling site. The fluctuations with time in the density of oviposition marks on trees indicated that the epidemic cycle was synchronous at all sites. However, the defoliator's density was significantly higher at the inland site, located 150 km from Hudson Bay, than at the coastal sites, where climatic conditions may be less favorable to insect activity and survival. No significant effects on tree growth were detected at the coastal sites, whereas larch from the inland site recorded a significant reduction in radial growth in 1984, the 1st year of outbreak conditions. Larch from this site also showed significant effects on foliage development after 4 years of defoliation (1988), when a virtual stop on elongative growth became apparent. Larch trees reproduced actively during the outbreak, with peak reproductive effort in 1984 and 1987 correlating with lows in elongative growth. Delayed or minimal effects of this outbreak characterized by the presence of hundreds of sawfly colonies per tree and defoliation levels comparable to those observed in the boreal forest during similar outbreaks indicate that subarctic larch may be highly tolerant to prolonged defoliation by the larch sawfly.