Variation in wood structure of eastern larch defoliated by the larch sawfly in subarctic Quebec, Canada

We studied the wood structure of eastern larch (Larix laricina (Du Roi) K. Koch) affected by a larch sawfly (Pristiphora erichsonii (Htg.)) infestation that occurred in 1939 in the Lac a l'Eau-Claire and Riviere-aux-Melezes areas, subarctic Quebec. The objective was to document variation in wood characteristics in relation to defoliation at three study sites. Lumen diameter of tracheids and cell wall thickness were measured from thin sections representing the 1931-1955 period. Significant changes in the wood structure of larch was noticed only at the Atkinson Island site, where the radial growth pattern was typical of the insect activity. A light ring was formed the 1st year of the infestation (1939). During the 2nd year (1940), the size of tracheids (lumen) decreased abruptly along with radial growth, while the proportion of latewood increased. The variation in wood anatomy among the three study sites paralleled that in radial growth. Wood structure appears to be a good indicator of the end of an infestation.