Dan Quiring, Don Ostaff and Eldon Eveleigh
Faculty of Forestry and Environmental Management, University of New Brunswick

SERG Project #2000/05
Year of Project: 2000
Report Received: 2006

Pre-commercial spacing or thinning is a management tool that removes a portion of the canopy to accelerate diameter increment of residual trees. Wood supply models usually assume that pre-commercially thinned stands will produce increased volume and allow harvest at an earlier age. Such models usually do not consider the possibility that growth reductions resulting from defoliating insects could be greater in pre-commercially thinned than unthinned stands if thinned stands are subjected to higher levels of defoliation. Increasing the mean distance between trees influences many factors, such as increased soil temperature and thus organic matter decomposition (Piene, 1978; François et al., 1985; Wickman and Torgersen, 1987; Thibodeau et al., 2000), foliar chemistry (Bauce, 1996) and illumination (Pothier and Margolis, 1991), some of which may influence insect development (eg., Mason et al., 1992; Bauce, 1996). The effects of thinning on defoliation by insects are, however, quite variable. For example, defoliation by the spruce budworm (Choristoneura fumiferana (Clem)) increased in young balsam fir (Abies balsamea (L.) Mill.) stands (Piene, 1989) whereas thinning in older coniferous stands had either no (Crook et al., 1979) or a negative effect on defoliation (Batzer, 1967, Carlson et al., 1985).

Until the recent past, most large outbreaks of defoliating insects in North America occurred in naturally regenerated and largely “untended” forest stands. However, many recent insect outbreaks of economic and ecological concern have occurred in plantations and in naturally regenerated stands where the densities of plants have been reduced by planting practices or pre-commercial thinning, respectively (e.g., Quiring, 1990; Piene et al., 2001, Moreau 2004).

The balsam fir sawfly [Neodiprion abietis (Harr.)] is part of a little-studied complex distributed throughout Canada and on the east and west coasts of the United States (Ross, 1955; Wallace and Cunningham, 1995). Although it has been recorded on many hosts, N. abietis feeds principally on old (i.e., > one-year-old) foliage of balsam fir and occasionally on white (Picea glauca [Moench.] Voss) and black (P. mariana [Mill.] B.S.P) spruce (Martineau, 1984) in Canada. Previous periodic outbreaks of N. abietis were localized and of short duration, but current outbreaks of this insect in western Newfoundland and Nova Scotia encompass extensive areas. Natural, low-density stands of balsam fir also appear to be more heavily defoliated by N. abietis than high-density stands (Atwood, 1960; Martineau, 1984). If N. abietis prefers to lay eggs in stands with low host density, like many other species of sawflies (Heitland and Pschorn-Walcher, 1993), the artificial reduction of balsam fir density through pre-commercial thinning could generate conditions favoring N. abietis and thus higher levels of defoliation by this forest defoliator.

In 2000, a study was established to determine: (1) the influence of pre-commercial thinning (hereafter referred to as thinning) on susceptibility of young balsam fir stands to defoliation by N. abietis; (2) if percent defoliation is a good indicator of tree vigour and thus the ability of trees to recover after insect outbreaks; and (3) determine the influence of thinning on survival of BFS in thinned and unthinned stands. In this report we discuss our results relating to objectives 1 and 2. Objective 3 will be discussed in next year’s report. Much of this present report will be published in an article by Ostaff et al. (2006).