The Influence of Forest and Stand Conditions on Spruce Budworm Defoliation in New Brunswick, Canada

The Influence of Forest and Stand Conditions on Spruce Budworm Defoliation in New Brunswick, Canada ABSTRACT. The species composition of stands and surrounding forest have been suggested as important factors influencing the amount of spruce budworm (Choristoneura fumiferana [Clem.]) defoliation and, consequently, budworm-induced growth loss and mortality. We measured spruce budworm defoliation from 1989–1993 in 40 spruce (Picea sp.) and balsam fir (Abies balsamea [L.] Mill.) stands in north-central New Brunswick, Canada, and evaluated the influence of surrounding forest type (softwood, mixedwood), species group (balsam fir, spruce), and site quality (wet/nutrient poor, moist/nutrient rich) on defoliation. Surrounding forest type had a significant effect on the amount of defoliation in white spruce (Picea glauca [Moench] Voss) stands; stands in softwood forests sustained 11% more (P = 0.0485) defoliation than those in mixedwood. There was evidence that hardwood species have to be within, rather than surrounding, a spruce-fir stand to significantly influence defoliation. Site quality had a significant effect (P = 0.0039) on balsam fir defoliation, with stands on moist/rich sites sustaining 19% more defoliation than those on wet/poor sites. In softwood forest, white spruce stands sustained an average of 16% more defoliation than red-black spruce (Picea rubens Sarg.-Picea mariana [Mill.] B.S.P.) stands. FOR. SCI. 49(5): 657–667. Key Words: Choristoneura fumiferana, Abies balsamea, Picea sp., balsam fir, spruce, softwood, mixedwood, site quality. S PRUCE BUDWORM (Choristoneura fumiferana [Clem.]), herein referred to as budworm, is a forest defoliator that kills its host trees through repeated consumption of foliage. Severe, widespread damage caused by periodic budworm outbreaks results in uncertainty about short- and long-term availability of forest resources for consumptive and nonconsumptive uses (MacLean 1985, 1990). Sterner and Davidson (1982) estimated that 44 million m3/yr of spruce (Picea sp.) and balsam fir (Abies balsamea [L.] Mill.) timber volume were lost to budworm in Canada from 1977 to 1981. Budworm-induced volume losses can have dramatic effects in areas where forest management operates under a tight balance between timber supply and demand. Aerial application of insecticides has been used routinely in the past to protect current foliage and prevent tree mortality caused by budworm outbreaks (Kettela 1975, Webb and Irving 1983). However, with increasing environmental awareness, the products that are acceptable to protect against budworm damage are limited, and only the biological insecticide, Bacillus thuringiensis (B.t.), and the insect growth regulator, Mimic® (tebufenozide), are currently used in Canada. It is clear that other methods of reducing budworm damage are urgently needed. Scientists have proposed that various forest management activities could be used to reduce the amount or impact of budworm defoliation. These include (a) selective harvesting Wayne E. MacKinnon, Forestry Officer, Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, P.O. Box 4000, Fredericton, N.B. E3B 5P7 Canada—Phone: 506-451-6096; Fax: 506-452-3525; E-mail: . David A. MacLean, Dean, Faculty of Forestry and Environmental Management, University of New Brunswick, P.O. Box 44555, Fredericton, N.B. E3B 5A3 Canada—E-mail: . Acknowledgments: This article formed part of the M.Sc.F. thesis of Wayne MacKinnon at the University of New Brunswick. The research was funded by the Government of Canada Green Plan and the Canadian Forest Service. The authors thank David Gray, Kevin Porter, and two anonymous referees for helpful comments on a draft of the manuscript. Manuscript received Dec. 3, 2001, accepted Oct. 24, 2002. Copyright © 2003 by the Society of American Foresters Forest Science 49(5) 2003 657 Wayne E. MacKinnon and David A. MacLean 658 Forest Science 49(5) 2003 Materials and Methods We selected two surrounding forest types that were classified as softwood (≥70% spruce/fir with minor components of other softwood species) or mixedwood (<70% spruce/fir and ≥30% hardwood) based on vegetation within a 1 km radius of the sample stand. Sample stands were all ≥75% spruce-fir, and two species groups were classified as balsam fir (fir ≥50%) or spruce (spruce ≥50%). Site quality was classified as wet/poor or moist/rich based on soil drainage and plant indicators of soil nutrients, using the New Brunswick site classification system (Zelazny et al. 1989). The experimental plot design consisted of randomly selecting five replicate stands for every combination of the two surrounding forest types (softwood, mixedwood) × two species groups (balsam fir, spruce) × two site quality classes (wet/ poor, moist/rich). Therefore, a total of 40 stands were sampled. Study Area Description The study area spanned three ecoregions as defined by Loucks (1962): Maritime Uplands, Restigouche-Bras d’Or, and New Brunswick Highlands Ecoregion. The Maritime Uplands Ecoregion includes hardwood species such as sugar maple (Acer saccharum Marsh), yellow birch (Betula alleghaniensis Britton [Betula lutea Michx. f.]), and red maple (Acer rubrum L.) intermixed with spruce and fir on the slopes. The Restigouche-Bras d’Or Ecoregion includes slopes and valley bottoms dominated by spruce and fir, with poor quality hardwoods occupying the higher areas. The New Brunswick Highlands is predominantly characterized by coniferous forests composed of balsam fir, white spruce, black spruce, white birch (Betula papyrifera Marsh.), and white pine (Pinus strobus L.) on the well-drained slopes, and fir and black spruce on the lower, poorly drained areas (Loucks 1962). Sample Stand Selection Sample stands were restricted to an area in northern New Brunswick, Canada, that had sustained budworm defoliation from 1989–1993 (Figure 1). Preliminary location of potential sample stands was done using the geographic information system (GIS) software ARC/INFO to query New Brunswick Department of Natural Resources and Energy (NBDNRE) forest inventory and site classification databases. The forest inventory data consisted of individual stand delineations with attribute information for each stand, including a unique stand identifier, species composition, species percentages, crown closure, and maturity class. The site classification data consisted of soil drainage, underlying bedrock, soil parent material, soil type, vegetation type, and site class (treatment unit) by ecoregion. ARC/INFO was used to overlay the site information with the forest inventory data to select stands with Treatment Units 2, 3, 7, or 8 (explained further in the “Stand and site measurements” section). Forty of the initially identified sample stands were selected after verification by field sampling. Stand and Site Measurements Twenty spruce and 20 balsam fir stands were sampled during the summer of 1994. An initial survey of each stand was (...truncated)