The Severity of Budworm-Caused Growth Reductions in Balsam Fir/Spruce Stands Varies with the Hardwood Content of Surrounding Forest Landscapes (pdf)

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The Severity of Budworm-Caused Growth Reductions in Balsam Fir/Spruce Stands Varies with the Hardwood Content of Surrounding Forest Landscapes

The Severity of Budworm-Caused Growth Reductions in Balsam Fir/ Spruce Stands Varies with the Hardwood Content of Surrounding Forest Landscapes Elizabeth M. Campbell, David A. MacLean, and Yves Bergeron Keywords: budworm impacts, defoliation, dendroecology, insect outbreaks P ERIODIC SPRUCE BUDWORM (Choristoneura fumiferana Clem.) (Lepidoptera: Tortricidae) outbreaks occur about every 35 years in the boreal and mixedwood forest regions of eastern North America (Royama 1984, Jardon et al. 2003). During outbreaks, repeated annual defoliation by budworm larvae reduces tree growth and often kills its hosts, particularly balsam fir (Abies balsamea [L.] Mill.), over vast areas (MacLean 1980, Hardy et al. 1986, Bergeron et al. 1995). Although budworm outbreaks are natural forest disturbances that play a critical role in directing large-scale ecological processes (e.g., forest succession and nutrient cycling), they also reduce timber supplies and generate substantial socioeconomic losses. From 1977 to 1987, 17–22% of the annual timber harvest level was lost to spruce budworm in Canada (Sterner and Davidson 1982, Power 1991, National Forestry Database 1995). During the spruce budworm outbreaks of the 1970s, 1980s, and early 1990s, aerial application of insecticide was used throughout much of eastern Canada to reduce budworm damage and economic losses. Broad-spectrum chem- ical insecticides (e.g., fenitrothion), as well as the biological insecticide Bacillus thuringiensis (B.t.), were applied. Although chemical insecticides are no longer considered an environmentally acceptable option for controlling insect outbreaks in Canada, B.t., and the insect growth regulator, Mimic, can be used to reduce budworm populations during outbreaks to protect foliage and prevent host deaths. Past management efforts to reduce the impact of spruce budworm outbreaks in eastern Canada have relied heavily on the application of insecticides, but given concern about the potential negative ecological impacts of spraying B.t., for example, dramatic reductions of Lepidoptera that could significantly alter forest insect communities and affect a range of other species and ecosystem processes (Crawford and Jennings 1989, Miller 1990), additional approaches to reducing the economic impacts of budworm outbreaks are necessary. The fact that damage caused by spruce budworm outbreaks varies with forest species composition has long been recognized. On the basis of these observations, researchers Elizabeth M. Campbell, Centre d’Étude de la Forêt and Département des Sciences Biologiques, C.P. 8888, Succ. Centre-ville, Université du Québec à Montréal, Québec H3P 3P8, Canada. Current address: Research Branch, British Columbia Ministry of Forests and Range, PO Box 9519, Stn Prov Govt, Victoria, BC V8W 1N1, Canada—Phone: (250) 387-6712; Fax: (250) 387-0046; David A. MacLean, Faculty of Forestry and Environmental Management, University of New Brunswick, PO Box 44555, Fredericton, New Brunswick E3B 6C2, Canada. Yves Bergeron, Chaire industrielle CRSNG-UQAT-UQAM en aménagement forestier durable, Université du Québec en Abitibi-Témiscamingue, C.P. 700, Rouyn-Noranda, Québec J9X 5E4, Canada. Acknowledgments: This project was funded by National Science and Engineering Research Council (NSERC) and Quebec–Canadian Provinces University Cooperation Program grants and through scholarships obtained by E.C. from the Sustainable Forest Management Network and the NSERC/Université du Québec à Montréal/Université du Québec en Abitibi-Témiscamingue Industrial Chair in Sustainable Forest Management. We thank Wayne MacKinnon, Canadian Forest Service (Fredericton) for geographic information system work, assistance with core collections, and stand data from 12 of his research sites. Guillaume Sauzède and Peter Goulet helped with fieldwork. Vince Zelazny (New Brunswick Department of Natural Resources) provided the ArcView map layer of ecoregions onto which we overlaid our sample plots. We gratefully acknowledge two anonymous reviewers whose comments helped to significantly improve this manuscript. Manuscript received April 18, 2006, accepted September 21, 2007 Copyright © 2008 by the Society of American Foresters Forest Science 54(2) 2008 195 Abstract: Tree-ring data collected from balsam fir (Abies balsamea [L.] Mill.) in 31 spruce (Picea spp.)/firdominated stands in northern New Brunswick were used to evaluate how radial growth reductions during the last spruce budworm (Choristoneura fumiferana Clem.) outbreak (1972–1992) varied according to the hardwood content of surrounding forest landscapes, insecticide spray history and elevation. Stepwise regression analysis was used to build 12 models describing relationships between mean percent growth reduction (i.e., deviation from mean tree growth averaged over the lifespan of all trees in a stand) and the explanatory variables for years when ⬎80% of stands had growth reductions (1976 –1978, 1982–1984, 1990, 1991, and 1994) and for three growth reduction periods (1972–1980, 1981–1985, and 1986 –1993). The hardwood content of surrounding forest landscapes entered into all significant regression models (P ⬍ 0.05), explaining 12– 41% of the variance in budworm-caused growth reductions. Differences in growth reductions among stands were greatest during the initial years of the budworm outbreak (1972–1980), particularly in 1976 when mean reductions were ⬃40% in stands with ⬍50% hardwood and content and ⬃20% in stands in which surrounding hardwood content was ⬎50%. Relationships between variation in growth reductions among stands and elevation or insecticide spray history were weak and less consistent than with landscape hardwood content. Our findings suggest that increasing the hardwood content of forest landscapes could help to reduce the impact of outbreaks on timber supplies. FOR. SCI. 54(2):195–205. 196 Forest Science 54(2) 2008 of defoliation calculated from aerial surveys) (e.g., Hardy et al. 1986). Studies such as this one and others that quantify stand-level impacts on the basis of measurements of individual trees (e.g., MacKinnon and MacLean 2004) better reflect local budworm population densities and thus provide an improved basis for understanding relationships between forest composition, outbreak impacts, and the mechanisms that drive budworm population dynamics. This information could have significant implications for the way in which forests are managed to reduce future socioeconomic losses caused by budworm outbreaks. Methods Study Area Thirty-one stands were sampled in northern New Brunswick. Most stands occurred within the Northern Uplands bioclimatic ecoregion, but a few stands occurred in the Highlands and the Central Uplands ecoregions (Figure 1). Elevations ranged from 177 to 549 m, with a mean of 335 m. The Northern Uplands is climatically intermediate to the colder Highlands and the warmer, wetter Central Uplands (TECWG 2003). Mean annual, sea (...truncated)