Growth and Survival of Picea glauca following Thinning of Plantations Affected by Eastern Spruce Budworm

intervals throughout eastern North America (Taylor and MacLean 2008) and continued for up to 15 years (Kucera and Orr 1981, Magnussen et al. 2005, Hennigar et al. 2007). In the past, pesticides and biological control agents were commonly used to maintain the productivity of infested sites (Mason and Paul 1996, Sheehan 1996); however, the minimal long-term success and the cost of these treatments have led to the search for less costly alternatives, such as thinning. Unfortunately, few studies have examined the effectiveness of thinning at minimizing mortality from SBW. Also, these studies have focused on the primary host species, balsam fir (Crook et al. 1979, Bauce 1996), leaving a limited understanding of the effectiveness of this approach for spruce-dominated systems. Furthermore, studies evaluating the effectiveness of thinning in reducing the impact of SBW have found varying results. For example, studies in young balsam fir stands have suggested that thinning treatments may elevate the severity of SBW by increasing host vigor (MacLean and Piene 1995). On the other hand, studies in older mixed conifer stands in the western United States have demonstrated that thinning may have beneficial effects in reducing western SBW populations in infested stands (Carlson et al. 1985). There is a well-documented history of SBW outbreaks in eastern North America, with each lasting 7–17 years and separated by 30–60 years of no activity (Colson and Witter 1984). Interestingly, the most recent outbreak in Minnesota started in 1954 and has lasted across the forested landscapes in this region for over 55 years (Minnesota Department of Natural Resources [MNDNR] 2008). Potential explanations for the extended duration of this outbreak include an increased abundance of balsam fir due to the maturation of postlogging mixed aspen-balsam fir stands, as well as the fragmentation and homogenization of the landscape due to past land use (Sturtevant et al. 2004). In the 1990s, SBW began targeting white spruce, causing substantial top kill and mortality, particularly in white spruce plantations across Minnesota and other portions of the Lake States region (MNDNR 1996, Wisconsin Department of Natural Resources [WDNR] 2006). White spruce plantations cover over 72,000 ha of commercial forestland across the northern Lake States (Michigan, Minnesota, and Wisconsin; Rauscher 1984), suggesting a need to evaluate silvicultural strategies that might minimize effects of SBW outbreaks on stand productivity in a cost-effective manner. To address this need, this study investigates the relationships between thinning treatments applied to white spruce plantations affected by SBW and changes in individual tree and stand-level growth, canopy structure, and survivorship. Specific questions included the following: (1) Does thinning increase survivorship in stands affected by SBW? (2) Does thinning increase stand volume growth in stands affected by SBW through changes in foliage patterns (live crown ratio and foliage density)? (3) What factors are most important for predicting postthinning growth responses of individual white spruce in plantations affected by SBW?