Effects of forest management legacies on spruce budworm (Choristoneura fumiferana) outbreaks

The silvicultural hypothesis of spruce budworm (Choristoneura fumiferana Clem.) dynamics postulates that increasing severity of spruce budworm outbreaks over the last century resulted from forest conditions created by past management activities. Yet, definitive tests of the hypothesis remain elusive. We examined spruce budworm outbreak dynamics (synchrony, periodicity, and intensity) in the 20th century using historical reconstruction from tree-ring chronologies sampled within 19 sites in a large ecoregion located on the border of Minnesota and Ontario. The study encompassed three areas affected by contrasting management legacies: a fine-grained area (Minnesota, six sites, average cut size = 17 ha), a coarse-grained area (Ontario, six sites, average cut size 10 times that of Minnesota), and a conservation zone (seven sites) with little recent harvest activity overlapping the border. Results suggest important differences in outbreak dynamics between the forest management zones that cannot be explained by differences in climate among sample sites. Budworm outbreaks within the conservation zone were more synchronous, with more trees per site affected and less frequent outbreaks than sites sampled within fine-scale managed areas. Outbreak dynamics within forests managed at coarser scales suggest a mixture of the conservation and fine-scale management zone outbreak patterns. Potential factors affecting differences in the observed outbreak patterns include forest pattern, composition, and age. Our study generally supports the silvicultural hypothesis and emphasizes that management legacy effects on spruce budworm dynamics should be observable at landscape scales, as well as at local scales.