Changes in Host Abundance Following Harvesting Desynchronize Forest Insect Pest Outbreaks
Humans shape landscape composition and structure with activities including fire suppression and timber harvests, leading to changes in fundamental ecosystem processes, but the extent to which they influence insect outbreak dynamics is unknown. A Forest Service scientist led an international team that studied 20th-century spruce budworm outbreak dynamics over a 6-million hectare (about 15 million acres) ecoregion straddling the border of Ontario,Canada, and Minnesota,USA. This ecoregion contains three contrasting forestland management legacies that have resulted in differences in forest composition and structure: fine-grained forest harvesting in Minnesota, coarse-grained forest harvesting in Ontario, and an unmanaged wilderness zone situated between the two with little recent harvest activity. The researchers reconstructed spruce budworm outbreak histories across management zones and climatic gradients using tree-ring chronologies at 16 locations. They found that forest composition and configuration explained 14 and 11 percent of the variance in outbreak variability, whereas climate explained only 0.2 percent. Results suggest that diverse forest landscapes will have lower probability of intense budworm outbreaks compared to landscapes dominated by host species, namely balsam fir and spruce. This study demonstrates the unintended impacts of human induced landscape-scale changes in forest structure on natural ecosystem functioning in general, and on insect outbreak dynamics in particular.
Forest Service Partners