Impacts of Douglas-fir Beetles on Overstory and Understory Conditions of Douglas-fir Stands

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Journal/Book Title/Conference

USDA Forest Service

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USDA Forest Service Renewable Resources

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Douglas-fir beetle (Dendroctonus pseudotsugae) infestations frequently result from disturbance events that create large volumes of weakened Douglas-fir (Pseudotsuga menziesii) trees. Although research has focused on measuring and predicting the amount of tree mortality caused by Douglas-fir beetle infestations following disturbance events, there has been an inadequate amount of work on the consequent changes in both the overstory and understory. In 1988, extensive wildfires occurred in Yellowstone National Park and the Shoshone National Forest. Populations of Douglas-fir beetle increased in firescorched trees caused by the wildfires. Subsequent generations of the beetles moved from these injured trees to undamaged trees in neighboring stands on the Shoshone National Forest, Wyoming. In 1999, Forest Health Management personnel quantified changes in forest stand conditions and subsequent responses in the understory caused by the Douglas-fir beetle infestation using transect sampling (20 miles) and 25 pairs of previously infested and uninfested plots. Significant effects of the Douglas-fir beetle infestation included: 1) Basal area was reduced by 40 - 70 percent, tree diameter decreased by 8 - 40 percent, and the Douglas-fir component of the overstory decreased by more than 15 percent. 2) Conifer seedling regeneration increased nearly four-fold in the infested plots and 90 percent of the regeneration was Douglas-fir. 3) The understory vegetation (forbs, grass, and shrubs) had a three-fold increase in the infested plots compared with uninfested plots. In addition, basal area of Douglas-fir killed by the Douglas-fir beetle was significantly correlated with initial Douglas-fir basal area and percent of Douglas-fir, but not tree diameter or trees per hectare. Significant inverse relationships were also found between post-infestation basal area and the abundance of forbs, grass, shrubs, and understory height. Based on these results, Douglas-fir beetle infestations, although causing significant shortterm impacts in both the overstory and understory, probably are not changing the long-term successional patterns. Management alternatives are presented to control Douglas-fir beetle impacts for areas where the beetle is jeopardizing forest objectives. Information gathered from this study will be useful to the Shoshone National Forest, and other national forests both inside and outside the region, and the Yellowstone National Park.