In recent decades, bark beetle disturbances are increasing in extent and severity across western forests. Causes and consequences of spruce beetle (Dendroctonus rufipennis) infestation are important to the management of Engelmann spruce (Picea engelmannii) forests. Increased rates of tree mortality have been attributed to the influence of warming temperatures on bark beetle life cycles, but more research is needed to determine whether other potential factors exist and how those factors will change with the climate.
To address this need, Forest Service scientists modeled the effects of increased temperatures and changing forest stand conditions, such as density and species composition, on the likelihood of spruce beetle infestation over time.
Global climate change scenario models were used to determine future temperatures, which were combined with forest stand conditions to make predictions of future beetle infestation. Results show that under all climate scenarios, the percentage of spruce forest likely to be infested by spruce beetle increased, and the increase was most pronounced in the year 2080, when anywhere from 16 to 32 percent of forests in the Interior West could be infested.
Temperature and stand conditions were equally important predictors. In particular, the amount of mature Engelmann spruce in a stand, in combination with increasing temperatures, greatly increases the likelihood of spruce beetle infestation. Although the amount of potential spruce beetle activity is predicted to increase, it is important to note that no model predicted total mortality of spruce.
Results corroborate previous studies that were conducted on the mountain pine beetle in western North America. Findings from this study are being incorporated into management guidelines for silviculturists who wish to mitigate spruce beetle infestation by modifying the density or composition of Engelmann spruce forests in the Interior West.