Western spruce budworm outbreaks often last for decades in the interior West. However, their impact on fire behavior is poorly understood. Insects are often thought to increase fire hazard or behavior owing to high tree mortality, but the western spruce budworm kills few trees outright. Scientists have little understanding of how removing foliage alone affects fire behavior, and it is difficult to make precise links between historical western spruce budworm outbreaks and fire.
To isolate the effect that defoliation has on crown fuels, researchers simulated single-tree torching across a range of crown fuel changes that occur during western spruce budworm infestation. By isolating the effects on a single tree and simulating the tree in a three-dimensional fire model called the Wildland-Urban Interface Fire Dynamics Simulator (WFDS), these researchers were able to identify precise links between western spruce budworm disturbance and fire behavior changes. Development of WFDS was a joint project between the U.S. Forest Service Pacific Northwest Research Station and the National Institute of Standards and Technology (NIST) in Maryland. In addition, tree ring analysis was used to reveal historical fires and outbreaks of western spruce budworm from central Oregon to western Montana.
Tree rings revealed many historical fires and outbreaks of western spruce budworm outbreaks along a 1,000-mile transect from central Oregon to western Montana, but no consistent relationship in their timing. Western spruce budworm was associated with the ends of droughts, whereas fire was associated with single drought years. Results indicated that defoliation, when modeled as reduced foliar density, dampens crown fire behavior by inhibiting fire spread between crowns and increasing the surface fire intensity necessary to ignite a single tree crown. This change in crown ignition is unique because it results from reduced crown fuel without a change to crown fuel base height.This suggests that areas heavily defoliated by western spruce budworm may inhibit crown fire spread and may thus promote non-lethal surface fires.
For more information, please visit: http://www.firelab.org/project/western-spruce-budworm.