A recently published study in the Canadian Journal of Forest Research indicates that fuel reduction treatments should simultaneously take place in the overstory, understory, and on the ground to adequately reduce fire severity. Thinning trees without treating surface fuels does not reduce mortality adequately because mortality can occur from hot fires on the ground, as well as fires that burn through the tree crowns.
The study, “Fuel Treatments Alter the Effects of Wildfire in a Mixed-Evergreen Forest, Oregon, USA,” is co-authored by Crystal Raymond, University of Washington and David L. Peterson, Pacific Northwest Research Station/USDA Forest Service.
The 2002 Biscuit Fire in southwestern Oregon impacted forest resources, threatened lives, and costs millions of dollars to suppress. It also provided a rare opportunity to study previous fuel treatments (thinning of forest stands and underburning of surface fuels) to see which projects actually reduced tree damage when the Biscuit Fire burned through.
The research team measured the effects of fuel treatments in a Douglas-fir–tanoak forest. Scientists were fortunate to have data collected before the Biscuit Fire with which to directly quantify the relationship between forest structure and fire severity. The effectiveness of two fuel treatments at reducing tree damage and mortality was measured by comparing treated and untreated forests that burned in the fire.
“The number of trees killed in the Biscuit Fire was highest in the thinned areas we studied, most likely due to slash left after the thinning treatment,” Raymond explains. “Overstory tree mortality was lowest in sites that were thinned and then underburned, and moderate in sites that were not treated prior to the Biscuit Fire. Thinning ladder fuels is just the first step in effective fuel treatment for most forests.” Ladder fuels are the small trees that carry fire from the ground to the overstory tree crowns.
Fuel treatments intended to minimize damage to the overstory are more effective if fine fuels on the ground are reduced following removal of understory trees. “We have known this in principle for many years,” says Peterson, a research biologist at the Station, “and the Biscuit Fire gave us a chance to validate the effectiveness of on-the-ground fuel treatments.”
Federal agencies are mandated to reduce fuel accumulations in dry forests throughout the West. However, studies like the one conducted by Raymond and Peterson that validate the effectiveness of fuel treatments are rare. “Data from the Biscuit Fire provide new scientific evidence that will help improve techniques for treating fuels. As more wildfires burn through treated areas, we will have additional opportunities to document how well those treatments are working,” adds Peterson.