Conventional wisdom in fire management holds that forested stands containing trees that are killed by insects, disease, or fire will remain at high fire hazard for decades after the disturbance. The foliage and fine woody material that falls from the trees killed by the disturbance agents will be highly flammable and create landscapes that have high risk for abnormally severe fire, including crown fire. This assumption, however, is currently being debated for many ecosystems across the western United States. There may be only a small period of time when the disturbance-killed tree foliage is markedly more flammable than the remaining live foliage, especially during moisture conditions when surface fuels can burn.
Dead foliage may fall to the ground within a year or two. Therefore, some think that the fine woody material from dead trees will fall to the ground and create surface fuel conditions that could foster wildfires of high intensity and severity. Recent litterfall studies have shown that only small amounts of fine fuel are deposited each year and much of this material is lost to duff in a decade or two.
Caption: Litter traps are used to collect dead foliage for monitoring. Photo credit Bob Keane
To help resolve these uncertainties, researchers investigated the effect of disturbance events, namely fire and beetles, on future fire hazard and risk in conifer forests of the Northern Rocky Mountains. The study included four sites that had experienced fire, and 11 sites disturbed by mountain pine beetle. The sites contained Douglas fir, lodgepole pine, ponderosa pine, whitebark pine, subalpine fir, spruce, and/or larch. From 2001 - 2011, scientists and staff annually measured stand and fuel characteristics at every site. They measured surface fuel deposition and the resulting decomposition rates for 11 forest types across the Northern Rocky Mountains after wildfire events and beetle epidemics to quantitatively describe fuel dynamics. Using the BehavePlus fire modeling system for predicting surface fire behavior and NEXUS for estimating crown fire behavior, the researchers used collected or measured tree and fuels data, local weather summaries, and other data to quantify fire behavior for each plot. A summary reporting fuel deposition and decomposition rates in these disturbed stands during the 10-year period is forthcoming. The report will also describe the fire hazard in these stands for each of the 10 years.
Caption: Litter traps were installed at this site after stand-replacing fires to measure fuel characteristics. Photo credit Bob Keane
Results from this study will inform managers and researchers about the effects of stand-replacement disturbances on fuel, future fire behavior, and future fire effects. This research may lead to new methods of prioritizing fuel treatments after major insect, disease, or fire events, and will provide important parameters and values for fuel sampling efforts.
For more information, please visit http://firelab.org/project/fueldist.