Search

Wildland fires impact ecosystems and communities worldwide. Many wildfires burn in living or a mixture of living and senescent vegetation. Therefore, it is necessary to understand the burning behavior of living fuels, in contrast to just dead or dried fuels, to more effectively support fire management decisions. In this study, the ignition and burning behaviors of needles placed in convective heat flux were evaluated. The species included longleaf pine (Pinus palustris), Douglas-fir (Pseudotsuga menziesii), western red cedar (Thuja plicata), ponderosa pine (Pinus ponderosa), western larch (Larix occidentalis), pacific yew (Taxus brevifolia), white spruce (Picea glauca), and sagebrush (Artemisia tridentate). The ignition and burning behaviors were related to live fuel moisture content (LFMC), pilot flame temperatures, and convective heat fluxes. The different phases of ignition and burning were captured using high-speed imaging. In general, four burning phases can be observed: droplet ejection and burning, a transition phase, flaming combustion, and smoldering combustion. Ejection and subsequent burning of droplets can occur prior to sustained flaming ignition only in live fuels. For some species (e.g., longleaf pine, ponderosa pine, white spruce) droplet ejection and burning can reduce ignition times relative to dried fuel with lower LFMC. In general, the transition phase tends to take longer than the flaming and droplet phases (when these occur). During the transition phase, the fuels are heated and pyrolysis occurs. Time-scales to ignition and the different phases of ignition and burning vary more among live fuels than dead and dried fuels. This conclusion indicates that other parameters, such as chemical composition and structural morphology of the fuel, can significantly influence the burning of live fuels.

Burn severity is the ecological change resulting from wildland fires. It is often mapped by using prefire and postfire satellite imagery and classified as low, moderate, or high. Areas burned with high severity are of particular concern to land managers and others because postfire vegetation, soil, and other important ecosystem components can be highly altered. In this study, we developed Random Forest statistical models describing the occurrence of high burn severity across the contiguous United States. We divided our work into 17 regions in the western United States and 8 regions in the eastern United States, and further subdivided them by forest and nonforest vegetation settings, resulting in 50 separate models. Our predictor variables represented prefire vegetation, topography, and fuel moisture, and our response variable was satellite-derived burn severity from the U.S. interagency Monitoring Trends in Burn Severity program. We created statistical models from 6,663 fires in the western United States (1984-2007) and 5,295 fires in the eastern United States (2000-2013). We then mapped our model predictions as a Severe Fire Potential (SFP) index ranging from 0 to 100. Our Random Forest models performed reasonably well (area under the receiver operating characteristic curve [AUC] = 0.71-0.93), though independent validation of our spatial predictions was less promising (AUC = 0.50-0.79). We analyzed spatial distributions of our SFP predictions, as well as satellite-based burn severity observations, across mapping regions and broad landform classes. We present observations about the spatial distribution of high severity fire, the variables influencing burn severity, and suggestions for applications of our SFP data that we believe managers and scientists interested in burn severity will find useful. This is the first national effort to understand where and why fires burn severely. Our consistent set of burn severity maps, field observations, and predictor variables together move us toward a better understanding of the "ecological footprint" of fires in forests and rangelands across the contiguous United States. [ABSTRACT FROM AUTHOR]