- Wildland fire refers to an unpredictable and uncontrollable fire that happens in wildland areas. Regardless of the beneficial impacts of wildland fires on ecosystems, the destructive impacts of wildfires on air quality, economy, public health, etc. are considerable. An essential goal in the fire community is that the wildland fires get controlled and managed when they may have destructive impacts. To achieve this goal, in the first step, it is necessary to study the ignition and burning behavior of live fuels. Many fires burn live and a combination of live and dead fuels. Therefore, more parameters are involved in the ignition and burning of live fuels than those in dead fuels. Ignition and burning of live fuel are still potential subjects for further research due to a lack of knowledge regarding its behavior. This study will seek to identify the physical and chemical processes in live fuel ignition and burning. Hence, this work is comprised of two studies focused on identifying key physical and chemical processes in the ignition and burning of live fuels.
In the first study, the ignition and burning behaviors of live fuels in different convective heat fluxes were evaluated. The species evaluated consist of different species such as 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. Individual needles were suspended above a flat flame burner, and the resulting ignition process was recorded using a high-speed camera. In general, four burning stages can be observed: droplet burning, transition, flaming, 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 stage tends to take longer than the flaming and droplet stages (when these occur). During the transition stage, the fuels are heated, and pyrolysis occurs. Time-scales to ignition and the different stages 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.
In the second study, the influence of chemical composition and seasonal variability on the ignition and burning of four living conifer species were investigated. The species studied include long-leaf pine (Pinus palustris), ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii), and western larch (Larix occidentalis). Individual needles were held 2 cm above a flat flame burner, and the resulting ignition and burning processes were recorded using a high-speed camera to identify the time of droplet, transition, and flaming/smoldering stages for each species. After finding the time of each stage, then the samples were put on the burner for a specific time to collect enough droplet, transition, and flaming/smoldering samples for chemistry analysis. Live fuel moisture content (LFMC), chemical composition, and time-to-ignition of live fuels at the constant heating condition are reported. Finally, the variation of chemical composition, live fuel moisture content, time-to-ignition, and the time-scale of live fuel burning are measured to evaluate the impact of seasonal variability.