Ignition and burning behavior of live woody fuels

This study evaluated the ignition and burning behavior of woody fuels (e.g. stems and branches >6 mm in diameter with all additional stems and foliage removed) for four species: Douglas-fir, ponderosa pine, manzanita, and sagebrush. Samples were held above a burner while the process was recorded with a DSLR camera. Four burning stages were observed: micro-explosions (eruptive jetting and droplet burning), transition, flaming combustion, and smoldering combustion. Micro-explosions caused bursts of flames and ejected volatiles up to 15 cm away from samples, suggesting that adjacent fuels could be heated from this process. Micro-explosions can occur in both live and dead woody fuels; this contrasts with previous results for foliage, where droplet ejection has only been observed for live samples. The amount of moisture present at ignition depends on fuel size and heat flux. Some samples had no moisture remaining at ignition (how wildland fuels have been classically modeled), while others retained some moisture like live fine fuels. The ignition times showed sensitivity to the fuel diameter and fuel moisture content; an increase in either diameter or fuel moisture content increased the ignition time. Woody fuels up to 25 mm in diameter, which are generally thought of as thermally thick fuels, showed ignition behavior consistent with aspects of thermally thin and thick fuels.