  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.
