Ignition and burning behavior of live foliage: single elements vs. groups

This study investigated the ignition and burning behavior of single foliage elements and naturally occurring foliage groups under controlled convective heating. Across all species, droplet ejection and combustion was observed, with more frequent ejection and burning occurring in foliage groups. The combustion of ejected droplets represented a distinct ignition phenomenon and contributed to early heat release, particularly in conifers. In some cases, the burning of ejected droplets occurred simultaneously with the burning of the foliage. Ignition behavior differed between single foliage elements and foliage groups. At low heat fluxes, foliage groups were more likely to ignite than single elements due to higher local equivalence ratios from greater pyrolyzate mass flux, while no sensitivity to foliage grouping was observed at higher fluxes. Foliage groups exhibited longer and more variable flaming durations than single elements, reflecting sequential ignition and larger available fuel mass. Unlike ignition time, flaming duration showed no consistent dependence on heat flux, indicating that fuel traits and arrangement exert stronger influence. These findings highlight the potential importance of incorporating fuel geometry and foliage grouping effects into fire behavior models, particularly for ignition under low heating conditions.