Patterns of duff ignition and smoldering beneath old Pinus palustris: Influence of tree proximity, moisture content, and ignition vectors

Smoldering fires are common in deep forest floor fuels in long-unburned forests. These “duff fires” that burn through lower forest floor horizons (Oe and Oa) are often implicated in postfire stress and mortality of large conifers, long-duration soil heating, and noxious emissions. We present data on duff ignition and consumption patterns from burn studies in long-unburned Pinus palustris forests in northern Florida, USA. In small experimental and large operational-scale burns, ignition probability and duff consumption were greatest near tree bases, despite moisture condition or the presence of ignition vectors (pine cones and woody fuels). Upper duff (Oe) horizons dried throughout the 45 days of small-scale experiments, whereas lower duff (Oa) moisture was more constant. Duff adjacent to trees, however, was deeper and typically drier than beyond duff mounds. Oa horizons were least dense at tree bases, whereas Oe bulk density did not differ with tree proximity. Our results highlight the propensity for duff to ignite at the base of mature longleaf pine trees and significant basal duff consumption to occur at moisture contents <60%. Restoring fire to long-unburned coniferous forests may be a challenge where basal duff has accumulated and will require special attention to forest floor moisture to minimize negative effects.