Reintroducing fire into long-unburned coniferous forests often results in substantial overstory mortality; the result of smoldering fire within the forest floor often termed “duff fires.” Contrary to previous reports of mortality diminishing with increased tree size, duff fires tend to kill the largest individuals more often than smaller trees. These low temperature, long residence time fires penetrate the protective bark layers to damage vascular tissues and surficial organic and mineral soil horizons to damage active roots. At Eglin Air Force Base in Florida, we conducted a study to determine the role of duff moisture on longleaf pine (Pinus palustris) mortality. We installed large scale (10-ha) burning treatments to four long-unburned longleaf pine stands with three replicates each of dry, moist, and wet. Mean moisture content of pre-treatment duff measured 54.33 %, 87.15 %, 114.50 %, respectively. One-year overstory pine mortality ranged from 0 to 42 %, with dry duff moisture burns having the greatest mortality across all three sites. Likewise, duff consumption followed the same pattern, with consumption highest (mean=69%) in dry burns, lowest (mean=38%) in wet burns. Our on-going work is monitoring delayed mortality, determining mechanistic links between smoldering fire and tree damage and physiology, and designing prescription parameters for safely reintroducing prescribed fire to long-unburned southern pine forests.