Fuel accumulations were measured in duff mounds around the bases of 19 mature Pinus ponderosa Laws. (ponderosa pine) in a 200-year-old stand in Glacier National Park, Montana. Tree diameter at breast height ranged from 50 to 114 cm (mean = 80 cm). The stand burned at intervals between 13 to 58 years prior to European settlement. This stand had not burned for 69 years. The duff depth 30 cm from the tree bole ranged from 3 to 39 cm (mean = 18 cm). Duff depth increased with tree diameter and decreased with distance from the bole. Duff depth 90 cm from the bole averaged one-half the depth 30 cm from the bole. Duff consumption and its effect on cambium mortality were quantified following a late summer, low intensity fire. Duff moisture contents on a dry weight basis were: fermentation (20%) and humus (36%). Smoldering combustion consumed 98% of the duff beneath the trees. Two patterns of duff burning were documented: downward spreading and lateral spreading. Temperatures near the root crown were above 300°C for 2 to 4 hours, resulting in mortality of 45% of the cambium samples (n = 76) tested at the root crown. The probability of cambium mortality increased with duff depth and tree diameter. However, cambium mortality was lower than expected from analysis of thermal diffusion through bark. Cooling by mass transport through phloem and xylem is suggested as a possible explanation for the low cambium mortality.