Fire is one of the key disturbances affecting trembling aspen (Populus tremuloides Michx.) forest ecosystems within western Canadian wildlands, including Elk Island National Park in central Alberta, Canada. Although prescribed fire is a tool available to modify aspen forests, a clear understanding of its potential impact is necessary to successfully manage this disturbance. Undesirable social and ecological consequences of severe, deep-burning ground fires include smoke generation and impaired vegetation regrowth. Data on the duff moisture conditions under which ground or subsurface fires may ignite and spread in aspen forest duff layers are presented, as well as experimental test fire results. Different topographic positions, plant communities and seasonality were factored into the research design. The Duff Moisture Code (DMC) and Drought Code (DC) components of the Canadian Forest Fire Weather Index System were calculated and factors including duff moisture content, bulk density and inorganic content measured before ignition of experimental test fires. Probability of sustained smouldering combustion models were developed for the duff layer in the aspen forest fuel type in Elk Island National Park, with values of 27 for DMC and 300 for DC at the 50% probability level.