In Canada, the importance of seasonality in forest fire danger rating associated with phenological changes in deciduous tree leaves and lesser ground vegetation has historically been taken into account by dividing the fire season into three distinct periods (i.e., spring, summer, and fall). During the mid-1980s, the developers of the Canadian Forest Fire Behavior Prediction (FBP) System did not envision that the M-2 Boreal Mixedwood - Green fuel type with 100% hardwood composition would eventually be explicitly interpreted by field users and other researchers to represent a trembling aspen (Populus tremuloides Michx.) fuel type in the summer following green-up or flushing of the overstory canopy and understory vegetation. Interest in what has become to be known as the D-2 FBP System fuel type to represent leafed-out trembling aspen stands during the summer fire season has steadily increased since. Formal recognition of such a fuel type may very well constitute an example of overextending the original basis and heuristics associated with the rate of fire spread model for the M-2 FBP System fuel type. Thus, the assumptions underlying a D-2 fuel type are explicitly restated here for the benefit of fire managers and researchers alike. Furthermore, an interim guideline is presented with respect to the threshold condition in fuel dryness necessary for surface fire spread in the D-2 fuel type to occur based on existing empirical observations garnered from experimental fires, prescribed burns and wildfires. This criterion was deduced from existing information and knowledge, and is expressed in terms of the Buildup Index (BUI) component of the Canadian Forest Fire Weather Index System. The rationale for the descriptive name assigned to the D-2 fuel type and the corresponding fuel strata characteristics are given. Improvements in the present basis of the D-2 fuel type could be realized from monitoring selected wildfires and operational prescribed fires and/or by carrying out an experimental burning study.