One of the goals of ecosystem management has been to maintain plant specie diversity. Emulating the "natural” (pre-European) fire regime is often proposed as a means of accomplishing this goal in fire-influenced boreal forest ecosystems. Here we examine this hypothesis as it applies to upland vascular plant species in the mixedwood boreal forest of Saskatchewan, Canada. Results from standardized multiple regressions indicate that neither~ time-since-fire nor canopy coverage (light availability) is the most important in accounting for species richness, evenness, and diversity. Instead, the most important factors are hillslope position and surficial geology (i.e., the moisture and nutrient gradients), and of secondary importance is the interaction between time-since-fire and canopy coverage (light availability). Diversity and species richness increase downslope as the moisture and nutrients increase Glacial till hillslopes have higher richness than glaciofluvial hillslopes. The upper parts of~ hillslopes have lower richness, wider species tolerances, and lower overlap of species frequencies. Lower parts of hillslopes have higher richness, narrower species tolerance, anc more overlap of species frequencies. Of secondary importance in influencing hillslope diversity is the interaction between time-since-fire and canopy coverage. Younger forests with~ lower canopy coverage have higher diversity, and older forests with more complete canopy coverage have lower diversity. Thus, upland species diversity in this fire-dominated ecosystem~ is determined by the system of hillslope and surficial geology and the interaction between time-since-fire and canopy coverage. Finally, species richness can be predicted using hillslope~ position, time-since-fire, and canopy coverage, respectively, from a Digital Elevation Mode~ (DEM), a time-since-fire map, and a LANDSAT TM image. ©Ecological Society of America. Abstract reproduced by permission.