Fire has an important role for regeneration of many boreal forest tree species, and this includes both wildfire and prescribed burning following clear-cutting. Depending on the severity, fire can have a variety of effects on above- and below-ground properties that impact tree seedling establishment. Very little is known about the impacts of ground fire severity on post-fire seedling performance, or how the effects of fire severity interact with those of canopy structure. We conducted a full-factorial experiment that manipulated surface-burn severity (no burn; light, medium, or heavy burn; or scarification) and canopy (closed forest or open clear-cut) to reveal their interactive effects on ecophysiological traits of establishing broadleaf and conifer seedlings in a Swedish boreal forest. Medium and heavy surface burns increased seedling growth, photosynthesis, respiration, and foliar N and P concentrations, and these effects were most apparent in open clear-cuts. Growth rates of all species responded similarly to surface-burn treatments, although photosynthesis, foliar P, and specific leaf area were more responsive to burning treatments for broadleaf species than for conifers. Our study demonstrates that the positive impacts of fire on tree seedling physiology are dependent on a minimum severity threshold and are more effective when combined with clear-cutting.