Boreal forest ecosystems are important drivers of the global carbon (C) cycle by acting as both sinks and sources of atmospheric CO(2). While several factors have been proposed as determining the ability of boreal forest to function as C sinks, little is known about their relative importance. In this study, we applied structural equation modelling to a previously published dataset involving 30 boreal-forested islands that vary greatly in their historic fire regime, in order to explore the simultaneous influence of several factors believed to be important in influencing above-ground, below-ground and total ecosystem C accumulation. We found that wildfire was a major driver of ecosystem C sequestration, and exerted direct effects on below-ground C storage (presumably through humus combustion) and indirect effects on both above-ground and below-ground C storage through altering plant-community composition. By contrast, plant diversity influenced only below-ground C storage (and even then only weakly), while net primary productivity and decomposition had no detectable effect. Our results suggest that while boreal forests have great potential for storing significant amounts of C, traits of dominant plant species that promote below-ground C accumulation and the absence of wildfire are the most important drivers of C sequestration in these ecosystems.