Boreal forest dynamics and biodiversity are mainly governed by natural disturbances such as fire. Because boreal forest communities are typically species-poor and composed predominantly of wide-ranging circumboreal species, all measurements of biodiversity using the most common species richness-based indices are likely to underestimate vegetation diversity at the stand level. To estimate vegetation diversity differences, we introduce a spatial diversity index (SDI), which accounts not only for species richness and species abundance, but also for the spatial occupancy of species, a neglected although important component of plant diversity. We tested the SDI along a postfire successional gradient of the lichen woodland zone in northern Quebec using eleven sites with different postfire ages. The SDI allowed us to statistically differentiate three species' spatial occupancy patterns, which correspond to three successional stages (pioneer, expansion and stabilization). In our study, we were unable to discriminate between these three successional structural phases using only Simpson and Shannon diversity indices. We conclude that indices based only upon species richness and species abundance may fail to differentiate vegetation diversity between sites in the boreal forest, whereas the spatial diversity index has succeeded because it incorporates species space occupancy. © Ecoscience.