Spatial vegetation diversity index along a postfire successional gradient in the northern boreal forest

Boreal forest dynamics and biodiversity are mainly governed by natural dist urbances such as fire. Because boreal forest communities are typically spec ies-poor and composed predominantly of wide-ranging circumboreal species, a ll measurements of biodiversity using the most common species richness-base d indices are likely to underestimate vegetation diversity at the stand lev el. To estimate vegetation diversity differences, we introduce a spatial di versity index (SDI), which accounts not only for species richness and speci es abundance, but also for the spatial occupancy of species, a neglected al though important component of plant diversity. We tested the SDI along a po stfire successional gradient of the lichen woodland zone in northern Quebec using eleven sites with different postfire ages, The SDI allowed us to sta tistically differentiate three species' spatial occupancy patterns. which c orrespond to three successional stages (pioneer, expansion and stabilizatio n). In our study. we were unable to discriminate between these three succes sional structural phases using only Simpson and Shannon diversity indices. We conclude that indices based only upon species richness and species abund ance may fail to differentiate vegetation diversity beta een sites in the b oreal forest, whereas the spatial diversity index has succeeded because it incorporates species space occupancy.