plant diversity and composition in boreal black spruce understory communities was assessed 15 years after the application of three harvest disturbance treatments on loam, sand and peat site types. the study was carried out on replicated uncut, tree-length, full-tree, and full-tree blade harvest treatments applied on 30 x 30 m treatment plots in six research areas in northwestern ontario in 1995/96. a significant treatment x site type interaction effect on species diversity and abundance was found. species richness and diversity increased after canopy removal on all site types. species evenness after canopy removal was significantly different only on the loam sites. the retention of slash (tree-length) compared to its removal lead to a significant decrease in species diversity on loam sites; the reverse was observed on peat sites. species composition shifts were most evident on sites where the surface o-layer was removed (full-tree blade) with increases in herbaceous cover including grasses still evident 15 years after treatments. species evenness was only significantly different after full-tree blade compared to other harvest treatments on sand sites. mineral soil exposure and the presence of ruderal species were still evident on both the loam and sand site types. comparison of the soil seed bank with the composition of the 15 year old communities did not detect the presence of invasive or exotic species. species diversity response did not comply with the intermediate disturbance hypothesis. there was some evidence of compliance with the mass ratio hypothesis on peat sites, with the continued dominance of the pre-harvest ericaceous shrub and sphagnum components.