Question What are the determinants of fine-scale plant spe-cies richness (SR)?Location: Île-aux-Grues, Québec, Canada.Methods: Elevation, soil organic matter, soil pH, irradiance, tree basal area (B A) and plant SR (herbs, shrubs, and trees) were determined in 100 contiguous 25 m2 quadrats in a deciduous forest. Each variable was analysed for spatial autocorrelation using Moran's I. Path analysis was used to determine the effects of different variables on tree, shrub and herb SR in a hierarchical modelling approach.Results: Most of the variables, except tree BA, PPFD (photo-synthetic photon flux density) and shrub SR, were positively autocorrelated at a scale of ca. 20 m or less. The path analyses explained ca. 11%–40% of the variance in plant SR; however, the model for shrub SR was not significant. Tree SR was posi-tively associated, but herb SR was negatively associated with tree BA. Tree SR had a positive influence on shrub SR, but herb SR remained unaffected by tree or shrub SR.Conclusion: The positive association of tree BA and tree SR suggests that the data from the study site correspond to the left (ascending) portion of the SR-biomass relationship (un-dersaturated SR). The negative effect of tree BA on herb SR is direct and not mediated through reduced PPFD. High tree BA might cause high resource use, induce high litter production and affect soil properties, all of which might significantly affect herb SR. Several factors not considered here might influence fine-scale SR, such as interspecific interactions, fine-scale disturbances and heterogeneity (both spatial and temporal) in resources and abiotic conditions. Within-site variations of SR might be difficult to model with precision because of the relative importance of stochastic vs deterministic processes at this spatial scale.