(1) Variation in the physical environment was investigated at scales of 0.1-50 m in the understorey of a southern Quebec forest, dominated by Acer saccharum and Fagus grandifolia, to determine if the spatial heterogeneity necessary to account for the evolution and maintenance of genetically diverse populations of forest plants exists in this old-growth forest. (2) To characterize spatial heterogeneity in the understorey during the late summer, soil pH and the availability of K+ and NO3(-1) ions in the soil solution were measured. The sampling programme involved analysis of replicate soil cores at 555 points dispersed throughout a 50-m X 50-m area in a valley previously undisturbed by human activities. (3) A geostatistical analysis demonstrated that all three edaphic measures are predictably similar up to about 2 m, but that at greater distances the predictability is negligible and the environmental regime is best estimated by the mean for the site as a whole. (4) These results demonstrate environmental variance at scales relevant to seed dispersal and genetic neighbourhood size in understorey herbs such as Impatiens. This spatial pattern of environmental variation may contribute to the maintenance of genetic variation through selection of locally adapted subpopulations.