The hypothesis that the intensity of belowground competition varies with community standing crop was tested in forest and prairie in central Canada. Three plots were studied in each habitat. Forest had significantly higher soil nitrate, ammonium, water, and root biomass than prairie, but significantly lower root:shoot ratios. Transplants of a grass (Bouteloua gracilis) and a tree (Populus deltoides) were grown singly for one summer in both habitats with live neighbor roots either absent or present. Live neighbor roots were removed by cutting roots along the perimeter of a 10 cm diameter circle to a depth of 15 cm and then inserting a plastic tube (10 cm diameter, 15 cm long) vertically into the soil. Transplants grown with live neighbor roots also had neighbor roots cut along the perimeter of a 10 cm diameter circle, but no tube was installed. There were ten replicates of each combination of species and root treatment in each plot. The aboveground biomass of transplants showed a significant interactive effect between habitat and competition treatment. Biomass was significantly lower in the presence of neighbor roots in prairie, but not in forest. Neighbor roots significantly decreased the survivorship of the tree but not the grass. Belowground competition was most intense in prairie, where the supply of soil resources was lowest.