A current, widespread example of vegetation change is the invasion of grassland by woody plants. This is associated with an increase in soil heterogeneity, and it has been argued that woody plants both cause and benefit from high heterogeneity. We know of no experimental demonstrations of differences between grasses and woody plants in their effects on heterogeneity. Here we compare heterogeneity between mixed‐grass prairie and aspen forest, and we report the results of a soil transplant experiment that tested for differences between these vegetation types in their effects on soil resource heterogeneity. We measured the heterogeneity of resources and plant mass along 10 transects in both prairie and aspen forest in spring and summer. Light and available nitrogen (N; sum of ammonium and nitrate) were significantly more variable in forest than prairie, as were root and understory shoot mass. The variability of soil moisture and topography did not differ between prairie and forest. In our experiment, N and water in cores of prairie soil moved to forest attained the relatively high variability of forest soils. Further, forest soils moved to prairie attained the relatively low variability of prairie soils. In summary, both the biomass heterogeneity measurements and the soil transplant experiment suggested that plant uptake contributed to greater heterogeneity in forests.