1. Plant invasions can be promoted by environmental heterogeneity, but the opposite effect, the impact of plant invasion on heterogeneity, has received little attention. Grassland invasions might contribute to decreased spatial heterogeneity because invaders tend to be larger than native vegetation. Lowered heterogeneity may contribute to the low diversity of invaded communities, as well as to the persistence of invasive populations. 2. We compared the spatial heterogeneity of roots and resources in uninvaded native grassland and in stands invaded by a relatively large exotic grass (Agropyron cristatum), in four combinations of mowing and nitrogen (N) addition. We focused on roots because they account for the majority of primary production in grasslands. 3. The spatial heterogeneity of root length (m root / m2 rhizotron image) and root production was significantly lower beneath A. cristatum than uninvaded grassland. This result was consistent in all combinations of mowing and N addition. 4. Beneath the invader, root length was significantly greater, and the proportion of samples that contained roots was significantly higher. This suggests that the invader decreased spatial heterogeneity by more completely filling the soil volume with roots. 5. Resource heterogeneity varied significantly between vegetation types in just one out of four cases examined, suggesting that invader effects on resource heterogeneity were small relative to its effects on root heterogeneity. 6. These results suggest a novel mechanism promoting invader success and persistence: high root heterogeneity, lower root length, and empty soil volumes in native grassland may make it relatively vulnerable to invasion, while reduced heterogeneity and greater root length in invaded grasslands may sustain stable, low-diversity communities dominated by the invader. Lowered heterogeneity accompanying invasion may partly account for the wide-spread occurrence of low diversity, invader dominated grasslands in North America.