Formerly widespread Midwest savannas have become a rare vegetation type mainly due to fire exclusion and development of fire resistance as trees increase in size. As a result, restoration management including burning and thinning is a high conservation priority. We examined temporal changes in a thinned and fire-managed mesic oak savanna remnant in the Chicago region of northeast Illinois. Before 1960, this remnant was structured with an open canopy and apparently an oak grub layer. It was initially sampled in 1995 after near canopy closure due to fire exclusion, and was resampled in 2013–14 after management. Burning did not increase canopy openness. We asked how thinning to increase canopy openness affected ground layer diversity at multiple scales and whether species light adaptations and traits could predict responses to thinning. Thinning reduced stem density and increased canopy openness to > 30%, but also reduced canopy heterogeneity as large canopy gaps were not restored. Alpha diversity increased significantly due to increases in density and cover of light-adapted rhizomatous woodland species. Beta diversity declined, reflecting reduced canopy heterogeneity and decline of shade-adapted species. Gamma diversity of light-adapted woodland species also increased. Despite close proximity of a species pool of C4 grasses and prairie forbs, a truncated canopy light gradient apparently prevented increases in these shade-intolerant species. Oak sprouts from cut stumps appear to be capable of regenerating a grub layer. Management implications include the need to apply additional thinning to further increase canopy openness and heterogeneity as well as diversity of ground layer vegetation at multiple scales, and to apply frequent fire to maintain an oak grub layer and enhance herbaceous species establishment.